The Atlas3D Project -- XI. Dense molecular gas properties of CO-luminous early-type galaxies

TL;DR

This study investigates dense molecular gas properties in 18 CO-bright early-type galaxies from the ATLAS3D sample, revealing correlations between molecular line ratios and galaxy characteristics, and suggesting optical depth effects influence gas conditions.

Contribution

First comprehensive analysis of dense molecular gas in early-type galaxies, linking line ratios to galaxy properties and optical depth variations, expanding understanding beyond spiral galaxies.

Findings

Line ratios correlate with stellar mass, age, gas ratio, dust temperature.

Interacting and starbursting galaxies have more optically thin gas.

Some galaxies show very low 12CO/13CO ratios indicating stable, less efficient star-forming gas.

Abstract

Surveying eighteen 12CO-bright galaxies from the ATLAS3D early-type galaxy sample with the Institut de Radio Astronomie Millim\'etrique (IRAM) 30m telescope, we detect 13CO(1-0) and 13CO(2-1) in all eighteen galaxies, HCN(1-0) in 12/18 and HCO+(1-0) in 10/18. We find that the line ratios 12CO(1-0)/13CO(1-0) and 12CO(1-0)/HCN(1-0) are clearly correlated with several galaxy properties: total stellar mass, luminosity-weighted mean stellar age, molecular to atomic gas ratio, dust temperature and dust morphology. We suggest that these correlations are primarily governed by the optical depth in the 12CO lines; interacting, accreting and/or starbursting early-type galaxies have more optically thin molecular gas while those with settled dust and gas discs host optically thick molecular gas. The ranges of the integrated line intensity ratios generally overlap with those of spirals, although we note some outliers in the 12CO(1- 0)/13CO(1-0), 12CO(2-1)/13CO(2-1) and HCN/HCO+(1-0) ratios. In particular, three galaxies are found to have very low 12CO(1-0)/13CO(1-0) and 12CO(2-1)/13CO(2-1) ratios. Such low ratios may signal particularly stable molecular gas which creates stars less efficiently than 'normal' (i.e. below Schmidt-Kennicutt prediction), consistent with the low dust temperatures seen in these galaxies.