Gravitational Potential and Surface Density Drive Stellar Populations -- II. Star-Forming Galaxies

TL;DR

This study investigates how stellar population properties in star-forming galaxies relate to structural parameters, finding that age correlates with surface mass density and metallicity with gravitational potential, suggesting causal links.

Contribution

It extends previous work on early-type galaxies to star-forming galaxies, identifying the primary structural regulators of age and metallicity in these systems.

Findings

Age correlates best with stellar surface mass density.

Metallicity [Z/H] correlates best with gravitational potential.

Gravitational potential influences metallicity via gas escape velocity.

Abstract

Stellar population parameters correlate with a range of galaxy properties, but it is unclear which relations are causal and which are the result of another underlying trend. In this series, we quantitatively compare trends between stellar population properties and galaxy structural parameters in order to determine which relations are intrinsically tighter, and are therefore more likely to reflect a causal relation. Specifically, we focus on the galaxy structural parameters of mass $M$, gravitational potential $\Phi\sim M/R_e$, and surface mass density $\Sigma\sim M/R_e^2$. In Barone et al. (2018) we found that for early-type galaxies the age-$\Sigma$ and [Z/H]-$\Phi$ relations show the least intrinsic scatter as well as the least residual trend with galaxy size. In this work we study the ages and metallicities measured from full spectral fitting of 2085 star-forming galaxies from the SDSS Legacy Survey, selected so all galaxies in the sample are probed to one effective radius. As with the trends found in early-type galaxies, we find that in star-forming galaxies age correlates best with stellar surface mass density, and [Z/H] correlates best with gravitational potential. We discuss multiple mechanisms that could lead to these scaling relations. For the [Z/H]--$\Phi$ relation we conclude that gravitational potential is the primary regulator of metallicity, via its relation to the gas escape velocity. The age--$\Sigma$ relation is consistent with compact galaxies forming earlier, as higher gas fractions in the early universe cause old galaxies to form more compactly during their in-situ formation phase, and may be reinforced by compactness-related quenching mechanisms.