A Large Population of Massive Compact Post-Starburst Galaxies at z>1: Implications for the Size Evolution and Quenching Mechanism of Quiescent Galaxies

TL;DR

This study reveals a significant population of massive, recently quenched post-starburst galaxies at z>1, providing insights into the size evolution and quenching mechanisms of quiescent galaxies over cosmic time.

Contribution

It identifies a large population of massive post-starburst galaxies at z>1 and challenges the idea that size growth is driven solely by transformations of larger star-forming galaxies.

Findings

Large population of post-starburst galaxies at z>1

Size evolution inconsistent with simple transformation models

Mergers likely contribute to size growth from z=2 to present

Abstract

We study the growth of the red sequence through the number density and structural evolution of a sample of young and old quiescent galaxies at 0<z<2. The galaxies are selected from the NEWFIRM Medium-Band Survey (NMBS) in the Cosmic Evolution Survey (COSMOS) field. We find a large population of massive young recently quenched ("post-starburst") galaxies at z>1 that are almost non-existent at z<1; their number density is 5 x 10^{-5} Mpc^{-3} at z=2, whereas it is a factor of 10 less at z=0.5. The observed number densities of young and old quiescent galaxies at z>1 are consistent with a simple model in which all old quiescent galaxies were once identified as post-starburst galaxies. We find that the overall population of quiescent galaxies have smaller sizes and slightly more elongated shapes at higher redshift, in agreement with other recent studies. Interestingly, the most recently quenched galaxies at 1<z<2 are not larger, and possibly even smaller, than older galaxies at those redshifts. This result is inconsistent with the idea that the evolution of the average size of quiescent galaxies is largely driven by continuous transformations of larger, star-forming galaxies: in that case, the youngest quiescent galaxies would also be the largest. Instead, mergers or other mechanisms appear to be required to explain the size growth of quiescent galaxies from z=2 to the present.