Thyagarajan, N., Helfand, D. J., White, R. L., & Becker, R. H. 2011, ApJ, 742, 49
A comprehensive search for variable and transient radio sources has been conducted using ~55,000 snapshot images of the Faint Images of the Radio Sky at Twenty-cm survey. We present an analysis leading to the discovery of 1627 variable and transient objects down to mJy levels over a wide range of timescales (a few minutes to years). Variations observed range from 20% to a factor of 25. Multi-wavelength matching for counterparts reveals the diverse classes of objects exhibiting variability, ranging from nearby stars and pulsars to galaxies and distant quasars. Interestingly, more than half of the objects in the sample have either no classified counterparts or no corresponding sources at any other wavelength and require multi-wavelength follow-up observations. We discuss these classes of variables and speculate on the identity of objects that lack multi-wavelength counterparts.
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Zeimann, G. R., White, R. L., Becker, R. H., Hodge, J. A., Stanford, S. A., & Richards, G. T. 2011, ApJ, 736, 57
We present the discovery of only the second radio-selected z ~ 6 quasar. We identified SDSS J222843.54+011032.2 (z = 5.95) by matching the optical detections of the deep Sloan Digital Sky Survey Stripe 82 with their radio counterparts in the Stripe 82 Very Large Array Survey. We also matched the Canadian-France-Hawaiian Telescope Legacy Survey Wide with the Faint Images of the Radio Sky at Twenty cm survey but have yet to find any z ~ 6 quasars in this survey area. The discovered quasar is optically faint, z = 22.3 and M 1450 ~ -24.5, but radio bright, with a flux density of f 1.4 GHz, peak = 0.31 mJy and a radio loudness of R ~ 1100 (where R ≡ f 5 GHz/f 2500). The i - z color of the discovered quasar places it outside the color selection criteria for existing optical surveys. We conclude by discussing the need for deeper wide-area radio surveys in the context of high-redshift quasars.
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Hodge, J. A., Becker, R. H., White, R. L., Richards, G. T., & Zeimann, G. R. 2011, AJ, 142, 3
We present a high-resolution radio survey of the Sloan Digital Sky Survey (SDSS) Southern Equatorial Stripe, a.k.a. Stripe 82. This 1.4 GHz survey was conducted with the Very Large Array primarily in the A-configuration, with supplemental B-configuration data to increase sensitivity to extended structure. The survey has an angular resolution of 1.8 arcsec and achieves a median rms noise of 52 μJy beam-1 over 92 deg2. This is the deepest 1.4 GHz survey to achieve this large of an area, filling a gap in the phase space between small, deep and large, shallow surveys. It also serves as a pilot project for a larger high-resolution survey with the Expanded Very Large Array. We discuss the technical design of the survey and details of the observations, and we outline our method for data reduction. We present a catalog of 17,969 isolated radio components, for an overall source density of ~195 sources deg-2. The astrometric accuracy of the data is excellent, with an internal check utilizing multiply observed sources yielding an rms scatter of 0.19 arcsec in both right ascension and declination. A comparison to the SDSS DR7 Quasar Catalog further confirms that the astrometry is well tied to the optical reference frame, with mean offsets of 0.02 arcsec ± 0.01 arcsec in right ascension, and 0.01 arcsec ± 0.02 arcsec in declination. A check of our photometry reveals a small, negative CLEAN-like bias on the level of 35 μJy. We report on the catalog completeness, finding that 97% of FIRST-detected quasars are recovered in the new Stripe 82 radio catalog, while faint, extended sources are more likely to be resolved out by the resolution bias. We conclude with a discussion of the optical counterparts to the catalog sources, including 76 newly detected radio quasars. The full catalog as well as a search page and cutout server are available online at http://third.ucllnl.org/cgi-bin/stripe82cutout.
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DiPompeo, M. A., Brotherton, M. S., Becker, R. H., Tran, H. D., Gregg, M. D., White, R. L., & Laurent-Muehleisen, S. A. 2010, ApJS, 189, 83
We report spectropolarimetry of 30 radio-selected broad absorption line (BAL) quasars with the Keck Observatory, 25 from the sample of Becker et al. Both high- and low-ionization BAL quasars are represented, with redshifts ranging from 0.5 to 2.5. The spectropolarimetric properties of radio-selected BAL quasars are very similar to those of radio-quiet BAL quasars: a sizeable fraction (20%) shows large continuum polarization (2%-10%) usually rising toward short wavelengths; emission lines are typically less polarized than the continuum; and absorption line troughs often show large polarization jumps. There are no significant correlations between polarization properties and radio properties, including those indicative of system orientation, suggesting that BAL quasars are not simply normal quasars seen from an edge-on perspective.
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McGreer, I. D., Helfand, D. J., & White, R. L. 2009, AJ, 138, 1925
We have conducted a pilot survey for z > 3.5 quasars by combining the FIRST radio survey with the Sloan Digital Sky Survey (SDSS). While SDSS already targets FIRST sources for spectroscopy as quasar candidates, our survey includes fainter quasars and greatly improves the discovery rate by using strict astrometric criteria for matching the radio and optical positions. Our method allows for selection of high-redshift quasars with less color bias than with optical selection, as using radio selection essentially eliminates stellar contamination. We report the results of spectroscopy for 45 candidates, including 29 quasars in the range 0.37 < z < 5.2, with 7 having redshifts z > 3.5. We compare quasars selected using radio and optical criteria, and find that radio-selected quasars have a much higher fraction of moderately reddened objects. We derive a radio-loud quasar luminosity function at 3.5 < z < 4.0, and find that it is in good agreement with expectations from prior SDSS results.
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Hodge, J. A., Zeimann, G. R., Becker, R. H., & White, R. L. 2009, AJ, 138, 900
We detect and study the properties of faint radio active galactic nuclei (AGNs) in luminous red galaxies (LRGs). The LRG sample comprises 760,000 objects from a catalog of LRG photometric redshifts constructed from the Sloan Digital Sky Survey (SDSS) imaging data, and 65,000 LRGs from the SDSS spectroscopic sample. These galaxies have typical 1.4 GHz flux densities in the 10s-100s of μJy, with the contribution from a low-luminosity AGN dominating any contribution from star formation. To probe the radio properties of such faint objects, we employ a stacking technique whereby FIRST survey image cutouts at each optical LRG position are sorted by the parameter of interest and median-combined within bins. We find that median radio luminosity scales with optical luminosity (L opt) as L 1.4 GHz vprop L β opt, where β depends on the redshift being probed. Above z sime 0.4, β appears to decrease from β sime 1 at z = 0.4 to β sime 0 at z = 0.7, a result which could be indicative of AGN cosmic downsizing. We also find that the overall LRG population, which is dominated by low-luminosity AGNs, experiences significant cosmic evolution between z = 0.2 and z = 0.7. A simultaneous fit to untangle the redshift and luminosity dependences yields redshift evolution of the form L 1.4 GHz vprop (1 + z)3.15±0.07, implying a considerable increase in total AGN heating for these massive ellipticals with redshift. By matching against the FIRST catalog, we investigate the incidence and properties of LRGs associated with double-lobed (FR I/II) radio galaxies.
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Urrutia, T., Becker, R. H., White, R. L., Glikman, E., Lacy, M., Hodge, J., & Gregg, M. D. 2009, ApJ, 698, 1095
We present results on a survey to find extremely dust-reddened Type 1 quasars. Combining the FIRST radio survey, the 2MASS Infrared Survey and the Sloan Digital Sky Survey, we have selected a candidate list of 122 potential red quasars. With more than 80% spectroscopically identified objects, well over 50% are classified as dust-reddened Type 1 quasars, whose reddenings (E(B - V)) range from approximately 0.1 to 1.5 mag. They lie well off the color selection windows usually used to detect quasars and many fall within the stellar locus, which would have made it impossible to find these objects with traditional color selection techniques. The reddenings found are much more consistent with obscuration happening in the host galaxy rather than stemming from the dust torus. We find an unusually high fraction of broad absorption line (BAL) quasars at high redshift, all but one of them belonging to the low-ionization BAL (LoBAL) class and many also showing absorption in the metastable Fe II line (FeLoBAL). The discovery of further examples of dust-reddened LoBAL quasars provides more support for the hypothesis that BAL quasars (at least LoBAL quasars) represent an early stage in the lifetime of the quasar. The fact that we see such a high fraction of BALs could indicate that the quasar is in a young phase in which quasar feedback from the BAL winds is suppressing star formation in the host galaxy.
Based on observations obtained with the W. M. Keck Observatory, which is jointly operated by the California Institute of Technology and the University of California.
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Hodge, J. A., Becker, R. H., White, R. L., & de Vries, W. H. 2008, AJ, 136, 1097
We investigate the radio emission of ~185,000 quiescent (optically unclassifiable) galaxies selected from the Sloan Digital Sky Survey (SDSS). By median-stacking Faint Images of the Radio Sky at Twenty Centimeter (FIRST) cutouts centered on the optically-selected sources, we are able to reach flux densities down to the 10s of μJy. The quiescent galaxy sample is composed of two subgroups inhabiting vastly different regimes: those targeted for the SDSS MAIN galaxy sample (~55%) and those targeted for the Luminous Red Galaxy (LRG) sample (~45%). To investigate the star formation of these quiescent galaxies, we calibrate a radio to star-formation rate (SFR) conversion using a third sample of star-forming (SF) galaxies. We confirm a tight power-law dependence for the SF sample, where L 1.4GHz ~ (SFR)1.37. Comparing this SFR-indicator with indicators in the optical and UV, we derive conflicting SFR estimates for the MAIN sample quiescent galaxies. These radio-derived SFRs intersect those calculated using the 4000 Å break (D4000) around an SFR of 1 M sun yr-1 and agree to within a factor of 3 over the range of SFRs. However, we find that the radio-derived SFRs are too high relative to the SFRs estimated for similar populations of galaxies using analysis of UV emission, implying either contamination of the radio by Active Galactic Nuclei (AGNs) or incomplete dust modeling. If AGN activity is dominant in these galaxies, then a relation between AGN radio luminosity and galaxy mass is required to explain the observed trends. For the LRGs, on the other hand, we find the radio luminosity to be relatively high (compared to the SF galaxies) and independent of SFR as derived from D4000, indicating that an AGN component dominates their radio emission. AGN-based radio emission often implies the existence of radio jets, providing evidence of a mechanism for low-level feedback in these quiescent LRGs.
