Massive Compact Quiescent Galaxies in the $M_\star$ vs. $\sigma_\mathrm{e}$ Plane: Insights from stellar Population Properties

TL;DR

This study analyzes the stellar populations of 1858 massive compact galaxies, revealing they are old, alpha-enhanced, and metal-rich, with metallicity correlating with velocity dispersion, and differences from typical quiescent galaxies suggesting unique formation histories.

Contribution

It provides new insights into the stellar population properties of massive compact galaxies and compares them to typical quiescent galaxies, highlighting differences in metallicity, age, and potential dark matter content.

Findings

Massive compact galaxies are old, alpha-enhanced, and have solar to super-solar metallicities.

Metallicity increases with velocity dispersion, while age and alpha-enhancement do not vary significantly.

Compared to typical quiescent galaxies, MCGs have lower metallicities and, at fixed velocity dispersion, lower stellar masses.

Abstract

We investigated the stellar population properties of a sample of 1858 massive compact galaxies (MCGs) extracted from the SDSS survey. Motivated by previous results showing that older compact galaxies tend to have larger velocity dispersion at fixed stellar mass, we used the distance to the $\sigma_\mathrm{e}$ vs. $R_\mathrm{e}$ and $M_\star$ vs. $\sigma_\mathrm{e}$ relations as selection criteria. We found that MCGs are old ($\gtrsim 10$ Gyr), $\alpha$-enhanced ([$\alpha/\mathrm{Fe}] \sim 0.2$) and have solar to super-solar stellar metallicities. Metallicity increases with $\sigma_\mathrm{e}$, while age and [$\alpha$/Fe] do not vary significantly. Moreover, at fixed $\sigma_\mathrm{e}$, metallicity and stellar mass are correlated. Compared to a control sample of typical quiescent galaxies, MCGs have, on average, lower metallicities than control sample galaxies (CSGs) of similar $\sigma_\mathrm{e}$. For $\sigma_\mathrm{e} \lesssim 225$ km/s, MCGs are older and more $\alpha$-enhanced than CSGs, while for higher $\sigma_\mathrm{e}$ ages and $\alpha$-enhancement are similar. The differences in age and $\alpha$-enhancement can be explained by lower-$\sigma_\mathrm{e}$ CSGs being an amalgam of quiescent galaxies with a variety of ages. The origin of the differences in metallicity, however, is not clear. Lastly, we compared the stellar mass within the region probed by the SDSS fiber finding that, at fixed fiber velocity dispersion, MCGs have lower stellar masses on average. Since the velocity dispersion is a tracer of the dynamical mass, this raises the possibility that MCGs have, on average, a bottom heavier initial mass function or a larger dark matter fraction within the inner $\sim 1-2$ kpc.