2024A&A...691A.289I

This paper summarizes all presently available J_upp ≥ 5 ¹³CO and accompanying ¹²CO measurements of galaxy centers including new J = 6-5 ¹³CO and ¹²CO observations of eleven galaxies with the Atacama Pathfinder EXperiment (APEX) telescope and also Herschel high-J measurements of both species in five galaxies. The observed J = 6-5/J = 1-0 ¹²CO integrated temperature ratios range from 0.10 to 0.45 in matching beams. Multi-aperture data indicate that the emission of ¹³CO(6-5) is more centrally concentrated than that of ¹²CO(6-5). The intensities of ¹²CO(6-5) suggest a correlation with those of HCO⁺ but not with those of HCN. The new data are essential in refining and constraining the parameters of the observed galaxy center molecular gas in a simple two-phase model to approximate its complex multi-phase structure. In all galaxies except the Seyfert galaxy NGC 1068, high-J emission from the center is dominated by a dense (n ∼ 10⁵ cm^–3) and relatively cool (20-60 K) high-pressure gas. In contrast, the low-J lines are dominated by low-pressure gas of a moderate density (n ∼ 10³ cm^–3) and more elevated temperature (60-150 K) in most galaxies. The three exceptions with significant high-pressure gas contributions to the low-J emission are all associated with active central star formation.