Central molecular zones in galaxies: ¹³CO(6–5) and molecular gas conditions in bright nearby galaxies

This paper summarizes all presently available J upp ≥ 5 13 CO and accompanying 12 CO measurements of galaxy centers including new J = 6–5 13 CO and 12 CO 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 12 CO integrated temperature ratios range from 0.10 to 0.45 in matching beams. Multi-aperture data indicate that the emission of 13 CO(6–5) is more centrally concentrated than that of 12 CO(6–5). The intensities of 12 CO(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 ∼ 10 5 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 ∼ 10 3 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.