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Auger, M. W., Becker, R. H., & Fassnacht, C. D. 2008, AJ, 135, 1311
In the local universe, high-power radio galaxies live in lower-density environments than low-luminosity radio galaxies. If this trend continued to higher redshifts, powerful radio galaxies would serve as efficient probes of moderate redshift groups and poor clusters. Photometric studies of radio galaxies at 0.3 lsim z lsim 0.5 suggest that the radio luminosity-environment correlation disappears at moderate redshifts, though this could be the result of foreground/background contamination affecting the photometric measures of environment. We have obtained multi-object spectroscopy of in the fields of 14 lower luminosity (L 1.4 GHz < 4 × 1024 W Hz-1) and higher luminosity (L 1.4 GHz > 1.2 × 1025 W Hz-1) radio galaxies at z ≈ 0.3 to spectroscopically investigate the link between the environment and the radio luminosity of radio galaxies at moderate redshifts. Our results support the photometric analyses; there does not appear to be a correlation between the luminosity of a radio galaxy and its environment at moderate redshifts. Hence, radio galaxies are not efficient signposts for group environments at moderate redshifts.
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Urrutia, T., Lacy, M., & Becker, R. H. 2008, ApJ, 674, 80
We present Hubble Space Telescope ACS images of 13 dust-reddened type 1 quasars selected from the FIRST/2MASS Red Quasar Survey. These quasars have high intrinsic luminosities after correction for dust obscuration (-23.5>=MB>=-26.2 from K-magnitude). The images show strong evidence of recent or ongoing interaction in 11 of the 13 cases, even before the quasar nucleus is subtracted. None of the host galaxies are well fit by a simple elliptical profile. The fraction of quasars showing interaction is significantly higher than the 30% seen in samples of host galaxies of normal, unobscured quasars. There is a weak correlation between the amount of dust reddening and the magnitude of interaction in the host galaxy, measured using the Gini coefficient and the concentration index. Although few host galaxy studies of normal quasars are matched to ours in intrinsic quasar luminosity, no evidence has been found for a strong dependence of merger activity on host luminosity in samples of the host galaxies of normal quasars. We thus believe that the high merger fraction in our sample is related to their obscured nature, with a significant amount of reddening occurring in the host galaxy. The red quasar phenomenon seems to have an evolutionary explanation, with the young quasar spending the early part of its lifetime enshrouded in an interacting galaxy. This might be further indication of a link between AGNs and starburst galaxies.
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Glikman, E., Helfand, D. J., White, R. L., Becker, R. H., Gregg, M. D., & Lacy, M. 2007, ApJ, 667, 673
Combining radio observations with optical and infrared color selection, demonstrated in our pilot study to be an efficient selection algorithm for finding red quasars, we have obtained optical and infrared spectroscopy for 120 objects in a complete sample of 156 candidates from a sky area of 2716 deg2. Consistent with our initial results, we find that our selection criteria (J-K>1.7, R-K>4.0) yield a ~50% success rate for discovering quasars substantially redder than those found in optical surveys. Comparison with UVX- and optical color-selected samples shows that >~10% of the quasars are missed in a magnitude-limited survey. Simultaneous two-frequency radio observations for part of the sample indicate that a synchrotron continuum component is ruled out as a significant contributor to reddening the quasars' spectra. We go on to estimate extinctions for our objects assuming that their red colors are caused by dust. Continuum fits and Balmer decrements suggest E(B-V) values ranging from near zero to 2.5 mag. Correcting the K-band magnitudes for these extinctions, we find that for K<=14.0, red quasars make up between 25% and 60% of the underlying quasar population; owing to the incompleteness of 2MASS at fainter K-band magnitudes, we can only set a lower limit to the radio-detected red quasar population of >20%-30%.
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Croft, S., de Vries, W., & Becker, R. H. 2007, ApJ, 667, L13
We correlate the positions of 13,240 brightest cluster galaxies (BCGs) with 0.1<=z<=0.3 from the maxBCG catalog with radio sources from the FIRST survey to study the sizes and distributions of radio AGNs in galaxy clusters. We find that 19.7% of our BCGs are associated with FIRST sources, and this fraction depends on the stellar mass of the BCG, and to a lesser extent on the richness of the parent cluster (in the sense of increasing radio-loudness with increasing mass). The intrinsic size of the radio emission associated with the BCGs peaks at 55 kpc, with a tail extending to 200 kpc. The radio power of the extended sources places them on the divide between FR I and FR II type sources, while sources compact in the radio tend to be somewhat less radio-luminous. We also detect an excess of radio sources associated with the cluster, instead of with the BCG itself, extending out to ~1.4 Mpc.
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de Vries, W. H., Hodge, J. A., Becker, R. H., White, R. L., & Helfand, D. J. 2007, AJ, 134, 457
We investigate faint radio emission from low- to high-luminosity active galactic nuclei (AGNs) selected from the Sloan Digital Sky Survey. Their radio properties are inferred by co-adding large ensembles of radio image cut-outs from the FIRST survey, as almost all of the sources are individually undetected. We correlate the median radio flux densities against a range of other sample properties, including median values for redshift, [O III] luminosity, emission-line ratios, and the strength of the 4000 Å break. We detect a strong trend for sources that are actively undergoing star formation to have excess radio emission beyond the ~1028 ergs s-1 Hz-1 level found for sources without any discernible star formation. Furthermore, this additional radio emission correlates well with the strength of the 4000 Å break in the optical spectrum, and may be used to assess the age of the star-forming component. We examine two subsamples, one containing the systems with emission-line ratios most like star-forming systems, and one with the sources that have characteristic AGN ratios. This division also separates the mechanism responsible for the radio emission (star formation vs. AGNs). For both cases we find a strong, almost identical correlation between [O III] and radio luminosity, with the AGN sample extending toward lower, and the star formation sample toward higher luminosities. A clearer separation between the two subsamples is seen as function of the central velocity dispersion σ of the host galaxy. For systems at similar redshifts and values of σ, the star formation subsample is brighter than the AGN in the radio by an order of magnitude. This underlines the notion that the radio emission in star-forming systems can dominate the emission associated with the AGN.
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White, R. L., Helfand, D. J., Becker, R. H., Glikman, E., & de Vries, W. 2007, ApJ, 654, 99
We present a technique to explore the radio sky into the nanojansky regime by employing image stacking using the FIRST survey. We first discuss the nonintuitive relationship between the mean and median values of a non-Gaussian distribution that is dominated by noise, followed by an analysis of the systematic effects present in FIRST's 20 cm VLA snapshot images. Image stacking allows us to recover the properties of source populations with flux densities a factor of 30 or more below the rms noise level. Mean estimates of radio flux density, luminosity, etc. are derivable for any source class having arcsecond positional accuracy. We use this technique to compute the mean radio properties for 41,295 quasars from the SDSS DR3 catalog. There is a tight correlation between optical and radio luminosity, with the radio luminosity increasing as the 0.85 power of optical luminosity. This implies declining radio loudness with optical luminosity: the most luminous objects (MUV=-28.5) have average radio-to-optical ratios 3 times lower than the least luminous objects (MUV=-20). There is also a striking correlation between optical color and radio loudness: quasars that are either redder or bluer than the norm are brighter radio sources, with objects 0.8 mag redder than the SDSS composite spectrum having radio loudness ratios that are higher by a factor of 10. We explore the long-standing question of whether a radio-loud/radio-quiet dichotomy exists in quasars, finding that optical selection effects probably dominate the distribution function of radio loudness, which has at most a modest (~20%) inflection between the radio-loud and radio-quiet ends of the distribution. We also find, surprisingly, that broad absorption line quasars have higher mean radio flux densities, with the greatest disparity arising in the rare low-ionization BAL subclass.
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McGreer, I. D., Becker, R. H., Helfand, D. J., & White, R. L. 2006, ApJ, 652, 157
From examination of only 4 deg2 of sky in the NOAO Deep Wide-Field Survey (NDWFS) region, we have identified the first radio-loud quasar at a redshift z>6. The object, FIRST J1427385+331241, was discovered by matching the FLAMINGOS Extragalactic Survey (FLAMEX) IR survey to Faint Images of the Radio Sky at Twenty cm (FIRST) survey radio sources with NDWFS counterparts. One candidate z>6 quasar was found, and spectroscopy with the Keck II telescope confirmed its identification yielding a redshift z=6.12. The object is a broad absorption line (BAL) quasar with an optical luminosity of MB~-26.9 and a radio-to-optical flux ratio ~60. Two Mg II absorptions systems are present at redshifts of z=2.18 and z=2.20. We briefly discuss the implications of this discovery for the high-redshift quasar population.
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Reviglio, P., & Helfand, D. J. 2006, ApJ, 650, 717
Analysis of the frequency and physical properties of galaxies with star formation and active galactic nucleus (AGN) activity in different environments in the local universe is a cornerstone for understanding structure formation and galaxy evolution. We have built a new multiwavelength catalog for galaxies in a complete redshift survey (the 15R Survey), gathering information on their Hα, R-band, radio, far-infrared, and X-ray emission, as well as their radio and optical morphologies, and developed a classification scheme to compare different selection methods and accurately select samples of radio-emitting galaxies with AGN and star-forming activity. While alternative classification schemes do not lead to major differences for star-forming galaxies, we show that spectroscopic and photometric classifications of AGNs lead to incomplete samples. In particular, a large population of AGN-containing galaxies with absorption-line spectra, and in many cases extended radio structures (jets, lobes), is missed in the standard Baldwin-Phillips-Terlevich emission-line classification of active galaxies. This missed class of objects accounts for roughly half of the radio AGN population. Similarly, for X-ray-selected AGNs in our sample, we find that absorption-line AGNs account for half of the sample. Spectroscopically unremarkable, passive galaxies with AGN activity are not an exception, but the norm, and we show that although they exist in all environments, these systems preferentially reside in higher density regions. Because of the existence of this population, the fractional abundance of AGNs increases with increasing density, in contrast to the results based on emission-line AGNs extracted from the 15R, SDSS, and 2dF redshift surveys. Since emission-line radio AGNs are mostly associated with late-type galaxies and absorption-line radio AGNs with early-type galaxies, the trends found are connected to the well-known but poorly understood density-morphology relation.
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Schaefer, J. J., Brotherton, M. S., Shang, Z., Gregg, M. D., Becker, R. H., Laurent-Muehleisen, S. A., Lacy, M., & White, R. L. 2006, AJ, 132, 1464
We have detected FIRST J101614.3+520916 with the XMM-Newton X-ray observatory. FIRST J101614.3+520916, one of the most extreme radio-loud, broad absorption line (BAL) quasars so far discovered, is also a Fanaroff-Riley type II radio source. We find that, compared to its estimated intrinsic X-ray flux, the observed X-rays are likely suppressed and that the observed hardness ratio indicates significant soft X-ray photons. This is inconsistent with the simplest model, a normal quasar spectrum absorbed by a large neutral H I column density, which would primarily absorb the softer photons. More complex models, involving partial covering, an ionized absorber, ionized mirror reflection, or jet contributions need to be invoked to explain this source. The suppressed but soft X-ray emission in this radio-loud BAL quasar is consistent with the behavior displayed by other BAL quasars, both radio-loud and radio-quiet.
