The NRAO now operates 37 ``synthesis array elements", broken into a 27-element sub-array (the VLA) and a 10-element sub-array (the VLBA), with only one provision for cross-linking them (using one antenna or the whole VLA in its phased-array mode for VLBI). Much could be gained by allowing individual elements of the VLA and VLBA to join in observations with the other array, especially if we build the new antennas described earlier.
First, the information gathered
by an 
-element
synthesis array varies as the number of baselines, i.e., as
for large . Linking even four
VLA antennas to the VLBA would double the information content
of VLBA observations by doubling the number of correlations. This
doubled information content would translate into improved image
quality via (i) a wider field of view, (ii) improved sensitivity to
extended structures, and (iii) increased dynamic range (image
fidelity) for the VLBA. The more ambitious world-array VLBI
experiments have also shown that the apparent simplicity of typical
VLBI images reflects a lack of information gathered about the sources
more than any intrinsic simplicity of Nature on these scales. We can
expect many extragalactic sources to have complex structures on scales
from 1 to 100 milli-arcseconds. Physical interpretation of these
structures will require images with high dynamic range and wide fields
of view over a large range in frequency.
Figure 1.2:
(left) An image obtained by using the full u-v
coverage of the present VLBA at 
21cm to sample a model derived from
the radio structure of the supernova remnant Cassiopeia A, rescaled to
a distance of 1 Mpc. Only the most compact emission is represented at
this resolution (FWHM 
8 milli-arcsec).
(right) An image obtained by sampling the same model with the
full u-v coverage obtained by adding the Dusty, Bernardo,
Holbrook and Vaughn antennas to the VLBA. The FWHM of the synthesized
beam increases by only about 4% but the larger-scale structure of the
remnant is now correctly represented at this resolution. Neither
simulation is noise-limited.
Second, the VLA and VLBA, taken separately, leave the 40-400 km
range of baselines relatively unsampled. This ``coverage gap"
obstructs work on many astrophysical problems. For example, the VLA
now images well down to about 
FWHM at 2cm and proportionally lower
resolution at longer wavelengths. The VLBA images up to a few
milli-arcsec at 2cm. They fail to meet by about a
factor of ten in resolution at any frequency. The missing regime is
now of great interest to AGN jet dynamics, observations of galactic
X-ray transients, of circumstellar masers, proto-planetary disks, and
of supernova remnants in external galaxies.
Figure 1.2 illustrates how the
new antennas proposed for the A+ configuration could also improve
image quality when used to provide short-baseline u-v coverage
in observations with the VLBA. As in Section 1.2.5, we consider
observing a ``twin'' of the Cas A supernova remnant at a distance of
about 1 Mpc, so the angular extent of the remnant is about 
. The left panel shows
the image that could be obtained at 21cm with the present VLBA. The
right panel shows the dramatic improvement obtainable by adding the
data from the proposed new antennas.
Figure 1.3: Preliminary VLBA images of the nucleus of the active galaxy
3C84 at five frequencies. Note the differing sensitivities. The
extent to which the detailed differences between these images are real
frequency-dependent properties of the source (rather than differences
in the u-v coverage of the VLBA at these frequencies) would be
unclear even with matched sensitivities. The 43-GHz image clearly
under-samples the broader (>5 milli-arcsec) structure in the
lower-frequency images, however: it accounts for only about 3/4 of the
known total flux density. These uncertainties could be eliminated by
cross-linking the expanded VLA and the VLBA to provide a scaled-array
capability, especially at the highest frequencies.
Third, it is important to realize that the VLBA has very little ``scaled-array" capability because of its small number of antennas (image quality deteriorates rapidly if any antennas are removed at one frequency to ``match" the coverage at another) and because it is not reconfigurable. We can have multi-frequency coverage at a fixed angular resolution only by adding data from baselines shorter than 400 km at the higher frequencies (e.g., Figure 1.3). Cross-linking an expanded VLA with the VLBA will extend the scaled-array capability of both instruments to cover a huge domain of angular resolution, providing well-filled u-v coverage, scaled with wavelength over a wide frequency range. For example, when studying relativistic jets in AGNs or galactic X-ray transients, we must be able to image total and polarized emissions at the same resolution over a wide range of frequencies to determine spectral energy distributions, Faraday rotations and magnetic field directions, as well as the collimation and proper motion information that are available from single frequencies.
Although a foreign ``100-km" array (e.g., MERLIN) can provide some of the
``missing coverage" at >6cm for northern sources, the
high-frequency coverage that is critical to much of the astronomical
program, and the ``baseline synergy" with the VLA
or VLBA can be obtained only with antennas in the Southwestern
U.S.
Ultimately, to study how the properties of astronomical sources change with frequency without changing resolution, we should be able to select antenna combinations to create the appropriate set of matched spatial filters for any given multi-frequency experiment. We could do this at high angular resolution if we were able to choose ``sub-arrays" that include VLA and VLBA baselines almost interchangeably. The NRAO prepared for this by siting the VLBA antennas around the VLA as their centroid, and by co-locating the main VLA and VLBA operations centers. The VLA Expansion is needed to exploit the unique opportunity that this creates. It gives us a way to transform the scientific capabilities of two major instruments significantly at once.
To take full advantage of this, we should also plan to outfit the new antennas and a few existing VLA antennas with VLBA back-ends. The new antennas would be operated as part of the VLBA for much of the time that the VLA is in its more compact configurations and would need VLBA data acquisition systems. It is also desirable to have enough hardware at the VLA site itself to record signals from 4 VLA antennas at once. An upgrade of recording and formatter systems to reach 1 or 2 Gbps in a mode compatible with Mark IV is also desirable. Astrometric VLBI observations would benefit from a 2.4 GHz/8.4 GHz (S/X) dual frequency system.
This Science Workshop was directed only toward examining the astronomical motives for operating the new antennas as the VLA`s A+ configuration. A further working group is now being set up to explore the impact of the VLA Extension on science with the VLBA.
