A CO(1-0) AND CO(2-1) SURVEY OF NEARBY SPIRAL GALAXIES .2. PHYSICAL CONDITIONS IN THE NUCLEAR GAS

We present the analysis of the CO(1-0) and CO(2-1) survey of nearby galaxies carried out at the IRAM 30m telescope (Braine et al. 1992). The average CO(2-1) to CO(1-0) line ratio is 0.89+/-0.06 in galactic nuclei (inner kpc), and varies only between 0.82 for isolated to 1.11 for perturbed galaxies, with a large overlap. Since the excitation (as measured by the line ratio) of the gas is not highly different in isolated and perturbed galaxies, we assume that the standard NH2/WCO conversion ratio is valid for all galaxies (except for NGC 3310, where the line ratio is 2.6). Our data therefore indicate that disturbed galaxies have more H-2 gas in the central almost-equal-to kpc than galaxies without signs of perturbation. The observed line ratio of 0.9 is compatible with optically thick gas being the dominant component of the nuclear interstellar medium. Only in one galaxy, NGC 3310, do the physical conditions in the molecular gas appear to be different. As the sample contains all galaxies with B(T) < 12 and with distance and positional criteria only, this result illustrates the rarity of such objects. We show that even high line ratios (>1) do not necessarily imply different NH2/WCO conversion ratios. The differences in the CO emission due to galactic interactions are more pronounced than those due to morphological type, presence of bars, or nuclear activity.