An Introduction to Gas Accretion onto Galaxies

TL;DR

This paper reviews evidence and sources of gas accretion onto galaxies, discussing observational signatures, primary inflow sources, and the complexities of how accreted gas fuels star formation.

Contribution

It provides a comprehensive summary of observational and simulation-based evidence for gas accretion and discusses the primary sources and unresolved processes involved.

Findings

Gas accretion influences galaxy star formation and metallicity.

Multiple sources contribute to gas inflow, including intergalactic medium and satellite gas.

The process of gas settling into stars remains poorly understood.

Abstract

Evidence for gas accretion onto galaxies can be found throughout the universe. In this chapter, I summarize the direct and indirect signatures of this process and discuss the primary sources. The evidence for gas accretion includes the star formation rates and metallicities of galaxies, the evolution of the cold gas content of the universe with time, numerous indirect indicators for individual galaxies, and a few direct detections of inflow. The primary sources of gas accretion are the intergalactic medium, satellite gas and feedback material. There is support for each of these sources from observations and simulations, but the methods with which the fuel ultimately settles in to form stars remain murky.