Interferometric studies of CO have so far been limited to the lower
transitions of the molecule, tracing low-density gas. Other molecules
are better tracers of molecular gas temperature and density, and many
have been observed in external galaxies using single dishes. Some of
these molecules (CS, HC
N, H
CO) have been observed at the higher
transitions due to the limited range of frequencies attainable by the
millimeter telescopes; often if the lower transitions have been
observed, it was with low-sensitivity systems incapable of making
detections in external galaxies. In the enhanced VLA, several
observing bands (in particular the 22.5, 33, and 45 GHz bands) will be
sensitive enough to image several of the lower transitions of many
molecules in external galaxies. This will allow us to determine the
variations in physical conditions across galaxy disks.
CS(1
0) occurs at a frequency of 49 GHz, placing it in
Q Band; unfortunately other transitions fall outside the range of the
VLA receivers. The line is fairly strong; it should be useful in
probing molecular cloud dynamics. Besides Galactic detections, CS has
also been detected in external galaxies.
For NGC6946 and Maffei 2, peak main beam
temperatures were 
K for the CS(32) transition in
the nucleus; if the source is resolved, it would take about 13 hrs
with the enhanced VLA to make a 2
detection (assuming the same
peak temperature at the CS(10) transition) in the
D configuration. E configuration observations would of course go much
faster and still have resolution on the order of 
; this
configuration
could be used to mosaic larger regions of the galaxy disks.
Brighter CS(21)
peak temperatures have been found throughout M82, so integration times
would be
shorter for this galaxy.
Cyanoacetylene (HCN) is a molecule which gives information on
density distribution. Several transitions appear in the observing
bands of the VLA ranging from the J=10 transition at
9 GHz to the J=54 transition at 45 GHz. Higher
transitions of HCN have been observed in external galaxies;
a component
in the nearby spiral NGC253 emits its strongest radiation near
the J=54 line, which falls at 45 GHz. The peak main
beam temperatures is 
K for the HCN J=98
transition in the nucleus; if the source is fully resolved, it would
take about 6 hrs with the enhanced VLA to make a 3 detection
in HCN(54) the D configuration. For similar peak
temperatures in the upgraded 22.5 GHz band, a 3 detection in
HCN(21) would take 9 hrs.
Other molecules with transitions falling in the enhanced VLA band
include formaldehyde (HCO), methyl acetylene (CHCH), and methyl
cyanide (CHCN), each an excellent tracer of density and/or
temperature with at least three transitions falling in the 15. 22.5,
33, and 45 GHz bands. Sulfur monoxide (SO), which traces oxygen-rich
regions of clouds, also has several transitions which fall in the
enhanced VLA band.
These observations would not be possible without the sensitivity
proposed for the higher frequency bands in the
VLA Development Plan.
Also, at these frequencies we need large bandwidths to cover the
velocity range typically found in external galaxies (hundreds of
km/s); the larger channel widths of the new correlator are required to
ensure proper coverage. If flexibility is built into the correlator,
two or more of these lines can be observed simultaneously, allowing
quick determination of physical parameters across the galactic disk.
The E configuration will be needed to mosaic larger objects (since the
primary beam at 45 GHz is 
).