The most important discoveries that have been made using the POSS - quasars, starburst galaxies, the large-scale structure of the Universe - were not anticipated by those who carried out the original survey. And many of these discoveries have come as a result of comparing work done at other wavelengths with this unparalleled optical database. Likewise, we cannot anticipate all of the new source classes and the astrophysical insights that FIRST survey will yield, but we are confident that cross correlation of the maps and catalogs we produce with data from other wavelength regimes will be enormously productive. To maximize the utility of the survey, it has been designed to cover the same square degrees of sky as the Sloan Digital Sky Survey (SDSS) which will, by the end of the decade, have produced five-color CCD images of this entire area to and will be in the process of collecting spectra from of the objects therein. The subarcsecond positions our survey will produce will allow immediate optical identification of of our radio catalog from the SDSS database, further enhancing its utility in infrared and high energy astrophysics. While an exhaustive description of the scientific impact this project will have is beyond the scope of this paper, we outline below a few examples of areas in which the FIRST survey will make fundamental contributions.
At 20-cm flux densities above mJy, active galactic nuclei dominate radio source counts. In the thirty years since the discovery of quasars, nearly 10,000 galaxy systems with extraordinarily powerful, compact nuclei have been catalogued, and progress in understanding the mechanisms which power them has been substantial. Nonetheless, we still argue about density vs. luminosity vs. luminosity-dependent density evolution for quasars, and estimates of the typical quasar lifetime are uncertain by roughly two orders of magnitude. The connection between IRAS starburst galaxies and quasars, the importance of mergers, the origin of the powerful -ray emission recently discovered by CGRO, the contribution of various classes of objects to the cosmic X-ray background, the importance of relativistic beaming for BL Lacs and their connection to radio galaxies, as well as generalized ``unified models'' of AGN are all topics of current active research. Work toward answers for each of these outstanding questions will be materially aided by a deep, uniform, large-area radio survey.
Using the Cambridge University Automated Plate Machine (APM) scans of the POSS (McMahon, private communication) and data from a 25 deg pilot for our VLA survey, we have demonstrated that, using optical color selection plus radio identification, we will be able to generate immediately a complete, flux-limited catalog of radio-loud quasars brighter than for use in evolution studies. Since this sample will be optically bright, it will also be ideal for detailed spectroscopic studies. Cross-correlation with the ROSAT all-sky survey will increase the currently small sample of known BL Lacs by more than an order of magnitude from a little over 100 to several thousand; furthermore, since the 1 mJy limit of the survey is equivalent to the lowest flux density ever found from X-ray selected samples of these objects (Stocke 1990) and is far below the threshold for radio-selected objects, this sample will also be complete and flux-limited. The enormous benefit this new sample will have for the study of all objects in which relativistic beaming is important can be gauged by the progress brought about by the Kuhr et al. (1981) complete radio sample, the flux density limit of which is 1 Jy, times our threshold. In all, the FIRST survey will detect over half a million AGN, and most will have optical counterparts in the SDSS. None of the current problems in the field will be immune from attack with this powerful resource.
Below about 3 mJy, the radio source slope changes markedly, indicating the presence of a new population. Work over the past decade (Windhorst et al. 1985; Oort 1987; Thuan &Condon 1987) has demonstrated convincingly that this population consists largely of actively star-forming galaxies and, as such, holds an important key to the subject of galaxy evolution. At 1 mJy, the majority of our sources will be starburst galaxies at redshifts from 0.1 to 0.6 and, perhaps, far beyond (e.g., IRAS 10214+4724 at [Rowan-Robinson et al. 1991]). Every galaxy in the IRAS Faint Source Catalog will be detectable and, with the resultant subarcsecond radio positions, immediately identifiable. Many fainter objects, detectable only by future far-IR missions such as ISO and SIRTF, will also be identified, distinguished from AGN by their optical and radio extents. Questions such as the importance of mergers to star formation history, the shape of the IMF, the extent of mass-loss in supernova-driven winds, and the importance of local environment in galaxy evolution will be addressable with this enormous sample. Moreover, the combination of the radio data from our catalog with data from IRAS, CGRO, SDSS, etc., will allow the discovery of many rare objects such as IRAS 10214+4724, which often hold vital clues to the behavior of less extreme systems.
The evolution of large scale structure in the Universe, from the era probed by the COBE observations of the microwave background to the present, is one of the central issues of modern astrophysics and, owing to the important role of dark matter in this evolution, of particle physics as well. The enormous amount of work undertaken over the past decade to map the local large-scale structure has resulted in a relatively clear picture of the distribution of luminous matter at redshifts . The challenge now is to extend our knowledge of this structure to much higher redshifts so that its evolution becomes apparent. For high redshifts, our quasar sample will provide a useful start on the largest scales, but it is in the redshift range 0.1-0.5 that real progress will be forthcoming by tracing the matter distribution with our starburst sample. Other avenues of approach include the detection of a large cluster sample through selection of head-tail radio galaxies, and the selection of large-separation gravitational lens candidates via an optical filter on the sample of close doubles (demanding equal relative intensities for the two components in the two wavelength bands).
A half dozen categories of radio-emitting stars provide us with information on topics ranging from main sequence and pre-supernova mass loss to surface magnetic activity and its evolution. Only about 200 radio stars are known, and virtually all of these emerged from targeted surveys of optically selected objects. FIRST provides a unique opportunity to produce unbiased, flux-limited, and complete samples of stellar radio sources. The accurate positions of even the faintest radio sources will lead to immediate and unambiguous optical identifications from existing POSS and ESO plate scan databases, as virtually all detectable objects are brighter than these plate limits. (Note that although bright stellar objects are not dense on the sky, radio stars are so rare that one must have subarcsecond radio positions to identify them in a large area survey.) From enriching our understanding of the evolution of dynamo-generated magnetic fields on late type stars (which will emerge from a comparison with the ROSAT database), to identifying nearby, coeval, moving groups of red dwarfs, our survey will open a new era in stellar radio astronomy.
Other Galactic objects may also turn up in the survey. Recently, a 0.5 Jy radio source in the southern hemisphere was identified as the nearest millisecond pulsar yet discovered (Johnston et al. 1993). Comparison of our survey with the Texas 80 cm interferometric survey and the ongoing, low-frequency Westerbork survey will immediately identify good candidates for these fascinating objects from their uniquely steep spectral indices, providing the first wide-area pulsar survey unbiased by the problems of traditional search techniques such as low duty cycles, high dispersion measures, short periods, and binary accelerations.