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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.
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Up: The VLA's FIRST Survey
Previous: Data Analysis