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Gregg, M. D., Becker, R. H., & de Vries, W. 2006, ApJ, 641, 210
By combining the Sloan Digitized Sky Survey Third Data Release quasar list with the VLA FIRST survey, we have found five new objects having both broad absorption lines (BALs) in their optical spectra and FR II radio morphologies. We identify an additional example of this class from the FIRST Bright Quasar Survey, J1408+3054. Including the two previously known FR II-BAL quasars, J1016+5209 and LBQS 1138-0126, brings the number of such objects to eight. These quasars are relatively rare; finding this small handful has required the 45,000 large quasar sample of SDSS. The FR II-BAL quasars exhibit a significant anticorrelation between radio-loudness and the strength of the BAL features. This is easily accounted for by the evolutionary picture, in which quasars emerge from cocoons of BAL-producing material that stifle the development of radio jets and lobes. There is no such simple explanation for the observed properties of FR II-BALs in the unification-by-orientation model of quasars. The rarity of the FR II-BAL class implies that the two phases do not coexist for very long in a single quasar, perhaps less than 105 yr, with the combined FR II, high-ionization broad absorption phase being even shorter by another factor of 10 or more.
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Whalen, D. J., Laurent-Muehleisen, S. A., Moran, E. C., & Becker, R. H. 2006, AJ, 131, 1948
We present results from the analysis of the optical spectra of 47 radio-selected narrow-line Seyfert 1 (NLS1) galaxies. These objects are a subset of the First Bright Quasar Survey and were initially detected at 20 cm (flux density limit ~1 Jy) in the VLA FIRST Survey. We run Spearman rank correlation tests on several sets of parameters and conclude that, except for their radio properties, radio-selected NLS1 galaxies do not exhibit significant differences from traditional NLS1 galaxies. Our results are also in agreement with previous studies suggesting that NLS1 galaxies have small black hole masses that are accreting very close to the Eddington rate. We have found 16 new radio-loud NLS1 galaxies, which increases the number of known radio-loud NLS1 galaxies by a factor of ~5.
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de Vries, W. H., Becker, R. H., & White, R. L. 2006, AJ, 131, 666
We have combined a sample of 44,984 quasars selected from the Sloan Digital Sky Survey (SDSS) Data Release 3 with the FIRST radio survey. Using a novel technique in which the optical quasar position is matched to the complete radio environment within 450", we are able to characterize the radio morphological makeup of what is essentially an optically selected quasar sample, regardless of whether the quasar (nucleus) itself has been detected in the radio. About 10% of the quasar population has radio cores brighter than 0.75 mJy at 1.4 GHz, and 1.7% have double-lobed FR2-like radio morphologies. About 75% of the FR2 sources have a radio core (>0.75 mJy). A significant fraction (~40%) of the FR2 quasars are bent by more than 10°, indicating interactions of the radio plasma with either the intracluster medium or intergalactic medium. We found no evidence for correlations with redshift among our FR2 quasars; radio lobe flux densities and radio source diameters of the quasars have similar distributions at low (mean 0.77) and high (mean 2.09) redshifts. Using a smaller high-reliability FR2 sample of 422 quasars and two comparison samples of radio-quiet and non-FR2 radio-loud quasars matched in their redshift distributions, we constructed composite optical spectra from the SDSS spectroscopic data. Based on these spectra we can conclude that the FR2 quasars have stronger high-ionization emission lines compared to both the radio-quiet and non-FR2 radio-loud sources. This is consistent with the notion that the emission lines are brightened by ongoing shock ionization of ambient gas in the quasar host as the radio source expands.
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Brotherton, M. S., Laurent-Muehleisen, S. A., Becker, R. H., Gregg, M. D., Telis, G., White, R. L., & Shang, Z. 2005, AJ, 130, 2006
We report the results of a Chandra X-Ray Observatory survey of five formally radio-loud broad absorption line (BAL) quasars. These five objects include BAL quasars with a range of properties, including both high- and low-ionization BALs. All five BAL quasars are detected in 5 ks ACIS-S exposures, with counts ranging from 12 to 55. The X-ray count rates are down by factors of 40 or more compared to expectations based on the spectral energy distributions of normal, unabsorbed radio-loud quasars; this is the same sort of behavior seen in radio-quiet BAL quasars. Interestingly, the hardness ratios are rather soft and inconsistent with absorption from a neutral hydrogen column density large enough to suppress the X-rays as observed. We conclude that in many cases the X-rays emanating from BAL quasars must be reflected, scattered, or leaked through an ionized absorber or a neutral absorber that does not completely cover the X-ray source (covering >=98%), or that we are seeing an unabsorbed X-ray source perhaps associated with a radio jet. Much higher counts are required to distinguish among these possibilities. We note several suggestive correlations involving X-ray properties that require verification using larger samples. One source, FIRST J1556+3517, appears to be the X-ray brightest low-ionization BAL quasar known, other than the special case of the nearby Mrk 231. The very faint X-ray emission from FIRST J1044+3656 is consistent with significant obscuration, which strongly favors the multiphase X-ray shielding models of this object in the literature.
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Urrutia, T., Lacy, M., Gregg, M. D., & Becker, R. H. 2005, ApJ, 627, 75
We present results of a study of 12 dust-reddened quasars with 0.4<z<2.21 and reddenings in the range 0.15<E(B-V)<1.7. We obtained ACIS-S X-ray spectra of these quasars, estimated the column densities toward them, and hence obtained the gas:dust ratios in the material obscuring the quasar. We detect all but one of the red quasars in the X-rays. Even though there is no obvious correlation between the X-ray-determined column densities of our sources and their optical color or reddening, all of the sources show absorbed X-ray spectra. When we correct the luminosity for absorption, they can be placed among luminous quasars; therefore, our objects belong to the group of high-luminosity analogs of the sources contributing to the X-ray background seen in deep X-ray observations. Such sources are also found in serendipitous shallow X-ray surveys. There is a hint that the mean spectral slope of the red quasar is higher than that of normal, unobscured quasars, which could be an indication for higher accretion rates and/or an evolutionary effect. We investigate the number density of these sources compared to type 2 AGNs based on the X-ray background and estimate how many moderate-luminosity red quasars may be found in deep X-ray fields.
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Chang, T.-C., Refregier, A., & Helfand, D. J. 2004, ApJ, 617, 794
We present the first measurement of weak lensing by large-scale structure on scales of 1°-4° based on radio observations. We utilize the FIRST radio survey, a quarter-sky, 20 cm survey produced with the NRAO Very Large Array (VLA). The large angular scales afforded by the FIRST survey provide a measurement in the linear regime of the matter power spectrum, thus avoiding the necessity of applying uncertain nonlinear corrections. Moreover, since the VLA interferometer has a well-known and deterministic beam, our measurement does not suffer from the irreproducible effects of atmospheric seeing that limit ground-based optical surveys. We use the shapelet method described in an earlier paper to estimate the shear from the shape of radio sources derived directly from the interferometric measurements in the Fourier (u,v)-plane. With realistic simulations we verify that the method yields unbiased shear estimators. We study and quantify the systematic effects that can produce spurious shears, analytically and with simulations, and carefully correct for them. We measure the shear correlation functions on angular scales of 0.5d-40° and compute the corresponding aperture mass statistics. On 1°-4° scales, we find that the B-modes are consistent with zero and detect a lensing E-mode signal significant at the 3.0 σ level. After removing nearby radio sources with an optical counterpart, the E-mode signal increases by 10%-20%, as expected for a lensing signal derived from more distant sources. We use the E-mode measurement on these scales to constrain the mass power spectrum normalization σ8 and the median redshift zm of the unidentified radio sources. We find σ8(zm/2)0.6~=1.0+/-0.2, where the 1 σ error bars include statistical errors, cosmic variance, and systematics. This is consistent with earlier determinations of σ8 from cosmic shear, the cosmic microwave background, and cluster abundance, and with our current knowledge of the redshift distribution of radio sources. Taking the prior σ8=0.9+/-0.1 (68% CL) from the WMAP experiment, this corresponds to zm=2.2+/-0.9 (68% CL) for radio sources without optical counterparts, consistent with existing models for the radio source luminosity function. Our results offer promising prospects for precision measurements of cosmic shear with future radio interferometers such as LOFAR and the SKA.
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Glikman, E., Gregg, M. D., Lacy, M., Helfand, D. J., Becker, R. H., & White, R. L. 2004, ApJ, 607, 60
We have constructed a sample of bright near-infrared sources that are detected at radio wavelengths but undetected on the first-generation Palomar Observatory Sky Survey (POSSI) plates in order to search for a population of dust-obscured quasars. Optical and infrared spectroscopic follow-up of the sample has led to the discovery of 17 heavily reddened quasars (B-K>6.5), 14 of which are reported here for the first time. This has allowed us to define a region in the R-K, J-K color plane in which 50% of the radio-selected objects are highly reddened quasars. We compare the surface density of this previously overlooked population to that of ultraviolet-excess radio-selected quasars, finding that they make up ~20% of the total quasar population for K<~15.5.
This paper is available through ADS.
de Vries, W. H., Becker, R. H., White, R. L., & Helfand, D. J. 2004, AJ, 127, 2565
A sample of 123 radio sources that exhibit significant variations at 1.4 GHz on a 7 year baseline has been created using FIRST VLA B-configuration data from 1995 and 2002 on a strip at δ=0 near the south Galactic cap. This sample spans the range of radio flux densities from ~2 to 1000 mJy. It presents both in size and radio flux density range a unique starting point for variability studies of galaxies and quasars harboring lower luminosity active galactic nuclei (AGNs). We find, by comparing our variable and nonvariable control samples with the Sloan Digital Sky Survey, the following: (1) The quasar fraction of both the variable and nonvariable samples declines as a function of declining radio flux density levels; (2) our variable sample contains a consistently higher fraction of quasars than the nonvariable control sample, irrespective of radio flux; (3) the variable sources are almost twice as likely to be retrieved from the optical SDSS data than the nonvariable ones; (4) based on relative numbers, we estimate that quasars are about 5 times more likely to harbor a variable radio source than are galaxies; and (5) there does not appear to be any significant optical color offset between the two samples, even though the suggestive trend for sources to be bluer when a variable has been detected before and may be real. This leads us to conclude that both radio variability and radio flux density levels, in combination with accurate optical information, are important discriminators in the study of (radio) variability of galaxies. The latter start to dominate the source counts below ~20 mJy. In any case, variability appears to be an intrinsic property of radio sources and is not limited to quasars. Radio variability at low flux density levels may offer a unique tool in AGN unification studies.
This paper is available through ADS.
