Physical Conditions of the Molecular Gas in Seyfert Galaxies

We examine the physical conditions of the global molecular gas reservoir in the host galaxies of Seyfert 1 and Seyfert 2 nuclei. To do so, we acquired sensitive 12 CO, 13 CO, J = 1-0, and J = 2-1 observations and collected available data from the literature for a sample of 27 Seyfert galaxies. We find that Seyfert galaxies have an average value for the 12 CO/ 13 CO J = 1-0 and J = 2-1 line ratios of ⟨ R 10 ⟩ = 12 and ⟨ R 21 ⟩ = 13, respectively, with no discernible dependence on the Seyfert type. The r 12 = (2-1)/(1-0) line ratio for 12 CO does not reveal any significant difference between the two types, but Seyfert galaxies as a class seem to have systematically lower values of r 12 (~0.5-0.7) than do average spirals and starbursts. Moreover, for all the galaxies examined, but especially for Seyfert and starburst galaxies, we find that r 12 is likely to be smaller as the area of the galaxy sampled by the telescope beam becomes larger. This may be the consequence of a global gas excitation gradient in galaxies where warm ( T kin ≳ 20 K) gas lies confined, preferably in their central regions (≲1 kpc), while a colder ( T kin ≲ 10 K), and possibly subthermally excited, gas phase dominates the more extended CO emission in the disk. For Seyfert and starburst galaxies, there are indications that their gas excitation gradients may be quite similar. Examination of the properties of the molecular gas by using the r 12 , R 10 , and R 21 line ratios reveals that, unless r 12 ≲ 0.6, a single warm gas phase can account for the observed values of these ratios toward the central regions of an average Seyfert and/or starburst galaxy.