A Comparison of13CO and CS Emission in the Inner Galaxy

Abstract

We analyze 2 deg2 of the Galactic plane surveyed in CS J = 2 → 1 and 13CO J = 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 13CO 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 13CO clumps with emission above an integrated intensity of 15 K km s-1 (greater than 37 σ). The fraction of those infrared point sources detected as bright CS clumps above 1 K km s-1 (greater than 3 σ) is only 35%. The CS/13CO intensity ratio can be used as a measure of gas excitation conditions. We compared the observed CS and 13CO line intensities of the entire 2 deg2 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) 13CO flux threshold, is Tmb(CS)/Tmb(13CO) = 0.17 ± 0.06, with a peak value of ~0.5 toward two of the CS emission maxima. The ratio for the same cloud calculated using all 13CO positions with flux above 3 σ 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 deg2 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.