An indirect measurement of gas evolution in galaxies at $0.5\leq z \leq 2.0$

TL;DR

This paper introduces an indirect method to estimate cold gas content in high-redshift galaxies using star formation rates, revealing how gas fractions evolve with stellar mass and redshift, supporting galaxy downsizing.

Contribution

The study develops a novel indirect approach to measure galaxy gas content and applies it to a large galaxy sample, providing new insights into gas evolution over cosmic time.

Findings

Gas fractions decrease with increasing stellar mass and redshift.

Massive galaxies deplete their gas earlier than less massive ones.

The results support the downsizing scenario in galaxy evolution.

Abstract

One key piece of information missing from high redshift galaxy surveys is the galaxies' cold gas contents. We present a new method to indirectly determine cold gas surface densities and integrated gas masses from galaxy star formation rates and to separate the atomic and molecular gas components. Our predicted molecular and total gas surface densities and integrated masses are in very good agreement with direct measurements quoted in the literature for low and high-z galaxies. We apply this method to predict the gas content for a sample of $\sim 57000$ galaxies in the COSMOS field at $0.5 \leq z \leq 2.0$, selected to have $I_{AB} < 24$ mag. This approach allows us to investigate in detail the redshift evolution of galaxy cold and molecular gas content versus stellar mass and to provide fitting formulae for galaxy gas fractions. We find a clear trend between galaxy gas fraction, molecular gas fraction and stellar mass with redshift, suggesting that massive galaxies consume and/or expel their gas at higher redshift than less massive objects and have lower fractions of their gas in molecular form. The characteristic stellar mass separating gas- from stellar-dominated galaxies decreases with time. This indicates that massive galaxies reach a gas-poor state earlier than less massive objects. These trends can be considered to be another manifestation of downsizing in star formation activity.