Galaxy metallicities depend primarily on stellar mass and molecular gas mass

TL;DR

This study shows that galaxy metallicities are primarily determined by stellar mass and molecular gas mass, with little direct dependence on star formation rate, revealing an underlying equilibrium in galaxy evolution.

Contribution

It demonstrates that galaxy metallicity depends mainly on stellar and molecular gas mass, clarifying the physical basis of the fundamental metallicity relation.

Findings

Galaxies lie on a 2D plane in 4D parameter space.

Metallicity depends only on stellar and molecular gas mass.

Star formation rate's effect on metallicity is indirect, via molecular gas content.

Abstract

In this work we present an analysis of the behaviour of galaxies in a four-dimensional parameter space defined by stellar mass, metallicity, star formation rate, and molecular gas mass. We analyse a combined sample of 227 galaxies, which draws from a number of surveys across the redshift range 0 < z < 2 (> 90% of the sample at z~0), and covers > 3 decades in stellar mass.Using Principle Component Analysis, we demonstrate that galaxies in our sample lie on a 2-dimensional plane within this 4D parameter space, indicative of galaxies that exist in an equilibrium between gas inflow and outflow. Furthermore, we find that the metallicity of galaxies depends only on stellar mass and molecular gas mass. In other words, gas-phase metallicity has a negligible dependence on star formation rate, once the correlated effect of molecular gas content is accounted for. The well-known `fundamental metallicity relation', which describes a close and tight relationship between metallicity and SFR (at fixed stellar mass) is therefore entirely a by-product of the underlying physical relationship with molecular gas mass (via the Schmidt-Kennicutt relation).