White, R. L., Helfand, D. J., Becker, R. H., Gregg, M. D., Postman, M., Lauer, T. R., & Oegerle, W. 2003, AJ, 126, 706
We have constructed a sample of quasar candidates by comparing the FIRST radio survey with the 16 deg2 Deeprange I-band survey carried out by Postman and coworkers. Spectroscopic follow-up of this magnitude-limited sample [I<20.5, Fν(20cm)>1 mJy] has revealed 35 quasars, all but two of which are reported here for the first time. This sample contains some unusual broad absorption line (BAL) quasars, including the first radio-loud FR II BAL previously reported by Gregg and coworkers. Comparison of this sample with the FIRST Bright Quasar survey samples selected in a somewhat bluer band and with brighter magnitude limits reveals that the I-band-selected sample is redder by 0.25-0.5 mag in B-R and that the color difference is not explained by the higher mean redshift of this sample but must be intrinsic. Our small sample contains five quasars with unusually red colors, including three that appear very heavily reddened. Our data are fitted well with normal blue quasar spectra attenuated by more than 2.5 mag of extinction in the I band. These red quasars are only seen at low redshifts (z<1.3). Even with a magnitude limit I<20.5, our survey is deep enough to detect only the most luminous of these red quasars at z<~1 similar objects at higher redshifts would fall below our I-band limit. Indeed, the five most luminous objects (using dereddened magnitudes) with z<1.3 are all red. Our data strongly support the hypothesis that radio quasars are dominated by a previously undetected population of red, heavily obscured objects. Unless highly reddened quasars are preferentially also highly luminous, there must be an even larger, as yet undiscovered, population of red quasars at lower luminosity. We are likely to be finding only the most luminous tip of the red quasar iceberg. A comparison of the positions of the objects in our sample with the catalog of Deeprange cluster candidates reveals that five of our six z<1 quasars are associated with cluster candidates of similar estimated redshifts. This association is very unlikely to be the result of chance. It has some surprising implications, including the possibility that up to half of the Deeprange clusters at z~1 have associated quasars.
Based on observations obtained with the W. M. Keck Observatory, which is jointly operated by the California Institute of Technology and the University of California.
This paper is available through ADS.
Blanton, E. L., Gregg, M. D., Helfand, D. J., Becker, R. H., & White, R. L. 2003, AJ, 125, 1635
Using a combination of near-infrared and optical photometry, along with multiobject spectroscopy, we have confirmed the existence of a high-redshift cluster of galaxies at z=0.96. The cluster was found using a wide-angle-tailed radio source selected from the VLA FIRST survey as a cluster signpost. These types of radio sources are often found in clusters and are thought to attain their C-shaped morphologies from the relative motion between the radio source host galaxy and the intracluster medium. We present optical/near-infrared color-magnitude diagrams that show a concentration of cluster galaxies in color space. We also include spectroscopic results obtained from the Keck II Low Resolution Imaging Spectrometer. Ten galaxies are confirmed at the cluster redshift, with a line-of-sight velocity dispersion of σ∥=530+190-90 km s-1, typical of an Abell richness class 0 cluster. Using data from the ROSAT public archive, we limit the X-ray luminosity for the cluster to LX,bol<~3×1044 ergs s-1, consistent with the value expected from the LX-σ relation.
Based in part on observations obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.
This paper is available through ADS.
Bond, H. E., White, R. L., Becker, R. H., & O'Brien, M. S. 2002, PASP, 114, 1359
We have identified the 1.4 GHz radio source FIRST J102347.6+003841 (hereafter FIRST J1023+0038) with a previously unknown 17th magnitude Galactic cataclysmic variable (CV). The optical spectrum resembles that of a magnetic (AM Herculis or DQ Herculis type) CV. Five nights of optical CCD photometry showed variations on timescales of minutes to hours, along with rapid flickering. A reexamination of the FIRST radio-survey data reveals that the radio detection was based on a single 6.6 mJy flare; on two other occasions, the source was below the ~1 mJy survey limit. Several other magnetic CVs are known to be variable radio sources, suggesting that FIRST J1023+0038 is a new member of this class (and the first CV to be discovered on the basis of radio emission). However, FIRST J1023+0038 is several optical magnitudes fainter than the other radio-detected magnetic CVs. It remains unclear whether the source simply had a very rare and extraordinarily intense radio flare at the time of the FIRST observation, or is really an unusually radio-luminous CV; thus, further observations are urged.
This paper is available through ADS.
McMahon, R. G., White, R. L., Helfand, D. J., & Becker, R. H. 2002, ApJS, 143, 1
We describe a program to identify optical counterparts to radio sources from the VLA FIRST survey using the Cambridge APM scans of the POSS-I plates. We use radio observations covering 4150 deg2 of the north Galactic cap to a 20 cm flux density threshold of 1.0 mJy; the 382,892 sources detected all have positional uncertainties of <1" (radius of 90% confidence). Our description of the APM catalog, derived from the 148 POSS-I O and E plates covering this region, includes an assessment of its astrometric and photometric accuracy, a photometric recalibration using the Minnesota APS catalog, a discussion of the classification algorithm, and quantitative tests of the catalog's reliability and completeness. We go on to show how the use of FIRST sources as astrometric standards allows us to improve the absolute astrometry of the POSS plates by nearly an order of magnitude to ~0.15" rms. Matching the radio and optical catalogs yields counterparts for over 70,000 radio sources; we include detailed discussions of the reliability and completeness of these identifications as a function of optical and radio morphology, optical magnitude and color, and radio flux density. An analysis of the problem of radio sources with complex morphologies (e.g., double-lobed radio galaxies) is included. We conclude with a brief discussion of the source classes represented among the radio sources with identified counterparts.
This paper is available through ADS, in HTML, in compressed Postscript, and through the astro-ph e-print archives. There are also a number of data tables available online.
Brotherton, M. S., Croom, S. M., De Breuck, C., Becker, R. H., & Gregg, M. D. 2002, AJ, 124, 2575
We report the correct classification of an overlooked Fanaroff-Riley class II radio-loud quasar with broad absorption lines, only the second such object so identified. The rare properties of this quasar, LBQS 1138-0126, are twice overlooked. First, LBQS 1138-0126 was found in the Large Bright Quasar Survey but only noted as a possible broad absorption line quasar without additional follow-up. Later, LBQS 1138-0126 was rediscovered and classified as a radio-loud broad absorption line quasar but not recognized as an FR II radio source. We describe the radio, absorption line, and optical polarization properties of LBQS 1138-0126 and place it in context with respect to related quasars. In particular, spectropolarimetry shows that LBQS 1138-0126 has high continuum polarization increasing from 3% in the red (rest-frame 2400 Å) to over 4% in the blue (rest-frame 1650 Å), essentially confirming the intrinsic nature of the absorption. The polarization position angle rotates from ~-30° in the red to ~0° in the blue; the radio lobe position angle is ~52° for comparison. LBQS 1138-0126 is additionally notable for being one of the most radio-loud broad absorption line quasars and for having low-ionization broad absorption lines as well.
Based on observations carried out at the European Southern Observatory, Paranal, Chile (program 69.B-0078).
This paper is available through ADS.
Ivezić, Ž., et al. 2002, AJ, 124, 2364
We discuss the optical and radio properties of ~30,000 FIRST (radio, 20 cm, sensitive to 1 mJy) sources positionally associated within 1.5" with a Sloan Digital Sky Survey (SDSS) (optical, sensitive to r*~22.2) source in 1230 deg2 of sky. The matched sample represents ~30% of the 108,000 FIRST sources and 0.1% of the 2.5×107 SDSS sources in the studied region. SDSS spectra are available for 4300 galaxies and 1154 quasars from the matched sample and for a control sample of 140,000 galaxies and 20,000 quasars in 1030 deg2 of sky. Here we analyze only core sources, which dominate the sample; the fraction of SDSS-FIRST sources with complex radio morphology is determined to be less than 10%. This large and unbiased catalog of optical identifications provides much firmer statistical footing for existing results and allows several new findings. The majority (83%) of the FIRST sources identified with an SDSS source brighter than r*=21 are optically resolved; the fraction of resolved objects among the matched sources is a function of the radio flux, increasing from ~50% at the bright end to ~90% at the FIRST faint limit. Nearly all optically unresolved radio sources have nonstellar colors indicative of quasars. We estimate an upper limit of ~5% for the fraction of quasars with broadband optical colors indistinguishable from those of stars. The distribution of quasars in the radio flux-optical flux plane suggests the existence of the ``quasar radio dichotomy'' 8%+/-1% of all quasars with i*<18.5 are radio-loud, and this fraction seems independent of redshift and optical luminosity. The radio-loud quasars have a redder median color by 0.08+/-0.02 mag, and show a 3 times larger fraction of objects with extremely red colors. FIRST galaxies represent 5% of all SDSS galaxies with r*<17.5, and 1% for r*<20, and are dominated by red (u*-r*>2.22) galaxies, especially those with r*>17.5. Magnitude- and redshift-limited samples show that radio galaxies have a different optical luminosity distribution than nonradio galaxies selected by the same criteria; when galaxies are further separated by their colors, this result remains valid for both blue and red galaxies. For a given optical luminosity and redshift, the observed optical colors of radio galaxies are indistinguishable from those of all SDSS galaxies selected by identical criteria. The distributions of radio-to-optical flux ratio are similar for blue and red galaxies in redshift-limited samples; this similarity implies that the difference in their luminosity functions and resulting selection effects are the dominant cause for the preponderance of red radio galaxies in flux-limited samples. The fraction of radio galaxies whose emission-line ratios indicate an AGN (30%), rather than starburst, origin is 6 times larger than the corresponding fraction for all SDSS galaxies (r*<17.5). We confirm that the AGN-to-starburst galaxy number ratio increases with radio flux and find that radio emission from AGNs is more concentrated than radio emission from starburst galaxies.
This paper is available through ADS.
Lacy, M., Gregg, M., Becker, R. H., White, R. L., Glikman, E., Helfand, D., & Winn, J. N. 2002, AJ, 123, 2925
We report the discovery of a z=2.65 low-ionization iron broad absorption line quasar, FIRST J100424.9+122922, which is gravitationally lensed by a galaxy at z~0.95. The object was discovered as part of a program to find very red quasars by matching the FIRST radio survey with the Two Micron All Sky Survey in the near-infrared. J100424.9+122922 is the second lensed system to be found in this program, suggesting that many gravitational lenses are probably missed from conventional optical quasar surveys. We have made a simple lens model and a rough estimate of the reddening in the immediate environment of the quasar which suggests that the quasar is intrinsically very luminous and is accreting at close to the Eddington limit of its ~109 Msolar black hole. The lensing galaxy has a small amount of dust, which is responsible for some excess reddening observed in the fainter image of the quasar, but is otherwise a fairly typical massive elliptical galaxy. We model the selection effects working against the detection of red quasars in both lensed and unlensed samples. We show that these selection effects are very effective at removing even lightly reddened high-redshift quasars from magnitude-limited samples, whether they are lensed or not. This suggests that the red quasar population in general could be very large, and in particular the class of iron broad absorption line quasars of which J100424.9+122922 is a member may be much larger than their rarity in magnitude-limited samples would suggest.
This paper is available through ADS.
