The enhanced VLA will have a major impact in the area of unbiased
surveys in the neutral hydrogen line. For continuum observations the
VLA has been unsurpassed as a surveying instrument, with a collecting
area that is only second to Arecibo, combined with the large field of
view. For blind H I surveys a third dimension comes into play-the
instantaneous redshift range that can be covered. The new correlator
will be more powerful in that respect than any currently operational
or planned correlator on a synthesis instrument. Thus the VLA will
become the most powerful H I surveying machine by far. Extending the
1.4 GHz band down to 
800 MHz would allow the direct study of
galaxy evolution by imaging the H I emission of galaxies in 
. While the highest redshift at which H I has yet been detected
in emission is about 0.08, the enhanced VLA could detect thousands of
galaxies at redshifts from 0.2 to 0.8 in a 100-hr integration.
The large scale structure of the universe as we know it today, with its large filaments, sheets, walls and voids, is based almost entirely on observations of high-luminosity galaxies, observed at optical wavelengths. Biased galaxy formation predicts a separation between luminosity (bright galaxies) and mass (dark matter). Dwarf galaxies are expected to be more uniformly distributed. Since nearly all investigations of the properties and spatial distribution of galaxies begin with optically (or IRAS) selected galaxy catalogs, any population of gas clouds with very low optical luminosity or surface brightness could have largely escaped detection. Direct searches in the H I line circumvent these optical selection biases.
Examples of questions to be addressed by such surveys are the
abundance of gas rich dwarfs and the study of their spatial
distribution. Do they follow the optically bright galaxies or are they
distributed more uniformly as predicted in some theories of galaxy
formation. It will become possible to construct an unbiased H I mass
function for the first time. A complete inventory can be made of the
H I content in the local universe, the size and mass distribution of H I
disks. Galaxies may be found in the zone of avoidance and new light
may be shed on the parent population of nearby Ly 
absorbers. At larger redshifts, the structure of clusters of galaxies
and voids may may be determined. At even higher redshifts entirely new
ground will be trodden. The H I properties of galaxies
at 
0.1 will be studied for the first time, and it will be possible to
study the upper end of the H I mass function out to 
1.
This might settle once and for all whether the gas reservoirs around
galaxies increase with increasing redshift, and whether entirely new
gaseous galaxy types occur at these higher redshifts.
Below we present three examples of such surveys, one concerning the
local universe and employing the extreme surface brightness
sensitivity of the E configuration, one performing simultaneously an in-depth
study of a cluster at a redshift of 0.1, combined with a deep pencil
beam survey covering a range from 0 
0.2 to be done with the
C configuration. The third survey is aimed at studying the evolution of the
gas reservoirs around galaxies in the range from 0.2 0.8
with the resolution of the B configuration.
0.1In recent years all sky continuum surveys covering almost the entire electromagnetic spectrum have become available, giving us a good indication of the stellar, dust, hot gas and relativistic electrons distribution in the universe. However for the most abundant element, neutral hydrogen, this information is lacking. In fact, we don't even know the H I content of the local group of galaxies.
The high surface brightness sensitivity of the VLA E configuration, combined
with the large instantaneous velocity coverage of the correlator will
finally make it feasible to survey H I over the whole sky, simultaneously
covering a redshift from 0 to 0.1. This will give a complete
inventory of H I masses irrespective of a stellar association to
solar masses within the local group, to 
solar masses at
the distance of Virgo Cluster, 
solar masses in Coma, to
solar masses at the survey limit at a distance of 280
Mpc. Note that even in the nearest cluster, Virgo, only a few
percent of the total volume has yet been searched for HI. The total
search volume of this survey, 
, is
such that about 
galaxies should be detected with H I mass less
than 
solar masses .
The assumed survey parameters are an integration time of 3 minutes per
pointing, giving a column density sensitivity of 
cm
over a 
beam, using a total bandwidth of 125 MHz
and 2048 channels of 12.6 km/s. The entire sky north of 
deg
can then be observed to the 6
mass limits noted above
in a (mere) 2.7 yrs.
At higher redshifts, an entire cluster fits into one primary beam. For
example at z = 0.1, an Abell diameter is about 
, so we can
study an entire cluster in one pointing, giving the spatial and
kinematic distribution of all the gas-rich objects in and around
it. The resolution of the C configuration provides some spatial information
for individual galaxies. A 36-hr integration would (at z = 0.1),
allow a 6 detection of 
solar masses of H I with 100 km/s
line width. A total band of 62.5 MHz is needed,
corresponding to 13,000 km/s split up in 2048 channels of 6.3 km/s.
The spatial and velocity distribution of the gas-rich
galaxies will provide information on the dynamical state of the
cluster, whether it is relaxed or still collapsing. The velocity
coverage of 13,000 km/s is twice that needed to cover a typical
rich cluster, so many foreground and background spirals will also be
detected.. The 6 km/s accuracy of the velocity profiles will
make it possible to derive useful Tully-Fisher distances to the
galaxies and thus to probe deviations from the Hubble flow out to many
Mpc from the cluster. These observations will also provide a useful
data base to compare with cluster studies at higher redshifts aimed at
determining the evolution of the gas content in clusters.
In parallel with such deep integrations a sensitive pencil beam survey
can be carried out to sample the entire conical volume within the
primary beam between z = 0 - 0.2, using 250 MHz bandwidth and 1024 channels of
250 kHz (50 km/s). H I masses of 
solar masses will be
detectable at the survey limit of z = 0.2, implying that normal galaxies
with modest H I content can easily be detected. Note that currently the
highest redshift at which H I in galaxies has been detected in emission
is about 0.1. Within each search volume of 
we expect to detect a few hundred
galaxies with H I mass less than solar masses .
0.8
Recent Hubble Space Telescope results indicate that both field and
cluster galaxies are undergoing dramatic evolution at 
0.5. A
relative increase in the number of spiral-like galaxies is found,
representing interacting and merging galaxies in clusters and a
population of blue field galaxies unlike anything seen in the local
universe. A major question is how does the gas content of galaxies
evolve at these redshifts, it is expected to increase with redshift.
Extending the frequency coverage to 800-1200 MHz would allow us to
probe this critical phase in galaxy evolution in the redshifted H I
line. Making the conservative assumption that H I line fluxes at z =
0.5 are similar to the largest seen in the nearby universe, namely 
solar masses with a velocity width of 300 km/s, a 6
detection can be made in 100 hrs at that redshift. The largest possible
bandwidth should be used to probe the entire redshift interval from
z = 0.2 to 0.8 simultaneously, if the correlator capacity would permit
(500 MHz and 2048 channels). Each field observed would then sample a
total volume of 
, subject to limitations
imposed by RFI. An unknown number
of H I luminous objects might be detected in the interval 0.5 0.8,
but more importantly, assuming only local space densities and H I
luminosities we can expect detection of more than 
galaxies per
field in the interval z = 0.2 to 0.5, revolutionizing our
understanding of galaxy evolution. Such a survey would be done in the
B configuration, matching the beam size to the typical galaxy size at those
redshifts.