Origin of the Galaxy Mass-Metallicity-Star-Formation Relation

TL;DR

This paper presents an equilibrium model linking galaxy metallicity to stellar evolution, using SDSS data to constrain models of gas infall, outflow, and star formation, revealing that extragalactic gas infall plays a key role in galaxy evolution.

Contribution

It introduces a comprehensive equilibrium model that connects galaxy metallicity, stellar mass, and star formation rates with gas inflow and outflow, constrained by extensive SDSS data.

Findings

Extragalactic infall supplies about half of the gas for star formation across various galaxy masses.

Star formation likely begins when infalling gas doubles the mass of marginally stable interstellar clouds.

The model accounts for galaxy metallicity and suggests a constant infall ratio across a wide mass range.

Abstract

We describe an equilibrium model that links the metallicity of low-redshift galaxies to stellar evolution models. It enables the testing of different stellar initial mass functions and metal yields against observed galaxy metallicities. We show that the metallicities of more than 80,000 Sloan Digital Sky Survey (SDSS) galaxies in the low-redshift range $0.07\leq z\leq 0.3$ considerably constrain stellar evolution models that simultaneously relate galaxy stellar mass, metallicity, and star formation rates (SFRs) to the infall rate of low-metallicity extragalactic gas and outflow of enriched matter. A feature of our model is that it encompasses both the active star forming phases of a galaxy and epochs during which the same galaxy may lie fallow. We show that the galaxy-mass-metallicity-star-formation relation can be traced to infall of extragalactic gas mixing with native gas from host galaxies to form stars of observed metallicities, the most massive of which eject oxygen into extragalactic space. Most consequential among our findings is that, on average, extragalactic infall accounts for one half of the gas required for star formation, a ratio that is remarkably constant across galaxies with stellar masses ranging at least from $M* = 2 \times 10^9$ to $6\times 10^{10} M_{\odot}$. This leads us to propose that star formation is initiated when extragalactic infall roughly doubles the mass of marginally stable interstellar clouds. The processes described may also account quantitatively for the metallicity of extragalactic space, though to check this the fraction of extragalactic baryons will need to be more firmly established.