Richards, G. T., Gregg, M. D., Becker, R. H., & White, R. L. 2002, ApJ, 567, L13
We present a Keck Observatory/Echelle Spectrograph and Imager spectrum of FIRST 074711.2+273904, a K=15.4 quasar with a redshift of 4.11 that is detected by both the Faint Images of the Radio Sky at Twenty cm (FIRST) survey and the Two Micron All Sky Survey (2MASS). The spectrum contains at least 14 independent C IV absorption systems longward of the Lyα forest. These systems are found over a path length of Δz=0.984, constituting one of the highest densities per unit redshift of C IV absorption ever observed. One of the C IV systems is troughlike and resembles a weak broad absorption line (BAL)-type outflow. Two of the C IV are ``associated'' absorption systems with |v|<3000 km s-1. Of the 11 remaining systems with v>3000 km s-1, eight are either resolved or require multiple discrete systems to fit the line profiles. In addition to C IV absorption, there are two low-ionization Mg II absorption systems, along with two damped Lyα systems, at least one of which may be a C IV system. The overdensity of C IV absorption spans a redshift range of Δz~1. Superclusters along the line of sight are unlikely to cause an overdensity stretching over such a long redshift path; thus, the absorption may be an example of narrow, high-velocity intrinsic absorption that originates from the quasar. We suggest that this quasar is a member of a transitional class of BAL quasars, where we are just barely seeing the spatial, density, or temporal edge of the BAL-producing region (or period); the multiple high-velocity absorption systems may be the remnants (or precursors) of a stronger BAL outflow. If correct, then some simpler absorption-line complexes in other quasars may also be due to outflowing rather than to intervening material.
This paper is available through ADS.
Gregg, M. D., Lacy, M., White, R. L., Glikman, E., Helfand, D., Becker, R. H., & Brotherton, M. S. 2002, ApJ, 564, 133
In a survey of quasar candidates selected by matching the Faint Images of the Radio Sky at Twenty cm (FIRST) and Two Micron All-Sky Survey catalogs, we have found two extraordinarily red quasars. FIRST J013435.7-093102 is a 1 Jy source at z=2.216 and has B-K>~10, while FIRST J073820.1+275045 is a 2.5 mJy source at z=1.985 with B-K~8.4. FIRST J073820.1+275045 has strong absorption lines of Mg II and C IV in the rest frame of the quasar and is highly polarized in the rest-frame ultraviolet, strongly favoring the interpretation that its red spectral energy distribution is caused by dust reddening local to the quasar. FIRST J073820.1+275045 is thus one of the few low radio luminosity, highly dust-reddened quasars known. The available observational evidence for FIRST J013435.7-093102 leads us to conclude that it too is reddened by dust. We show that FIRST J013435.7-093102 is gravitationally lensed, increasing the number of known lensed, extremely dust-reddened quasars to at least three, including MG 0414-0534 and PKS 1830-211. We discuss the implications of whether these objects are reddened by dust in the host or lensing galaxies. If reddened by their local environment, then we estimate that between 10% and 20% of the radio-loud quasar population is reddened by dust in the host galaxy. The discovery of FIRST J073820.1+275045 and objects now emerging from X-ray surveys suggests the existence of an analogous radio-quiet red quasar population. Such objects would be entirely missed by standard radio or optical quasar surveys. If dust in the lensing galaxies is primarily responsible for the extreme redness of the lensed quasars, then an untold number of gravitationally lensed quasars are being overlooked.
This paper is available through ADS and through the astro-ph e-print archives.
Becker, R. H., et al. 2001, ApJS, 135, 227
We present the results of an extension of the FIRST Bright Quasar Survey (FBQS) to the South Galactic cap, and to a fainter optical magnitude limit. Radio source counterparts with SERC R magnitudes brighter than 18.9 which meet the other FBQS criteria are included. We supplement this list with a modest number of additional objects to test our completeness for quasars with extended radio morphologies. The survey covers 589 deg2 in two equatorial strips in the southern cap. We have obtained spectra for 86% of the 522 candidates and find 321 radio-selected quasars of which 264 are reported here for the first time. A comparison of this fainter sample with the FBQS sample shows the two to be generally similar. Fourteen new broad absorption line (BAL) quasars are included in this sample. When combined with the previously identified BAL quasars in our earlier papers, we can discern a break in the frequency of BAL quasars with radio loudness, namely that the relative number of high-ionization BAL quasars drops by a factor of 4 for quasars with a radio-loudness parameter R*>100. Based on observations obtained with the W. M. Keck Observatory, which is jointly operated by the California Institute of Technology and the University of California, and the Multiple Mirror Telescope Observatory, which is jointly operated by the University of Arizona and the Smithsonian Institution.
This paper is available through ADS and the astro-ph e-print archives. The figures with spectra in the astro-ph version are low-resolution; there is also a full-resolution gzipped postscript version of the paper available (1.6 Mbytes).
Blanton, E. L., Gregg, M. D., Helfand, D. J., Becker, R. H., & Leighly, K. M. 2001, AJ, 121, 2915
We present an optical spectroscopic and imaging study of the environments of a complete sample of moderate-redshift bent-double radio sources. More than half of the 40 radio galaxies in the sample are associated with clusters of Abell richness class 0 or greater at z<0.4. Most of the remaining objects are associated with groups, although a few appear to be hosted by nearly isolated elliptical galaxies. For the bent doubles appearing in poor environments, either dense gas must be associated with the systems to provide the ram pressure to bend the lobes or alternative bending mechanisms must be invoked to explain the radio morphologies. Correlation with the ROSAT All-Sky Survey bright and faint source catalogs reveals that the majority of the z<0.2 objects in our sample that we classify optically as clusters are also X-ray sources.
This paper is available through ADS and through the astro-ph e-print archives.
Leighly, K. M., Halpern, J. P., Helfand, D. J., Becker, R. H., & Impey, C. D. 2001, AJ, 121, 2889
We report the discovery of a new bright quasar, PHL 1811, as part of a follow-up program to identify bright quasars in the FIRST radio survey. With B=13.9, R=13.9, and z=0.192, this quasar is now the second-brightest quasar (in apparent magnitude) known beyond z=0.1. Optically classified as a narrow-line Seyfert 1 galaxy (NLS1), PHL 1811 is unusual for an object in this class in that it was not detected in X-rays in the ROSAT All-Sky Survey or in any previous X-ray survey. A follow-up BeppoSAX observation confirms that it is deficient in X-rays compared with other quasars, but poor signal-to-noise ratio prevents a definitive characterization of the X-ray spectrum. We consider three alternative hypotheses to explain why PHL 1811 is a weak X-ray source: (1) it is a broad absorption line quasi-stellar object and suffers X-ray absorption; (2) like several other luminous NLS1s, it exhibits high-amplitude X-ray variability and has been observed only when it is in an X-ray-quiescent state; and (3) it is intrinsically weak because it simply lacks an X-ray-emitting region.
This paper is available through ADS and through the astro-ph e-print archives.
Lacy, M., Laurent-Muehleisen, S. A., Ridgway, S. E., Becker, R. H., & White, R. L. 2001, ApJ, 551, L17
Several independent lines of evidence now point to a correlation between black hole mass Mbh and radio luminosity. In this Letter, we discuss the correlation for quasars from the FIRST Bright Quasar Survey (FBQS) using black hole mass estimates from Hβ line widths. The FBQS objects fill in the gap between the radio-loud and radio-quiet quasars in the radio luminosity-optical luminosity plane, and we find that they fill the corresponding gap in the Mbh-radio luminosity correlation. There is thus a continuous variation of radio luminosity with Mbh, and no evidence for a ``switch'' at some set of critical parameter values that turns on powerful radio jets. By combining the FBQS data with that for quasars from the Palomar-Green survey, we find evidence for a dependence of radio luminosity on accretion rate relative to the Eddington limit, L/LEdd, as well as on Mbh, consistent with the well-known radio-optical correlation for radio-loud quasars. We therefore suggest a new scheme to ``unify'' radio-loud and radio-quiet objects in which the radio luminosity scales proportional to M1.9+/-0.2bh(L/LEdd)1.0 for L/LEdd~0.1, with an apparently weaker accretion rate dependence at low L/LEdd. The scatter about this relation is +/-1.1 dex and may well hide significant contributions from other physical effects, such as the black hole spin and radio source environment.
This paper is available through ADS.
Helfand, D. J., Stone, R. P. S., Willman, B., White, R. L., Becker, R. H., Price, T., Gregg, M. D., & McMahon, R. G. 2001, AJ, 121, 1872
We have obtained single-epoch optical photometry for 202 quasars, taken from the FIRST Bright Quasar Survey, which span a wide range in radio loudness. Comparison with the magnitudes of these objects on the POSS-I plates provides by far the largest sample of long-term variability amplitudes for radio-selected quasars yet produced. We find the quasars to be more variable in the blue than in the red band, consistent with work on optically selected samples. The previously noted trend of decreasing variability with increasing optical luminosity applies only to radio-quiet objects. Furthermore, we do not confirm a rise in variability amplitude with redshift, nor do we see any dependence on radio flux or luminosity. The variability over a radio-optical flux ratio range spanning a factor of 60,000 from radio-quiet to extreme radio-loud objects is largely constant, although there is a suggestion of greater variability in the extreme radio-loud objects. We demonstrate the importance of Malmquist bias in variability studies and develop a procedure to correct for the bias in order to reveal the underlying variability properties of the sample.
This paper is available through ADS and through the astro-ph e-print archives.
Morgan, N. D., Becker, R. H., Gregg, M. D., Schechter, P. L., & White, R. L. 2001, AJ, 121, 611
We present our ground-based optical imaging, spectral analysis, and high-resolution radio mapping of the gravitational lens candidate FBQ 1633+3134. This z=1.52, B=17.7 quasar appears double on CCD images with an image separation of 0.66" and a relatively constant flux ratio of ~3:1 across B, V, R, and I filters. A single 0.27 mJy radio source is detected at 8.46 GHz, coincident within an arcsecond of both optical components, but no companion at radio wavelengths is detected for the system down to a flux level of 0.1 mJy (3 σ). Spectral observations reveal a rich metal-line absorption system consisting of a strong Mg II doublet and associated Fe I and Fe II absorption features, all at an intervening redshift of z=0.684, suggestive of a lensing galaxy. Point-spread function subtraction of ground-based images however shows no obvious signs of a third object between the two quasar images and places a detection limit of I>~23.0 if such an object exists. Although the possibility that FBQ 1633+3134 is a binary quasar cannot be ruled out, the evidence presented here is consistent with FBQ 1633+3134 being a single quasar lensed by a faint, metal-rich galaxy. Based on observations carried out in part at the MDM Observatory, the W. M. Keck Observatory, and the National Radio Astronomy Observatory (NRAO) Very Large Array. The NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
This paper is available through ADS.
Brotherton, M. S., Tran, H. D., Becker, R. H., Gregg, M. D., Laurent-Muehleisen, S. A., & White, R. L. 2001, ApJ, 546, 775
We present a very high signal-to-noise ratio composite spectrum created using 657 radio-selected quasars from the FIRST Bright Quasar Survey. The spectrum spans rest-frame wavelengths 900-7500 Å. Additionally, we present composite spectra formed from subsets of the total data set in order to investigate the spectral dependence on radio loudness and the presence of broad absorption lines. In particular, radio-loud quasars are red compared to radio-quiet quasars, and quasars showing low-ionization broad absorption lines are red compared to other quasars. We compare our composites with those from the Large Bright Quasar Survey. Composite quasar spectra have proved to be valuable tools for a host of applications, and in that spirit we make these publicly available via the FIRST survey web page.
