The age dependence of the size-stellar mass relation and some implications

TL;DR

This study reveals that older early-type galaxies are smaller and have higher velocity dispersions than younger ones of the same stellar mass, with implications for galaxy formation models and the evolution of the Fundamental Plane.

Contribution

It provides new empirical evidence on the age dependence of galaxy size and velocity dispersion, and discusses implications for galaxy formation and evolution models.

Findings

Older galaxies have 40% smaller sizes than younger ones at the same luminosity.

Older galaxies exhibit 25% higher velocity dispersions compared to younger counterparts.

The size and velocity dispersion variations are less than 15% for more luminous galaxies regardless of age.

Abstract

We use a sample of about 48,000 SDSS early-type galaxies to show that older galaxies have smaller half-light radii re and larger velocity dispersions sigma than younger ones of the same stellar mass Mstar. We use the age-corrected luminosity Lcorr as a proxy for Mstar to minimize biases: below Lcorr~1e11 Lsun, galaxies with age ~11 Gyrs have re smaller by 40% and sigma larger by 25%, compared to galaxies that are ~4 Gyrs younger. The sizes and velocity dispersions of more luminous galaxies vary by less than 15%, whatever their age, a challenge for current galaxy formation models. A closer check reveals that the lowering in the dispersion is caused by older galaxies that show a significant departure from the re--Lcorr and sigma--Lcorr relations at high Lcorr. Such features might find an explanation in models where more massive galaxies undergo more minor mergers than less massive galaxies at late times, thus causing a break in the homology. In terms of the Fundamental Plane of early-type galaxies, the data indicate that all galaxies show a significant and similar increase in the dynamical-to-stellar mass ratio with increasing mass, independent of their age. However, older galaxies have smaller Mdyn/Mstar ratios than objects which formed more recently. These findings may suggest that lower mass galaxies and, at fixed stellar mass, higher redshift galaxies, formed from gas-richer progenitors, thus underwent more dissipation and contraction.