The LEGA-C and SAMI Galaxy Surveys: Quiescent Stellar Populations and the Mass-Size Plane across 6 Gyr

TL;DR

This study compares stellar population properties of quiescent galaxies across redshifts, revealing that metallicity correlates with gravitational potential while age does not, due to galaxy compactness evolution over 6 billion years.

Contribution

It demonstrates the evolution of the age--surface density relation and links metallicity retention to gravitational potential, across a 6 Gyr redshift span.

Findings

Metallicity correlates with gravitational potential at both redshifts.

Age does not correlate with surface density at intermediate redshift.

Galaxies form more compactly at higher redshifts and quench at decreasing surface densities.

Abstract

We investigate the change in mean stellar population age and metallicity ([Z/H]) scaling relations for quiescent galaxies from intermediate redshift ($0.60\leq z\leq0.76$) using the LEGA-C Survey, to low redshift ($0.014\leq z\leq0.10$) using the SAMI Galaxy Survey. We find that, similarly to their low-redshift counterparts, the stellar metallicity of quiescent galaxies at $0.60\leq z\leq 0.76$ closely correlates with $M_*/R_\mathrm{e}$ (a proxy for the gravitational potential or escape velocity), in that galaxies with deeper potential wells are more metal-rich. This supports the hypothesis that the relation arises due to the gravitational potential regulating the retention of metals, by determining the escape velocity required by metal-rich stellar and supernova ejecta to escape the system and avoid being recycled into later stellar generations. On the other hand, we find no correlation between stellar age and $M_*/R_\mathrm{e}^2$ (stellar mass surface density $\Sigma$) in the LEGA-C sample, despite this being a strong relation at low redshift. We consider this change in the age--$\Sigma$ relation in the context of the redshift evolution of the star-forming and quiescent populations in the mass--size plane, and find our results can be explained as a consequence of galaxies forming more compactly at higher redshifts, and remaining compact throughout their evolution. Furthermore, galaxies appear to quench at a characteristic surface density that decreases with decreasing redshift. The $z\sim 0$ age--$\Sigma$ relation is therefore a result of building up the quiescent and star-forming populations with galaxies that formed at a range of redshifts and so a range of surface densities.