A catalog for the north and south Galactic caps
(946,432 sources), derived from the
1993 through 2011 observations, is
available as a gzip-compressed ASCII file
(catalog_14dec17.bin.gz)
and as a FITS binary table
(first_14dec17.fits.gz).
The file size is 41 MBytes compressed (156 MBytes uncompressed) for the
ASCII version and 66 MBytes compressed (113 MBytes uncompressed) for the
FITS version.
The catalog covers a total of about 10,575 square degrees of sky (8,444
square degrees in the north Galactic cap and 2,131 square degrees
in the south Galactic cap.)
See the coverage maps for more details of the area covered. Both the northern and southern areas were chosen to coincide approximately with the area covered by the Sloan Digital Sky Survey.
This catalog is identical to the previous version (14Mar04) except that it has more accurate data on which sources are not covered by the SDSS DR10 catalog. Approximately 1000 sources that were indicated as covered by DR10 in the previous version are now correctly marked as not covered. The source list, radio fluxes, etc., are all the same as the last version.
NOTE: In this version of the catalog, images taken in the the new EVLA configuration have been re-reduced using shallower CLEAN thresholds in order to reduce the "CLEAN bias" in those images. Also, the EVLA images are not co-added with older VLA images to avoid problems resulting from the different frequencies and noise properties of the configurations. That leads to small gaps in the sky coverage at boundaries between the EVLA and VLA regions. As a result, the area covered by this release of the catalog is about 60 square degrees smaller than the earlier release of the catalog (13Jun05), and the total number of sources is reduced by nearly 25,000. The previous version of the catalog does have sources in the overlap regions, but their flux densities are considered unreliable due to calibration errors. The flux densities should be more accurate in this catalog, biases are smaller, and the incidence of spurious sources is also reduced.
Over most of the survey area, the detection limit is 1 mJy. A region along the equatorial strip (RA = 21.3 to 3.3 hrs, Dec = -1 to 1 deg) has a deeper detection threshold because two epochs of observation were combined. The typical detection threshold in this region is 0.75 mJy. There are approximately 4,500 sources below the 1 mJy threshold used for most previous versions of the catalog.
The catalog is on-line and can be searched using the FIRST Search Engine. However, for large-scale investigations it will be necessary to obtain the complete catalog. This document describes the catalog format.
The catalog is sorted by decreasing declination and has the following format:
# ------SDSS------ -----2MASS---- Epoch Epoch Epoch # RA Dec P(S) Fpeak Fint RMS Maj Min PA fMaj fMin fPA Field # Sep i Cl # Sep K Mean-yr Mean-MJD rms-MJD 07 27 34.289 +64 40 59.80 0.197 1.00 1.12 0.139 2.13 1.58 0.2 5.80 5.63 0.2 07300+64243J -1 99.00 99.00 - 1 6.76 14.62 2002.687 2452526.1 0.002 07 38 39.304 +64 40 16.28 0.014 2.39 5.73 0.139 9.31 3.58 136.6 10.76 6.48 136.6 07360+64243J -1 99.00 99.00 - 0 99.00 99.00 2002.687 2452526.1 0.001 07 50 24.019 +64 40 01.21 0.014 22.09 24.00 0.140 1.96 1.13 6.8 5.74 5.52 6.8 07480+64243J -1 99.00 99.00 - 0 99.00 99.00 2002.687 2452526.1 0.002 07 38 45.622 +64 39 50.12 0.014 2.39 4.33 0.140 6.43 3.27 13.5 8.40 6.31 13.5 07360+64243J -1 99.00 99.00 - 0 99.00 99.00 2002.687 2452526.1 0.000 07 39 32.799 +64 39 18.03 0.082 1.41 1.25 0.139 2.63 0.00 37.5 6.01 4.30 37.5 07420+64243J -1 99.00 99.00 - 0 99.00 99.00 2002.687 2452526.1 0.002
The format of this catalog is the same as releases since 13Jun05 but differs from earlier versions of the catalog. The last 3 columns give information on the epoch of observation for each source and are described below in the observation epoch section. The P(S) column, which indicates the probability that the source is a sidelobe, replaces the previous binary sidelobe flag column. The columns following the field name give information on counterparts to the FIRST source in the SDSS DR10 catalog and the 2MASS catalog. These parameters are described in detail below in the description of the P(S) column and the counterparts columns. Other catalog columns are common with FIRST catalog releases extending back over the past decade.
unc(90% confidence) = Size (1/SNR + 1/20) arcsecwhere Size is either the major or minor axis fitted FWHM (fMaj or fMin) as given in the catalog and SNR is the peak flux density signal-to-noise ratio:
SNR = (Fpeak-0.25) / Rms
(The positional uncertainty is of course elliptical for elliptical sources.) The best possible positional uncertainty is limited to about 0.1 arcsec by our ability to fit source positions in maps with 1.8 arcsec pixels and by various calibration uncertainties. Systematic errors in the positions are smaller than 0.05 arcsec.
| Probability Range | Fraction of sources |
|---|---|
| 0.00 < PS < 0.05 | 76.3% |
| 0.05 < PS < 0.15 | 7.0% |
| 0.15 < PS < 0.25 | 4.4% |
| 0.25 < PS < 0.35 | 3.5% |
| 0.35 < PS < 0.45 | 2.1% |
| 0.45 < PS < 0.55 | 1.7% |
| 0.55 < PS < 0.65 | 1.6% |
| 0.65 < PS < 0.75 | 1.5% |
| 0.75 < PS < 0.85 | 1.1% |
| 0.85 < PS < 0.90 | 0.8% |
Sidelobe probabilities for this version of the catalog have been computed using an improved algorithm based on multiple voting oblique decision tree classifiers. The classifiers were trained using deep VLA fields that give reliable assessments of the reality of FIRST sources. The algorithm will be described in detail in a future paper on the final FIRST catalog; we believe it is a substantial improvement over the previous sidelobe flagging approach. Nevertheless, we still recommend checking the images using the FIRST Cutout Server if there is doubt about the reality of particular sources. This is easily done when using the FIRST Search Engine to search the catalog, since each source selected in the search has a link to the Cutout Server.