This paper is available through ADS, in compressed Postscript, and through the astro-ph e-print archives. The composite spectra are available for downloading.
Arav, N., Brotherton, M. S., Becker, R. H., Gregg, M. D., White, R. L., Price, T., & Hack, W. 2001, ApJ, 546, 140
We present detailed analyses of the absorption spectrum seen in QSO 2359-1241 (NVSS J235953-124148). Keck HIRES data reveal absorption from 20 transitions arising from He I, Mg I, Mg II, Ca II, and Fe II. Hubble Space Telescope data show broad absorption lines (BALs) from Al III λ1857, C IV λ1549, Si IV λ1397, and N V λ1240. Absorption from excited Fe II states constrains the temperature of the absorber to 2000<~T<~10,000 K and puts a lower limit of 105 cm-3 on the electron number density. Saturation diagnostics show that the real column densities of He I and Fe II can be determined, allowing us to derive meaningful constraints on the ionization equilibrium and abundances in the flow. The ionization parameter is constrained by the iron, helium, and magnesium data to -3.0<~log(U)<~-2.5, and the observed column densities can be reproduced without assuming departure from solar abundances. From comparison of the He I and Fe II absorption features, we infer that the outflow seen in QSO 2359-1241 is not shielded by a hydrogen ionization front and therefore that the existence of low-ionization species in the outflow (e.g., Mg II, Al III, Fe II) does not necessitate the existence of such a front. We find that the velocity width of the absorption systematically increases as a function of ionization and to a lesser extent with abundance. Complementary analyses of the radio and polarization properties of the object are discussed in a companion paper (Brotherton et al.).
This paper is available through ADS and through the astro-ph e-print archives.
Brotherton, M. S., Arav, N., Becker, R. H., Tran, H. D., Gregg, M. D., White, R. L., Laurent-Muehleisen, S. A., & Hack, W. 2001, ApJ, 546, 134
We report the discovery of a bright quasar (E=15.8, z=0.868) associated with the flat-spectrum radio source NVSS J235953-124148. This quasar, which we designate QSO 2359-1241, possesses a rare combination of extreme properties that make it of special interest. These properties include intrinsic high-velocity outflow seen in absorption for both high- and low-ionization species, high optical polarization (~5%), significant radio emission, and dust reddening. The dereddened absolute magnitude of QSO 2359-1241 places it among the three most optically luminous quasars known at z<1. High-resolution spectroscopy and a detailed analysis of the optical/ultraviolet absorption features will be given in a companion paper.
This paper is available through ADS and through the astro-ph e-print archives.
Najita, J., Dey, A., & Brotherton, M. 2000, AJ, 120, 2859
We present the first near-infrared spectroscopic observations of the radio-loud broad absorption line quasi-stellar object (QSO), FIRST J155633.8+351758. The spectrum is similar to that of a reddened QSO and shows strong emission lines of Hα and Hβ, as well as strong Fe II emission blends near Hβ. The redshift of the object, measured from the Hα and Hβ lines, is zBLR=1.5008+/-0.0007, slightly larger than the redshift of zmetal=1.48, estimated from the broad metal absorption features. Thus, the broad metal absorption features are blue shifted with respect to the systemic velocity. The width of the Hα emission line (FWHM~4100 km s -1) is typical of that observed in QSO broad-line regions, but the Balmer decrement (Hα/Hβ~5.8) is larger than that of most optically selected QSOs. Both the Balmer decrement and the slope of the rest-frame UV-optical continuum independently suggest a modest amount of extinction along the line of sight to the broad-line region (EB-V~0.5 for SMC-type screen extinction at the redshift of the QSO). The implied gas column density along the line of sight is much less than that implied by the weak X-ray flux of the object, suggesting that either the broad emission and absorption line regions have a low dust-to-gas ratio or that the rest-frame optical light encounters significantly lower mean column density lines of sight than the X-ray emission. From the rest-frame UV-optical spectrum, we are able to constrain the stellar mass content of the system (<3×1011 Msolar). Comparing this mass limit with the black hole mass estimated from the bolometric luminosity of the QSO, we find it possible that the ratio of the black hole to stellar mass is comparable to the Magorrian value, which would imply that the Magorrian relation is already in place at z=1.5. However, multiple factors favor a much larger black hole-to-stellar mass ratio. This would imply that if the Magorrian relation characterizes the late history of QSOs and if the situation observed for F1556+3517 is typical of the early evolutionary history of QSOs, central black hole masses develop more rapidly than bulge masses. Based on observations at the Kitt Peak National Observatory.
This paper is available through ADS and through the astro-ph e-print archives.
Gregg, M. D., Becker, R. H., Brotherton, M. S., Laurent-Muehleisen, S. A., Lacy, M., & White, R. L. 2000, ApJ, 544, 142
We have discovered a remarkable quasar, FIRST J101614.3+520916, whose optical spectrum shows unambiguous broad absorption features, while its double-lobed radio morphology and luminosity clearly indicate a classic Fanaroff-Riley type II radio source. Its radio luminosity places it at the extreme of the recently established class of radio-loud broad absorption line quasars. Because of its hybrid nature we speculate that FIRST J101614.3+520916 is a typical FR II quasar that has been rejuvenated as a broad absorption line (BAL) quasar with a compact steep spectrum core. The direction of the jet axis of FIRST J101614.3+520916 can be estimated from its radio structure and optical brightness, indicating that we are viewing the system at a viewing angle of >~40°. The position angles of the radio jet and optical polarization are not well aligned, differing by ~20°-30°. When combined with the evidence presented by Becker and coworkers for a sample of 29 BAL quasars showing that at least some BAL quasars are viewed along the jet axis, the implication is that no preferred viewing orientation is necessary to observe BAL systems in a quasar's spectrum. This, and the probable young nature of compact steep spectrum sources, leads naturally to the alternate hypothesis that BALs are an early stage in the lives of quasars.
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Becker, R. H., White, R. L., Gregg, M. D., Brotherton, M. S., Laurent-Muehleisen, S. A., & Arav, N. 2000, ApJ, 538, 72
In a spectroscopic follow-up to the VLA FIRST survey, the FIRST Bright Quasar Survey (FBQS) has found 29 radio-selected broad absorption line (BAL) quasars. This sample provides the first opportunity to study the properties of radio-selected BAL quasars. Contrary to most previous studies, we establish that a significant population of radio-loud BAL quasars exists. Radio-selected BAL quasars display compact radio morphologies and possess both steep and flat radio spectra. Quasars with low-ionization BALs have a color distribution redder than that of the FBQS sample as a whole. The frequency of BAL quasars in the FBQS is significantly greater, perhaps by as much as a factor of 2, than that inferred from optically selected samples. The frequency of BAL quasars appears to have a complex dependence on radio loudness. The properties of this sample appear to be inconsistent with simple unified models in which BAL quasars constitute a subset of quasars seen edge-on.
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Gregg, M. D., Wisotzki, L., Becker, R. H., Maza, J., Schechter, P. L., White, R. L., Brotherton, M. S., & Winn, J. N. 2000, AJ, 119, 2535
In the course of a Cycle 8 snapshot imaging survey with the Space Telescope Imaging Spectrograph (STIS), we have discovered that the z=1.565 quasar HE 0512-3329 is a double with image separation 0.644", differing in brightness by only 0.4 mag. This system is almost certainly gravitationally lensed. Although separate spectra for the two images have not yet been obtained, the possibility that either component is a Galactic star is ruled out by a high signal-to-noise composite ground-based spectrum and separate photometry for the two components: the spectrum shows no trace of any zero-redshift stellar absorption features belonging to a star with the temperature indicated by the broadband photometry. The optical spectrum shows strong absorption features of Mg II, Mg I, Fe II, Fe I, and Ca I, all at an identical intervening redshift of z=0.9313, probably due to the lensing object. The strength of Mg II and the presence of the other low-ionization absorption features is strong evidence for a damped Lyα system, likely the disk of a spiral galaxy. Point-spread function fitting to remove the two quasar components from the STIS image leads to a tentative detection of a third object, which may be the nucleus of the lensing galaxy. The brighter component is significantly redder than the fainter, due to either differential extinction or microlensing. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS 5-26555.
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Brinkmann, W., Laurent-Muehleisen, S. A., Voges, W., Siebert, J., Becker, R. H., Brotherton, M. S., White, R. L., & Gregg, M. D. 2000, A&A, 356, 445
We present the results of a correlation of the ROSAT All-Sky Survey with the April 1997 release of the VLA 20cm FIRST catalogue. We focus our analysis on the 843 X-ray sources which have unique radio counterparts. The majority of these objects (84%) have optical counterparts on the POSS 1 plates. Approximately 30% have been previously classified and we obtain new spectroscopic classifications for 85 sources by comparison with the ongoing FIRST Bright Quasar Survey and 106 additional sources from our own new spectroscopic data. Approximately 51% of the sources are presently classified, and the majority of the unclassified objects are optically faint. The newly classified sources are generally radio weak, exhibiting properties intermediate with previous samples of radio- and X-ray-selected AGN. This also holds for the subsample of 71 BL Lacs which includes many intermediate objects. The 146 quasars show no evidence for a bimodal distribution in their radio-loudness parameter, indicating that the supposed division between radio-quiet and radio-loud AGN may not be real. The X-ray and radio luminosities are correlated over two decades in radio luminosity, spanning the radio-loud and radio-quiet regimes, with radio-quiet quasars showing a linear correlation between the two luminosities. Many of the sources show peculiar or unusual properties which call for more detailed follow-up observations. We also give the X-ray and radio data for the 518 X-ray sources for which more than one radio object is found. Because of the difficulties inherent in identifying optical counterparts to these complex sources, we do not consider these data in the current analysis The tables are available in electronic form at the CDS via anonymous ftp from 130.79.128.5
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Blanton, E. L., Gregg, M. D., Helfand, D. J., Becker, R. H., & White, R. L. 2000, ApJ, 531, 118
Bent-double radio sources can act as tracers for clusters of galaxies. We present imaging and spectroscopic observations of the environments surrounding 10 of these sources (most of them wide-angle tails [WATs]) selected from the VLA FIRST survey. Our results reveal a previously unknown cluster associated with eight of the radio sources with redshifts in the range 0.33<z<0.85 furthermore, we cannot rule out that the other two bent doubles may be associated with clusters at higher redshift. Richness measurements indicate that these clusters are typical of the majority of those found in the Abell catalog, with a range of Abell richness classes from 0 to 2. The line-of-sight velocity dispersions are very different from cluster to cluster, ranging from approximately 300 to 1100 km s-1. At the upper end of these intervals, we may be sampling some of the highest redshift massive clusters known. Alternatively, the large velocity dispersions measured in some of the clusters may indicate that they are merging systems with significant substructure, consistent with recent ideas concerning WAT formation (Burns et al.). Based in part on observations obtained at the W. M. Keck Observatory.
