The Cool ISM in Elliptical Galaxies. II. Gas Content in the Volume - Limited Sample and Results from the Combined Elliptical and Lenticular Surveys

TL;DR

This study investigates the presence and origins of atomic and molecular gas in elliptical and lenticular galaxies, revealing differences in gas detection rates, origins, and a surprising bimodal distribution of gas ratios.

Contribution

It provides new observations of gas content in ellipticals, compares them with lenticulars, and uncovers a bimodal distribution of atomic to molecular gas ratios in early-type galaxies.

Findings

Detection rates are higher in luminous S0 galaxies.

Molecular gas likely has different origins in ellipticals and lenticulars.

Galaxies cluster around specific molecular-to-atomic gas ratios.

Abstract

We report new observations of atomic and molecular gas in a volume limited sample of elliptical galaxies. Combining the elliptical sample with an earlier and similar lenticular one, we show that cool gas detection rates are very similar among low luminosity E and SO galaxies but are much higher among luminous S0s. Using the combined sample we revisit the correlation between cool gas mass and blue luminosity which emerged from our lenticular survey, finding strong support for previous claims that the molecular gas in ellipticals and lenticulars has different origins. Unexpectedly, however, and contrary to earlier claims, the same is not true for atomic gas. We speculate that both the AGN feedback and merger paradigms might offer explanations for differences in detection rates, and might also point towards an understanding of why the two gas phases could follow different evolutionary paths in Es and S0s. Finally we present a new and puzzling discovery concerning the global mix of atomic and molecular gas in early type galaxies. Atomic gas comprises a greater fraction of the cool ISM in more gas rich galaxies, a trend which can be plausibly explained. The puzzle is that galaxies tend to cluster around molecular-to-atomic gas mass ratios near either 0.05 or 0.5.