The uncertainty in Fpeak is given by the rms noise at the source position, while the uncertainty in Fint can be considerably greater depending on the source size and morphology. For bright sources the accuracies of Fpeak and Fint are limited to about 5% by systematic effects. Note that for sources that are not well-described by an elliptical Gaussian model, Fint is not an accurate measure of the integrated flux density.
FITS images giving the rms noise as a function of position on the sky are available for the northern and the southern areas. These images give the rms in mJy/beam tabulated on a ~3 arcmin grid in RA and Declination. If there is no source in the catalog at a given position, the source peak flux density (before CLEAN bias correction) is less than 5 times the coverage map rms value at that position. The sky area covered is displayed in the images below (which are linked to postscript versions of the images):
The coverage in the southern Galactic cap is ragged (and discontiguous) at the northern edge because poor weather and equipment failures prevented the planned survey area from being filled in there. Small gaps can be seen around the edges of the area observed using the EVLA configuration in 2011 (see above for more details).
Note that the FIRST Search Engine gives the rms noise estimate at the specified sky position, as derived from these coverage maps. That is the most convenient way to determine the noise at a particular position in the survey (or to determine whether a position has been observed.)
Noise can cause the fitted values of the major and minor axes (before deconvolution) to be smaller than the beam. The corresponding deconvolved size is given as zero in that case.
The uncertainties in the deconvolved sizes depend on both the brightness and the sizes. Objects at the catalog flux density limit have uncertainties of about 2 arcsec in their sizes (so faint objects with Maj < 2 arcsec are consistent with point sources.) An simple empirical estimate of the uncertainty is
Sigma(Size) = 10 arcsec (1/SNR + 1/75)
where SNR is the signal-to-noise ratio defined above.
hhmmm+ddmmm is centered at RA=hh mm.m,
Dec=+dd mm.m. The images
are available from several archives and
through the FIRST
Cutout Server.
All field names in the current catalog end with a letter E through X, depending on the date of the last catalog release in which the image was modified. The W and X fields are new in this catalog, while sources extracted from the E-T fields are essentially identical to those in the previous version of the catalog. The W fields include contributions from EVLA data taken in Spring 2011 and have both a slightly different central frequency (1.335 GHz instead of 1.400 GHz) and typically higher noise levels than the older images. The X fields are images that neighbor the EVLA fields but differ from the previous release in that they omit nearby EVLA observations from the co-adding of overlapping grid images.
For each catalog there is a column giving the number of matches within a fiducial radius of 8 arcsec and, for the nearest match, the separation from the FIRST position in arcsec and the magnitude. For SDSS the morphological classification is also given (s=stellar, g=nonstellar/galaxy). A count of zero indicates there are no sources within this radius (which is also indicated by a separation of 99.00 and a classification of '-'). For SDSS, a count of -1 indicates that the FIRST source falls outside the SDSS DR10 survey area so that no SDSS data are available. All of these catalogs have multiple wavebands available; the given magnitudes are for i (SDSS) and K (2MASS). For SDSS, a magnitude of -1 indicates that the magnitude in the DR10 catalog was given as -9999. Note that these parameters can be used for catalog searches via the "Additional Constraints" field in the FIRST Search Engine web form. See the Additional Constraints Help Page for more information.
Some information summarizing the frequency of counterparts in the various catalogs is given in the table below:
| Survey | Characteristic | Percentage |
|---|---|---|
| SDSS | 1 or more counterparts < 8" | 54.5% |
| SDSS | Counterpart < 1.5" | 28.3% |
| SDSS | Covered by DR10 | 93.0% |
| 2MASS | 1 or more counterparts < 8" | 11.6% |
| 2MASS | Counterpart < 1.5" | 7.5% |
The Epoch Mean columns give the weighted mean of all the contributing pointing epochs at the position of the source. It is given both in decimal years and in MJD for convenience. The Epoch rms column (MJD) gives the weighted rms of the pointing epochs at the source position. It is a measure of the spread in epochs that contribute to the measurement. Many sources have small rms values of only a few minutes (dominated by a single 3-minute pointings or by 2 adjacent pointings), but values of days to weeks are also common (for sources observed in the overlap between declination strips), and some objects have rms values of years (for sources observed at the edges of regions in different observing seasons or that were observed multiple times due to data problems). The largest epoch rms in the survey is 6.8 years.
The epoch rms should be used as a guide to identify objects that do not have well-defined epochs. The table below gives an indication of the frequency of different epoch rms values in the catalog.
| Epoch rms range | Fraction |
|---|---|
| < 5 minutes | 5% |
| 5 min — 1 day | 29% |
| 1 — 10 days | 44% |
| 10 — 100 days | 11% |
| 100 days — 1 yr | 7% |
| 1 — 2 yrs | 1% |
| 2 — 5 yrs | 2% |
| > 5 yrs | 0.3% |
The catalog history page describes previously released versions of the FIRST catalogs, which are still available for historical purposes. We recommend that the new catalog be used where possible for all projects.