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White, R. L., et al. 2000, ApJS, 126, 133
We have used the Very Large Array (VLA) FIRST survey and the Automated Plate Measuring Facility (APM) catalog of the Palomar Observatory Sky Survey I (POSS-I) plates as the basis for constructing a new radio-selected sample of optically bright quasars. This is the first radio-selected sample that is competitive in size with current optically selected quasar surveys. Using only two basic criteria, radio-optical positional coincidence and optical morphology, quasars and BL Lac objects can be identified with 60% selection efficiency; the efficiency increases to 70% for objects fainter than 17 mag. We show that a more sophisticated selection scheme can predict with better than 85% reliability which candidates will turn out to be quasars. This paper presents the second installment of the FIRST Bright Quasar Survey (FBQS), with a catalog of 636 quasars distributed over 2682 deg2. The quasar sample is characterized and all spectra are displayed. The FBQS detects both radio-loud and radio-quiet quasars out to redshift z>3. We find a large population of objects of intermediate radio loudness; there is no evidence in our sample for a bimodal distribution of radio characteristics. The sample includes ~29 broad absorption line quasars, both high and low ionization, and a number of new objects with remarkable optical spectra.
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Gliozzi, M., Brinkmann, W., Laurent-Muehleisen, S. A., Takalo, L. O., & Sillanpää, A. 1999, A&A, 352, 437
We present the results of three ROSAT HRI observations of AGN expected to reside in clusters of galaxies. Although the exposures were truncated by the premature end of the ROSAT mission, valuable information could be achieved which greatly improved upon the previous PSPC results. For RGB 1745+398 we could separate the cluster emission from that of the BL Lac and could confirm the cluster parameters obtained from optical follow-up observations. In MRC 0625-536 the flux from the central point source contributes less than 3% to the total X-ray flux and the eastern component of the dumbbell galaxy seems to be the X-ray emitter. RXJ1234.6+2350 appears to be extended in X-rays. The X-ray flux is centered on a quasar, but optical spectroscopy indicates that the nearby radio galaxies reside in a previously unknown cluster at redshift z$sigma im$0.134.
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Arav, N., Becker, R. H., Laurent-Muehleisen, S. A., Gregg, M. D., White, R. L., Brotherton, M. S., & de Kool, M. 1999, ApJ, 524, 566
We find that the depth and shape of the broad absorption lines (BALs) in BALQSO 1603+3002 are determined largely by the fraction of the emitting source which is covered by the BAL flow. In addition, the observed depth of the BALs is poorly correlated with their real optical depth. The implication of this result is that abundance studies based on direct extraction of column densities from the depth of the absorption troughs are unreliable. Our conclusion is based on analysis of unblended absorption features of two lines from the same ion (in this case the Si IV doublet), which allows unambiguous separation of covering factor and optical depth effects. The complex morphology of the covering factor as a function of velocity suggests that the BALs are produced by several physically separated outflows. The covering factor is ion dependent in both depth and velocity width. We also find evidence that in BALQSO 1603+3002 the flow does not cover the broad emission line region.
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Brotherton, M. S., Gregg, M. D., Becker, R. H., Laurent-Muehleisen, S. A., White, R. L., & Stanford, S. A. 1999, ApJ, 514, L61
We report the discovery of a small-separation quasar pair (z=0.586, O=18.4, 19.2, and a separation of 2.3") associated with the radio source FIRST J164311.3+315618 (S_1400=120 mJy). The spectrum of the brighter quasar (A) has a much stronger narrow emission line spectrum than the other (B), and it also has stronger Balmer lines relative to the continuum. The continuum ratio of the spectra is flat in the blue (lambda_obs<6000 Å) at about 2.1 but falls to 1.5 at longer wavelengths. A K^' image shows two unresolved sources with a flux ratio of 1.3. The different colors appear to result from the contribution of the host galaxy of B, which is evident from Ca II and high-order Balmer absorption lines that are indicative of a substantial young stellar population. New 3.6 cm VLA observations show that the compact radio source is coincident with quasar A (B is only marginally detected). We rule out the lensing hypothesis because the optical flux ratio is A/B~1.5-2, while the radio flux ratio is A/B>~40, and conclude that this system is a binary. Moreover, the radio-loud quasar is a compact, steep spectrum source. FIRST J164311.3+315618A, B is the lowest redshift and smallest separation binary quasar yet identified.
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Helfand, D. J., Schnee, S., Becker, R. H., White, R. L., & McMahon, R. G. 1999, AJ, 117, 1568
Comparison of the VLA FIRST survey with various catalogs of bright stars allows an examination of the prevalence of stellar radio emission independent of optical selection criteria. This FIRST unbiased survey for radio stars covers nearly 5000 deg^2 of the northern sky to a flux density limit of 0.7 mJy at 20 cm. Using astrometric catalogs that include proper-motion information, we have detected 26 stellar radio sources, doubling the number of such objects previously known in this region of high-latitude sky. We also show that, in the absence of good proper motions, even the 1" precision of the FIRST positions is insufficient to avoid crippling chance coincidence rates. We calculate the fraction of radio detections as a function of stellar magnitude and show that, when proper motions from the Guide Star Catalog II become available, the number of stellar radio source detections should increase fourfold.
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Kamionkowski, M., Babul, A., Cress, C. M., & Refregier, A. 1998, MNRAS, 301, 1064
Weak lensing by large-scale mass inhomogeneities in the Universe induces correlations in the observed ellipticities of distant sources. We first review the harmonic analysis and statistics required of these correlations and discuss calculations for the predicted signal. We consider the ellipticity correlation function, the mean-square ellipticity, the ellipticity power spectrum and a global maximum-likelihood analysis to isolate a weak-lensing signal from the data. Estimates for the sensitivity of a survey of a given area, surface density, and mean intrinsic source ellipticity are presented. We then apply our results to the FIRST radio-source survey. We predict an rms ellipticity of roughly 0.011 in 1x1 deg^2 pixels and 0.018 in 20x20 arcmin^2 pixels if the power spectrum is normalized to sigma_8Omega^0.53=0.6, as indicated by the cluster abundance. The signal is significantly larger in some models if the power spectrum is normalized instead to the COBE anisotropy. The uncertainty in the predictions from imprecise knowledge of the FIRST redshift distribution is about 25 per cent in the rms ellipticity. We show that FIRST should be able to make a statistically significant detection of a weak-lensing signal for cluster-abundance-normalized power spectra.
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Brotherton, M. S., van Breugel, W., Smith, R. J., Boyle, B. J., Shanks, T., Croom, S. M., Miller, L., & Becker, R. H. 1998, ApJ, 505, L7
We report the discovery of five broad absorption line (BAL) QSOs in a complete sample of 111 ultraviolet excess (UVX) QSO candidates also detected in the NRAO VLA Sky Survey. All five BAL QSOs, which include two high-ionization BAL QSOs and three low-ionization BAL QSOs, are formally radio loud. Of QSOs with z>0.4 , 3%+/-2% show low-ionization BALs, and of QSOs with z>1.5 , all radio loud, 9%+/-5% show BALs; these frequencies are consistent with those of optical surveys. While the first reported radio-loud BAL QSO, FIRST J155633.8+351758, is likely to be heavily dust reddened and thus less radio loud than indicated by its observed radio-to-optical luminosity, these QSOs are UVX-selected and probably free of significant dust along the line of sight. We point out unusual features in two of the BAL QSOs and discuss the significance finding these heretofore rare objects.
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Hook, I. M., Becker, R. H., McMahon, R. G., & White, R. L. 1998, MNRAS, 297, 1115
We present the method and first results of a survey for high-redshift (z>3) radio-loud quasars, which is based on optical identifications of 2902 flat-spectrum radio sources with S_5GHz>=25 mJy. The radio sample was defined over a 1600 deg^2 region using the 5-GHz Green Bank survey and the 1.4-GHz VLA FIRST survey. 560 sources were identified to a limit of E = 19.5 on APM scans of POSS-I plates, and 337 of these optical counterparts are unresolved. From these a complete sample of 73 sources for spectroscopic follow-up was defined based on criteria of red (O-E>=1.2) optical colour. We have obtained spectra for 36 of these, and an additional 14 have redshifts in the literature, thus 70 per cent of the spectroscopic sample is completed. Six objects in the sample were found to be radio-loud quasars with z>3, of which two were previously known. The efficiency of the spectroscopic phase of the survey is therefore about 1 in 9, whereas without the colour selection criterion the efficiency would have been 1 in sim40. The six z>3 quasars were found in an effective area of 1100 deg^2, implying a surface density of one flat-spectrum z>3 radio-loud quasar per 190 deg^2 to limits of E = 19.5 and S_5GHz>=25mJy. This survey has also produced the first known radio-loud BAL quasar, 1556+3517 with z=1.48, which has been reported in an earlier paper. This object has a redder optical colour (O-E=2.56) than all the z>3 quasars found in this survey to date. In addition, we have obtained spectra of 22 GB/FIRST sources that are not part of the complete sample. We give positions, E (red) magnitudes, O-E colours, radio fluxes, radio spectral indices and redshifts where possible for objects for which we have obtained spectra. We give spectra and finding charts for the z>3 quasars.
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Cress, C. M., & Kamionkowski, M. 1998, MNRAS, 297, 486
We develop the formalism required to interpret, within a CDM framework, the angular clustering of sources in a deep radio survey. The effect of non-linear evolution of density perturbations is discussed, as is the effect of the assumed redshift distribution of sources. We also investigate what redshift ranges contribute to the clustering signal at different angular scales. Application of the formalism is focused on the clustering detected in the FIRST survey, but measurements made for other radio surveys are also investigated. We comment on the implications for the evolution of clustering.
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Schechter, P. L., Gregg, M. D., Becker, R. H., Helfand, D. J., & White, R. L. 1998, AJ, 115, 1371
The V = 16.9 quasar FBQ 0951+2635 at redshift z = 1.24 appears double on CCD exposures taken in subarcsecond seeing. The two objects are separated by 1.1" and differ in brightness by 0.9 mag. VLA observations show the radio source to be double with the same separation and position angle. Spectra taken with the Keck II Telescope show the two components to have nearly identical emission-line spectra, but with somewhat different absorption-line systems. Subtraction of two stellar point-spread functions from the pair of components consistently leaves a residual object. Depending upon whether this third object is extended or a point source, it may be as much as 1/10 or as little as 1/100 as bright as the brighter QSO component. The observations leave little doubt that the two brighter objects are gravitationally lensed images of the same quasar. The third object might be either the lensing galaxy or a third image of the quasar, but both interpretations have serious shortcomings. Observations reported in this paper were obtained in part at the MDM Observatory, a facility jointly operated by the University of Michigan, Dartmouth College, and the Massachusetts Institute of Technology.
