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

Abstract

This paper summarizes all presently available Jupp ≥ 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.