The physics of the fundamental metallicity relation

TL;DR

This paper introduces a simple, physics-based model explaining the local Fundamental Metallicity Relation across galaxy masses, accounting for star formation, inflows, and outflows, and successfully matches observed gas metallicity and gas fraction relations.

Contribution

The paper presents a novel, parameter-free analytic model that explains the FMR by incorporating key physical processes and matches multiple observational relations.

Findings

The model explains the constant metallicity in massive galaxies due to inflow-outflow balance.

In less massive galaxies, metallicity decreases with SFR due to efficient outflows.

The model successfully reproduces observed gas fraction and metallicity relations.

Abstract

We present a simple, redshift-independent analytic model that explains the local Fundamental Metallicity Relation (FMR), taking into account the physical processes of star formation, inflow of metal-poor intergalactic medium (IGM) gas, and the outflow of metal rich interstellar medium (ISM) gas. We show that the physics of the FMR can be summarised as follows: for massive galaxies with stellar mass M_* >= 10^11 solar masses, ISM metal enrichment due to star formation is compensated by inflow of metal poor IGM gas, leading to a constant value of the gas metallicity with star formation rate (SFR); outflows are rendered negligible as a result of the large potential wells of these galaxies. On the other hand, as a result of their smaller SFR, less massive galaxies produce less heavy elements that are also more efficiently ejected due to their shallow potential wells; as a result, for a given M_*, the gas metallicity decreases with SFR. For such galaxies, the outflow efficiency determines both the slope, and the knee of the metallicity-SFR relation. Without changing any parameters, this simple model is also successfully matched to the gas fraction - gas metallicity relation observed for a sample of about 260 nearby galaxies.