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

TL;DR

This paper compiles and analyzes high-J CO and 13CO measurements in galaxy centers, revealing distinct gas conditions and distributions, and providing new data to better understand molecular gas properties in bright nearby galaxies.

Contribution

It presents new APEX observations of 13CO(6-5) and 12CO(6-5), and analyzes their distribution and physical conditions in galaxy centers, advancing understanding of molecular gas phases.

Findings

High-J CO emission is concentrated in galaxy centers.

Most galaxies have dense, cool high-pressure gas dominating high-J emission.

Active star formation correlates with significant high-pressure gas in low-J emission.

Abstract

We summarize all available 13CO and accompanying 12CO measurements of local galaxy centers in transitions of J=5-4 and higher, including new APEX 13CO(6-5) and 12CO(6-5) observations of 11 galaxies. The observed integrated temperature ratios of 12CO(6-5) to 12CO(1-0) range from 0.10 to 0.45. Multi-aperture data indicate that 13CO(6-5) is more centrally concentrated than 12CO(6-5). The emission of 12CO(6-5) and HCO+ but not HCN may be correlated. The new data are essential to constrain the physical properties of the galaxy center molecular gas even in a simple two-phase model approximating the more complex multi-phase structure. In all galaxies, except the Seyfert AGN galaxy NGC 1068, high J emission from the center is dominated by a dense (n = 100 000) and relatively cool (T = 20 to 60 K) high-pressure gas. In contrast, the low-J lines are dominated in most galaxies by low-pressure gas of a moderate density (n = 1000) and more elevated temperature (T = 60 to 150 K). The three exceptions with significant high-pressure gas contributions to the low J emission are all associated with active central star formation.