1994A&A...290..259G

The transitions CS J=2-1 and J=3-2 have been observed in absorption towards background continuum sources in Sgr B2 and W49. Absorption profiles previously detected from foreground spiral arms have now been resolved into up to five narrow features per arm. Only weak features are seen at inter-arm velocities. A comparison of the spectra observed along two closely-spaced lines of sight suggests the absorbing gas is clumped on scales of a few pc, although associated with much larger diffuse clouds seen in emission in [13CO. To reproduce the observed number of features, the absorbing regions must be closely spaced, suggesting they may represent internal structure within the larger clouds, rather than discrete objects. Excitation temperatures were found to be very close to the 2.73K of the cosmic microwave background radiation, with an average upper limit of 4K. CS column densities are (1-8)x10¹³cm^–2 per foreground location, and radiative transfer modelling indicates upper limits to the gas density of 10⁴cm^–3, and 10^2–3 cm^–3 for the strongest lines. The low density and excitation conditions suggest that there is no star-formation in these clouds.