A comparison of 13CO and CS emission in the inner galaxy

We analyze 2 deg(2) of the Galactic plane surveyed in CSJ = 2 --> 1 and (13)COJ = 1 --> 0 emission lines as a part of the Boston University Five College Radio Astronomy Observatory Milky Way Galactic Ring Survey. Since the critical density of the CS molecule is large, strong CS emission originates only in dense molecular cloud cores. Yet, because high volume density regions traced by CS also tend to have large column densities, we find that (CO)-C-13 is just as useful as, and much more efficient than, CS for identifying potential dense, star-forming cores. Sixty-five percent of the star-forming sites in the survey region, selected using color criteria for embedded IRAS point sources, are detected as bright (CO)-C-13 clumps with emission above an integrated intensity of 15 K km s(-1) ( greater than 37 sigma). The fraction of those infrared point sources detected as bright CS clumps above 1 K km s(-1) (greater than 3 sigma) is only 35%. The CS/(CO)-C-13 intensity ratio can be used as a measure of gas excitation conditions. We compared the observed CS and (CO)-C-13 line intensities of the entire 2 deg(2) field as well as the average line ratios from two molecular clouds with very different physical properties. The average intensity ratio for GRSMC 45.46+0.05, a high volume density, star-forming molecular cloud, calculated with a high (26 K km s(-1)) (CO)-C-13 flux threshold, is T-mb(CS)/T-mb((CO)-C-13) = 0.17 +/- 0.06, with a peak value of similar to0.5 toward two of the CS emission maxima. The ratio for the same cloud calculated using all (CO)-C-13 positions with flux above 3 sigma is 0.06 +/- 0.01. For GRSMC 45.60+0.30, a low-density, quiescent molecular cloud, this ratio is even lower, 0.03 +/- 0.01. The average line ratio for the entire 2 deg(2) field is 0 : 04 0 : 01, similar to the value for the low-density cloud. Although the CS lines are brightest toward star-forming cores, ubiquitous, low-level CS emission dominates the emission in the survey region. This emission probably originates from subthermally excited, low-density gas.