Surface density: a new parameter in the fundamental metallicity relation of star-forming galaxies

TL;DR

This paper introduces surface density of stellar mass as a new parameter that significantly tightens the fundamental metallicity relation in star-forming galaxies, reducing scatter and improving understanding of galaxy evolution.

Contribution

It identifies surface density as a key fourth parameter in the FMR, enhancing the relation's precision and suggesting its importance in galaxy evolution models.

Findings

Surface density reduces metallicity dispersion by 50%.

Surface density is the fourth most important parameter in galaxy properties.

A four-dimensional relation forms a tighter hypersurface than previous models.

Abstract

Star-forming galaxies display a close relation among stellar mass, metallicity and star-formation rate (or molecular-gas mass). This is known as the fundamental metallicity relation (FMR) (or molecular-gas FMR), and it has a profound implication on models of galaxy evolution. However, there still remains a significant residual scatter around the FMR. We show here that a fourth parameter, the surface density of stellar mass, reduces the dispersion around the molecular-gas FMR. In a principal component analysis of 29 physical parameters of 41,338 star-forming galaxies, the surface density of stellar mass is found to be the fourth most important parameter. The new four-dimensional fundamental relation forms a tighter hypersurface that reduces the metallicity dispersion to 50\% of that of the molecular-gas FMR. We suggest that future analyses and models of galaxy evolution should consider the FMR in a four-dimensional space that includes surface density. The dilution time scale of gas inflow and the star-formation efficiency could explain the observational dependence on surface density of stellar mass.