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, HCN, HCO) 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(10) 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 ).