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Buchalter, A., Helfand, D. J., Becker, R. H., & White, R. L. 1998, ApJ, 494, 503
In previous attempts to measure cosmological parameters from the angular size-redshift ( theta -z) relation of double-lobed radio sources, the observed data have generally been consistent with a static Euclidean universe rather than with standard Friedmann models, and past authors have disagreed significantly as to what effects are responsible for this observation. These results and different interpretations may be due largely to a variety of selection effects and differences in the sample definitions destroying the integrity of the data sets, and inconsistencies in the analysis undermining the results. Using the VLA FIRST survey, we investigate the theta -z relation for a new sample of double-lobed quasars. We define a set of 103 sources, carefully addressing the various potential problems that, we believe, have compromised past work, including a robust definition of size and the completeness and homogeneity of the sample, and further devise a self-consistent method to assure accurate morphological classification and account for finite resolution effects in the analysis. Before focusing on cosmological constraints, we investigate the possible impact of correlations among the intrinsic properties of these sources over the entire assumed range of allowed cosmological parameter values. For all cases, we find apparent size evolution of the form l ~ (1 + z)c, with c ~ -0.8 +/- 0.4, which is found to arise mainly from a power-size correlation of the form l ~ P beta ( beta ~ - 0.13 +/- 0.06) coupled with a power-redshift correlation. Intrinsic size evolution is consistent with zero. We also find that in all cases, a subsample with c ~ 0 can be defined, whose theta -z relation should therefore arise primarily from cosmological effects. These results are found to be independent of orientation effects, although other evidence indicates that orientation effects are present and consistent with predictions of the unified scheme for radio-loud active galactic nuclei. The above results are all confirmed by nonparametric analysis. Contrary to past work, we find that the observed theta -z relation for our sample is more consistent with standard Friedmann models than with a static Euclidean universe. Though the current data cannot distinguish with high significance between various Friedmann models, significant constraints on the cosmological parameters within a given model are obtained. In particular, we find that a flat, matter-dominated universe ( Omega 0 = 1), a flat universe with a cosmological constant, and an open universe all provide comparably good fits to the data, with the latter two models both yielding Omega 0 ~ 0.35 with 1 sigma ranges including values between ~0.25 and 1.0; the c ~ 0 subsamples yield values of Omega 0 near unity in these models, though with even greater error ranges. We also examine the values of H0 implied by the data, using plausible assumptions about the intrinsic source sizes, and find these to be consistent with the currently accepted range of values. We determine the sample size needed to improve significantly the results and outline future strategies for such work.
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Becker, R. H., Gregg, M. D., Hook, I. M., McMahon, R. G., White, R. L., & Helfand, D. J. 1997, ApJ, 479, L93
We have discovered two low-ionization broad absorption line (BAL) quasars in programs to obtain optical spectra for radio-selected quasar candidates from the VLA Faint Images of the Radio Sky at Twenty Centimeters (FIRST) Survey. Both belong to the extremely rare class of BAL QSOs that exhibit narrow absorption lines from metastable excited levels of Fe II and Fe III. Until now, there was just a single object in this class, 0059-2735. In addition, one of our new objects is the first known radio-loud BAL QSO. The properties of these three unusual objects suggest a trend of increasing radio luminosity with the amount of absorption to the quasar, and are perhaps transition objects between radio-loud and radio-quiet quasars. The two new objects are from a radio-selected sample comprising less than 200 quasars; one is heavily attenuated at optical wavelengths in the observed frame. These objects would be easily overlooked by most optical QSO searches; their abundance in the radio sample suggests that they may be representatives of a largely undetected component of the quasar population, perhaps as numerous as ordinary low-ionization BAL QSOs, which constitute 1%--2% of all QSOs.
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White, R. L., Becker, R. H., Helfand, D. J., & Gregg, M. D. 1997, ApJ, 475, 479
We present a catalog of 138,665 radio sources derived from the initial 1550 deg2 of the FIRST survey. The survey parameters are reviewed, and a map depicting the coverage for the first two observing sessions is presented. We then describe in detail our algorithm for radio source detection and parameterization, as well as our procedures for constructing the final catalog. The results of extensive tests for astrometric and photometric accuracy, as well as for uncertainties in source extent and morphological characterization, are presented. Source positions are all good to better than 1", and the flux density scale is accurate to 5%; our elliptical Gaussian fitting procedure is shown both to provide a high-fidelity representation of source morphology for most objects and to characterize correctly bright sources extended on scales down to ~ \frac {1}{3} the synthesized beam size of 5."4. We use the catalog to construct a log N--log S relation for 20 cm radio sources over four decades of flux density, finding excellent agreement with previous determinations from smaller surveys. As one example of the catalog's utility, we present comparisons with the Guide Star Catalog, the IRAS Faint Source catalog, and the ROSAT WGA catalog, identifying radio counterparts to thousands of the objects. Complete instructions for access to this FIRST catalog and subsequent updates thereto are provided.
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Cress, C. M., Helfand, D. J., Becker, R. H., Gregg, M. D., & White, R. L. 1996, ApJ, 473, 7
The FIRST (Faint Images of the Radio Sky at Twenty centimeters) survey now covers 1550 deg^2^ of sky, where 07^h^16 <~ α <~ 17^h^40 and 28.3^deg^ <~ δ <~ 42^deg^. This yields a catalog of 138,665 sources above the survey threshold of 1 mJy, about one-third of which are in double-lobed and multicomponent sources. We have used these data to obtain the first high-significance measurement of the two-point angular correlation for a deep radio sample. We find that the correlation function between 0.02^deg^ and 2^deg^ is well fitted by a power law of the form Aθgamma^, where A ~ 3 x 10^-3^ and γ ~ -1.1. On small scales (θ < 0.2^deg^), double and multicomponent sources are shown to have a larger clustering amplitude than that of the whole sample. Sources with flux densities below 2 mJy are found to have a shallower slope than that obtained for the whole sample, consistent with there being a significant contribution from starbursting galaxies at these faint fluxes. The cross-correlation of radio sources and Abell clusters is determined. A preliminary approach to inferring spatial information is outlined.
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Gregg, M. D., Becker, R. H., White, R. L., Helfand, D. J., McMahon, R. G., & Hook, I. M. 1996, AJ, 112, 407
The FIRST radio survey provides a new resource for constructing a large quasar sample. With source positions accurate to better than 1" and a point source sensitivity limit of 1 mJy, it reaches 50 times deeper than previous radio catalogs. We report here on the results of the pilot phase for a FIRST Bright Quasar Survey (FBQS). Based on matching the radio catalog from the initial 300 deg^2^ of FIRST coverage with the optical catalog from the Automated Plate Machine (APM) digitization of Palomar Sky Survey plates, we have defined a sample of 219 quasar candidates brighter than E = 17.50. We have obtained optical spectroscopy for 151 of these and classified 25 others from the literature, yielding 69 quasars or Seyfert 1 galaxies, of which 51 are new. identifications. The brightest new quasar has an E magnitude of 14.6 and z = 0.91; four others are brighter than E = 16. The redshifts range from z = 0.12 to 3.42. Half of the detected objects are radio quiet with L_21-cm_ < 10^32.5^ ergs/s. We use the results of this pilot survey to establish criteria for the FBQS that will produce a quasar search program which will be 70% efficient and 95% complete to a 21-cm flux density limit of 1.0 mJy.
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Becker, R. H., White, R. L., & Helfand, D. J. 1995, ApJ, 450, 559
The FIRST survey to produce Faint Images of the Radio Sky at Twenty centimeters is now underway using the NRAO Very Large Array. We describe here the scientific motivation for a large-area sky survey at radio frequencies which has a sensitivity and angular resolution comparable to the Palomar Observatory Sky Survey, and we recount the history that led to the current survey project. The technical design of the survey is covered in detail, including a description and justification of the grid pattern chosen, the rationale behind the integration time and angular resolution selected, and a summary of the other considerations which informed our planning for the project. A comprehensive description of the automated data analysis pipeline we have developed is presented.
We also report here the results of the first year of FIRST observations. A total of 144 hr of time in 1993 April and May was used for a variety of tests, as well as to cover an initial strip of the survey extending between 07h 15m and 16h 30m in a 2°.8 wide declination zone passing through the local zenith (28.2 <δ < 31.0). A total of 2153 individual pointings yielded an image database containing 1039 merged images 46'.5 × 34'.5 in extent with 1".8 pixels and a typical rms of 0.13 mJy. A catalog derived from this 300 deg2 region contains 28,000 radio sources. We have performed extensive tests on the images and source list in order to establish the photometric and astrometric accuracy of these data products. We find systematic astrometric errors of < 0".05 individual sources down to the 1 mJy survey flux density threshold have 90% confidence error circles with radii of < 1". CLEAN bias introduces a systematic underestimate of point-source flux densities of ˜0.25 mJy; the bias is more severe for extended sources. Nonetheless, a comparison with a published deep survey field demonstrates that we successfully detect 39/49 sources with integrated flux densities greater than 0.75 mJy, including 19 of 20 sources above 2.0 mJy; the sources not detected are known to be very extended and so have surface brightnesses well below our threshold.
With 480 hr of observing time committed for each of the next three B-configuration periods, FIRST will complete nearly one-half of its goal of covering the 10,000 deg2 of the north Galactic cap scheduled for inclusion in the Sloan Digital Sky Survey. All of the FIRST data raw visibilities, self-calibrated UV data sets, individual pointing maps, final merged images, source catalogs, and individual source images are being placed in the public domain as soon as they are verified; all of the 1993 data are now available through the NRAO and/or the STScI archive. We conclude with a brief summary of the scientific significance of FIRST, which represents an improvement by a factor of 50 in both angular resolution and sensitivity over the best available large area radio surveys.
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Becker, R. H., White, R. L., & Helfand, D. J. 1994, ASPC, 61, 165
Over the next decade the VLA will carry out a systematic survey of the northern sky at 20 cm wavelength in two configurations. We have selected as a moniker the acronym FIRST, which in uncompressed form reads Faint Images of the Radio Sky at Twenty-cm. The high resolution survey will be done in 'B' configuration and hence will achieve an angular resolution of 5 arcsec. It will cover a 10**4 sq. degree region centered on the north Galactic pole. Each field will be observed for 3 minutes with a resulting RMS noise level of 0.2 mJy. The survey will result in a catalog of 1E6 discrete sources as well as detailed images composed of 4E10 pixels. To expedite the massive data analysis requirements this project entails, we have decided to utilize computers. It is our intention to release to the community compressed, machine-readable copies of all the images as well as an annotated catalog of sources. The challenge will be to accomplish this with a minimum of resources. We hope to maximally automate the data analysis pipeline to achieve this end. During our presentation, we will elaborate on the details of the methodology.
This paper is available through ADS. This is the first-published paper describing the FIRST project and is also available in HTML and compressed Postscript.
