A new scaling relation for HII regions in spiral galaxies: unveiling the true nature of the mass-metallicity relation

TL;DR

This paper uncovers a local relation between galaxy surface mass density, metallicity, and star-formation rate density, revealing that the global mass-metallicity relation in galaxies is a scaled-up version of local processes, supporting inside-out galaxy growth.

Contribution

It introduces a new local relation among galaxy properties that explains the global mass-metallicity relation as an integrated effect, advancing understanding of galaxy evolution.

Findings

The local relation extends over three orders of magnitude in mass density.

The global mass-metallicity relation can be reproduced from the local relation.

Supports inside-out growth and downsizing scenarios in galaxy evolution.

Abstract

We demonstrate the existence of a -local- relation between galaxy surface mass density, gas metallicity, and star-formation rate density using spatially-resolved optical spectroscopy of HII regions in the local Universe. One of the projections of this distribution, -the local mass-metallicity relation- extends over three orders of magnitude in galaxy mass density and a factor of eight in gas metallicity. We explain the new relation as the combined effect of the differential radial distributions of mass and metallicity in the discs of galaxies, and a selective star-formation efficiency. We use this local relation to reproduce -with remarkable agreement- the total mass-metallicity relation seen in galaxies, and conclude that the latter is a scale-up integrated effect of a local relation, supporting the inside-out growth and downsizing scenarios of galaxy evolution.