Not Dead Yet: Cool Circumgalactic Gas in the Halos of Early Type Galaxies

TL;DR

This study reveals that early type galaxies host substantial amounts of cool, photoionized circumgalactic gas, comparable to star-forming galaxies, challenging previous assumptions about galaxy quenching and gas depletion.

Contribution

It provides new observational evidence that early type galaxies retain significant cool circumgalactic gas, contrary to theoretical expectations of gas loss during quenching.

Findings

HI detection rates are similar in early type and star-forming galaxies.

The cool CGM gas has masses up to 10^11 solar masses.

Strong halo HI absorbers persist in passive galaxies.

Abstract

We report new observations of circumgalactic gas in the halos of early type galaxies obtained by the COS-Halos Survey with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope. We find that detections of HI surrounding early type galaxies are typically as common and strong as around star-forming galaxies, implying that the total mass of circumgalactic material is comparable in the two populations. For early type galaxies, the covering fraction for HI absorption above 10^16 cm^2 is ~40-50% within ~150 kpc. Line widths and kinematics of the detected material show it to be cold (T ~< 10^5 K) in comparison to the virial temperature of the host halos. The implied masses of cool, photoionized CGM baryons may be up to 10^9 --- 10^11 Msun. Contrary to some theoretical expectations, strong halo HI absorbers do not disappear as part of the quenching of star-formation. Even passive galaxies retain significant reservoirs of halo baryons which could replenish the interstellar gas reservoir and eventually form stars. This halo gas may feed the diffuse and molecular gas that is frequently observed inside ETGs.