Molecular gas as the driver of fundamental galactic relations

TL;DR

This study demonstrates that molecular gas content is a primary driver of the fundamental metallicity relation in galaxies, surpassing star formation rate in influence, across a broad redshift range.

Contribution

It provides the first large-scale analysis showing molecular gas mass as the key secondary parameter in the mass-metallicity relation, using a diverse galaxy sample from multiple surveys.

Findings

Metallicity offset negatively correlates with molecular and total gas mass.

Secondary dependence on gas mass is stronger than on star formation rate.

The fundamental metallicity relation involves stellar mass, metallicity, and gas mass.

Abstract

There has been much recent work dedicated to exploring secondary correlations in the mass-metallicity relation, with significant dependence on both the star formation rate and HI content being demonstrated. Previously, a paucity of molecular gas data (combined with sample selection bias) hampered the investigation of any such relation with molecular gas content. In this work, we assemble a sample of 221 galaxies from a variety of surveys in the redshift range 0 < z < 2, to explore the connection between molecular gas content and metallicity. We explore the effect of gas mass on the mass-metallicity relation, finding that the offset from the relation is negatively correlated against both molecular and total gas mass. We then employ a principle component analysis technique to explore secondary dependences in the mass-metallicity relation, finding that the secondary dependence with gas mass is significantly stronger than with star formation rate, and as such the underlying `Fundamental Metallicity Relation' is between stellar mass, metallicity, and gas mass. In particular, the metallicity dependence on SFR is simply a byproduct of the dependence on the molecular gas content, via the Schmidt-Kennicutt relation. Finally, we note that our principle component analysis finds essentially no connection between gas-phase metallicity and the efficiency of star formation.