Gas accretion regulates the scatter of the mass-metallicity relation

TL;DR

This study uses the GAEA semi-analytic model to show that cold gas content, driven by gas accretion fluctuations, significantly influences the scatter in the galaxy mass-metallicity relation, aligning with recent observations.

Contribution

It demonstrates that cold gas content is the third key parameter affecting the mass-metallicity relation scatter, explained by gas accretion rate fluctuations, a novel insight supported by the GAEA model.

Findings

Cold gas content influences the mass-metallicity relation scatter.

Gas accretion fluctuations cause metallicity offsets.

Late gas cooling affects low and intermediate mass galaxies.

Abstract

In this paper, we take advantage of the GAlaxy Evolution and Assembly (GAEA) semi-analytic model to analyse the origin of secondary dependencies in the local galaxy mass - gas metallicity relation. Our model reproduces quite well the trends observed in the local Universe as a function of galaxy star formation rate and different gas-mass phases. We show that the cold gas content (whose largest fraction is represented by the atomic gas phase) can be considered as the third parameter governing the scatter of the predicted mass-metallicity relation, in agreement with the most recent observational measurements. The trends can be explained with fluctuations of the gas accretion rates: a decrease of the gas supply leads to an increase of the gas metallicity due to star formation, while an increase of the available cold gas leads to a metallicity depletion. We demonstrate that the former process is responsible for offsets above the mass-metallicity relation, while the latter is responsible for deviations below the mass-metallicity relation. In low and intermediate mass galaxies, these negative offsets are primarily determined by late gas cooling dominated by material that has been previously ejected due to stellar feedback.