Evolution of Quiescent and Star-Forming Galaxies Since z~1.5 as a Function of Their Velocity Dispersions

TL;DR

This study investigates how the velocity dispersion function of quiescent and star-forming galaxies evolves since z~1.5, revealing a stable high-dispersion population and a significant increase in low-dispersion quiescent galaxies over time.

Contribution

It provides the first detailed analysis of the evolution of the velocity dispersion function for both galaxy populations since z~1.5, highlighting the growth of low-dispersion quiescent galaxies.

Findings

High velocity dispersions remain stable over time.

The VDF for star-forming galaxies is constant across redshifts.

Number density of low-dispersion quiescent galaxies increases by a factor of ~4.

Abstract

We measure stellar masses and structural parameters for 5,500 quiescent and 20,000 star-forming galaxies at 0.3<z\leq1.5 in the Newfirm Medium Band Survey COSMOS and UKIDSS UDS fields. We combine these measurements to infer velocity dispersions and determine how the number density of galaxies at fixed inferred dispersion, or the Velocity Dispersion Function (VDF), evolves with time for each population. We show that the number of galaxies with high velocity dispersions appears to be surprisingly stable with time, regardless of their star formation history. Furthermore, the overall VDF for star-forming galaxies is constant with redshift, extending down to the lowest velocity dispersions probed by this study. The only galaxy population showing strong evolution are quiescent galaxies with low inferred dispersions, whose number density increases by a factor of ~4 since z=1.5. This build-up leads to an evolution in the quiescent fraction of galaxies such that the threshold dispersion above which quiescent galaxies dominate the counts moves to lower velocity dispersion with time. We show that our results are qualitatively consistent with a simple model in which star-forming galaxies quench and are added to the quiescent population. In order to compensate for the migration into the quiescent population, the velocity dispersions of star-forming galaxies must increase, with a rate that increases with dispersion.