The Atlas3D project - XIV. The extent and kinematics of molecular gas in early-type galaxies

TL;DR

This study compares molecular gas properties in early-type and spiral galaxies, revealing similarities in scaled sizes, the impact of environment, and the kinematic behavior of molecular gas, with implications for galaxy dynamics and evolution.

Contribution

It provides a detailed analysis of molecular gas extent, surface brightness, and kinematics in early-type galaxies, highlighting the effects of environment and internal structures, and validating molecular gas as a tracer of galaxy dynamics.

Findings

Molecular gas extent is smaller in ETGs than in late-type galaxies in absolute terms.

Approximately half of ETGs have molecular gas profiles following stellar light.

Molecular gas in most ETGs is relaxed, cold, and extends beyond the circular velocity turnover.

Abstract

We use interferometric CO observations to compare the extent, surface brightness profiles and kinematics of the molecular gas in CO-rich Atlas3D early-type galaxies (ETGs) and spiral galaxies. We find that the molecular gas extent is smaller in absolute terms in ETGs than in late-type galaxies, but that the size distributions are similar once scaled by the galaxies optical/stellar characteristic scale-lengths. Virgo cluster ETGs have less extended molecular gas reservoirs than field counterparts. Approximately half of ETGs have molecular gas surface brightness profiles that follow the stellar light profile. These systems often have relaxed gas out to large radii, suggesting they are unlikely to have had recent merger/accretion events. A third of the sample galaxies show molecular gas surface brightness profiles that fall off slower than the light, and sometimes show a truncation. We suggest that ram pressure stripping and/or the presence of hot gas has compressed/truncated the gas in these systems. The remaining galaxies have rings, or composite profiles, that we argue can be caused by the effects of bars. We investigated the kinematics of the molecular gas using position-velocity diagrams, and compared the observed kinematics with dynamical model predictions, and the observed stellar and ionised gas velocities. We confirm that the molecular gas reaches beyond the turnover of the circular velocity curve in ~70% of our CO-rich Atlas3D ETGs. In general we find that in most galaxies the molecular gas is relaxed and dynamically cold. Molecular gas is a better direct tracer of the circular velocity than the ionised gas, justifying its use as a kinematic tracer for Tully-Fisher and similar analyses. (abridged)