Molecular gas in starburst galaxies: line intensities and physical conditions

In a study of molecular gas in IR-bright galaxies, we have observed the central position in ^12^CO J=1-0, J=2-1; ^13^CO J=1-0, J=2-1; C^18^O J=1-0 and HCN J=1-0 line emission. The sample (in total 32 galaxies) consists of starburst galaxies, interacting galaxies and two quiescent systems. We find a mean ^12^CO 2-1/1-0 line ratio of 0.93+/-0.22, and a mean ^13^CO 2-1/1-0 line ratio of 1.3+/-0.66. The mean ^12^CO/^13^CO 1-0 ratio (=R(1-0)) is =~13+/-6 and for ^12^CO/^13^CO 2-1 it is =~13+/-5. The mean ^12^CO/HCN 1-0 ratio is =~16. The ^12^CO/C^18^O 1-0 intensity ratio ranges from 20 to 140 in 6 galaxies. These values of ratios refer to central positions corrected for effects of beam-size and source-size. A considerable fraction of the ^13^CO emission may be saturated and originate in dense cores of smaller volume filling factor than the surrounding ^12^CO-emitting gas. High gas temperatures and turbulence work against large optical depths in the ^12^CO 1-0 line of the envelope gas. In contrast to what is usually assumed for the ^12^CO 1-0 line, we infer moderate optical depths, τ=~1, for the dominant ^12^CO-emitting structure. We have found that galaxies with large intensity ratios of [C II] 158 μm to ^12^CO 1-0 also have ^12^CO 2-1/1-0 ratios >=0.8, implying that the ^12^CO-emitting gas may be both dense and hot enough to excite the [C II] 158 μm line, a characteristic of warm (T_k_ > 100K), photon-dominated regions. We have not found a correlation between the intensity ratio, I(CO)/I(HCN), and disturbed morphology, far-infrared emission, or measures of star-forming activity in our sample of galaxies, in contrast to the findings of Solomon et al. (1992). The ^12^CO/HCN intensity ratio is quite uniform over a sample of 11 interacting galaxies and mergers. HCN is not detected in the one isolated spiral in the sample. Furthermore, we have observed R(1-0) in off-centre positions in 6 galaxies: NGC 1808, NGC 3256, NGC 4038/39, NGC 5055, NGC 6221 and NGC 7552. Offset positions in the ^12^CO/^13^CO 2-1 intensity ratio have been observed in 3 galaxies: NGC 660, NGC 2146 and Arp 299. We suggest that galaxies that display significant variation in the ratio, such as the merger NGC 3256, have at least two populations of molecular cloud ensembles: high-pressure clouds in the centre and an extended disk-component of more quiescent clouds. The molecular gas in the centres of luminous mergers with large gas surface densities (>10^4^M⊙/pc^2^) will be highly turbulent. In general, we suggest that R(1-0) is a measure of the cloud environment: the extreme values R(1-0)>20 originate in turbulent, high-pressure gas in the centres of luminous mergers; intermediate ratios 10<~R(1-0)<~15 originate in the inner kpc of more normal starburst galaxies; small ratios R(1-0)=~6 are a signature of a disk population of clouds. We address the notion that abundance anomalies cause the elevated values of R(1-0) in luminous merging galaxies. To some extent, this notion rests on the assumption of τ>>1 in the ^12^CO 1-0 line. We demonstrate how this assumption may be flawed and conclude that in order to measure abundances in the molecular medium, one must carefully model the physical properties of the molecular gas. Although abundances may be unusual in extreme mergers, they are only one aspect of the properties of the molecular gas in these regions.