On the depletion and accretion timescales of cold gas in local early-type galaxies

TL;DR

This paper investigates the timescales of gas accretion and depletion in local early-type galaxies by analyzing kinematic misalignments, revealing constraints on physical processes and suggesting rapid gas depletion or long relaxation timescales.

Contribution

It introduces a simple analytic model to constrain gas accretion, depletion, and relaxation timescales based on observed kinematic misalignments in early-type galaxies.

Findings

Lack of counter-rotating gas constrains relaxation and depletion timescales.

Fast gas depletion ($<10^8$ yr) or long relaxation timescales ($ oughly 1-5$ Gyr) can explain observations.

High merger rates are required to support rapid gas depletion or destruction.

Abstract

We consider what can be learnt about the processes of gas accretion and depletion from the kinematic misalignment between the cold/warm gas and stars in local early-type galaxies. Using simple analytic arguments and a toy model of the processes involved, we show that the lack of objects with counter-rotating gas reservoirs strongly constrains the relaxation, depletion and accretion timescales of gas in early-type galaxies. Standard values of the accretion rate, star formation efficiency and relaxation rate are not simultaneously consistent with the observed distribution of kinematic misalignments. To reproduce that distribution, both fast gas depletion ($t_{\rm dep} <10^8$ yr; e.g. more efficient star formation) and fast gas destruction (e.g. by active galactic nucleus feedback) can be invoked, but both also require a high rate of gas-rich mergers ($>1$ Gyr$^{-1}$). Alternatively, the relaxation of misaligned material could happen over very long timescales ($\simeq100$ dynamical times or $\approx1$-$5$ Gyr). We explore the various physical processes that could lead to fast gas depletion and/or slow gas relaxation, and discuss the prospects of using kinematic misalignments to probe gas-rich accretion processes in the era of large integral-field spectroscopic surveys.