The coldest molecular gas in our galaxy

Extract

Galactic CO emission surveys such as those carried out by Gordon and Burton (Ap. J., 208, 346, 1976) and references cited therein are sensitive only to CO with J = 1 → 0 excitation temperature ≳ 5 k “Cold” molecular (H₂) gas with CO excitation temperature ≳ 5 k would go undetected in such surveys. Low T_ex could be due to either low kinetic temperature or low H₂ density. Evidence for the possible existence of substantial amounts of cold gas (T_ex ∼ 3 K) towards the galactic center is apparent in spectra of Liszt et al. (Ap. J. 213, 38, 1977 and 198, 537, 1975) in the form of narrow gaps, suggestive of self-absorption, at velocities of 0, −30 and −55 km/s with respect to the LSR.