Cosmic evolution of gas content and accretion

TL;DR

This paper reviews the cosmic evolution of gas content in galaxies, focusing on how gas phases and star formation efficiency change over time and comparing observations with simulations.

Contribution

It provides a comprehensive review of high-redshift molecular gas observations and their comparison to theoretical models of galaxy evolution.

Findings

Gas fractions were higher in earlier galaxies.

The balance between molecular and atomic gas phases varies with redshift.

Star formation efficiency may change over cosmic time.

Abstract

In the present universe, the gas is a minor component of giant galaxies, and its dominant phase is atomic (HI). During galaxy evolution in cosmic times, models predict that gas fractions were much higher in galaxies, and gas phases could be more balanced between molecular (H2) and atomic (HI). This gaseous evolution is certainly a key factor to explain the cosmic evolution of the star formation rate density. Star formation efficiency might also vary with redshift, and the relative importance of these factors is not yet well known. Our current knowledge of cosmic evolution of gas from molecular observations at high-z is reviewed and confronted to simulations.