Circumgalactic Gas and the Precipitation Limit

TL;DR

This paper discusses how the circumgalactic medium's structure and role in galaxy evolution are governed by a precipitation limit, where feedback processes regulate the pressure and condensation of cold clouds.

Contribution

It proposes a simple, emergent principle that the CGM pressure is capped at the threshold for cold cloud precipitation, influencing galaxy evolution models.

Findings

CGM pressure limits cold cloud condensation

Feedback mechanisms regulate CGM pressure

Precipitation threshold influences galaxy evolution

Abstract

During the last decade, numerous and varied observations, along with increasingly sophisticated numerical simulations, have awakened astronomers to the central role the circumgalactic medium (CGM) plays in regulating galaxy evolution. It contains the majority of the baryonic matter associated with a galaxy, along with most of the metals, and must continually replenish the star forming gas in galaxies that continue to sustain star formation. And while the CGM is complex, containing gas ranging over orders of magnitude in temperature and density, a simple emergent property may be governing its structure and role. Observations increasingly suggest that the ambient CGM pressure cannot exceed the limit at which cold clouds start to condense out and precipitate toward the center of the potential well. If feedback fueled by those clouds then heats the CGM and causes it to expand, the pressure will drop and the "rain" will diminish. Such a feedback loop tends to suspend the CGM at the threshold pressure for precipitation. The coming decade will offer many opportunities to test this potentially fundamental principle of galaxy evolution.