Evidence for (and Against) Progenitor Bias in the Size Growth of Compact Red Galaxies

TL;DR

This study investigates whether the observed size growth of compact red galaxies from high redshift to the present is due to actual growth or progenitor bias, using stellar population analysis from the DEEP2 survey.

Contribution

It provides evidence that progenitor bias influences perceived size evolution, especially when galaxies are selected based on light distribution smoothness.

Findings

Compact galaxies selected by color and bulge ratio are younger than larger counterparts.

Selection based on light smoothness shows older ages for compact galaxies, indicating progenitor bias.

Size growth is partly due to progenitor bias, but also involves genuine galaxy evolution.

Abstract

Most massive passive galaxies are compact at high redshifts, but similarly compact massive galaxies are rare in the local universe. The most common interpretation of this phenomenon is that massive galaxies have grown in size by a factor of about five since redshift z=2. An alternative explanation is that recently quenched massive galaxies are larger (a "progenitor bias"). In this paper we explore the importance of progenitor bias by looking for systematic differences in the stellar populations of compact early-type galaxies in the DEEP2 survey as a function of size. Our analysis is based on applying the statistical technique of bootstrap resampling to constrain differences in the median ages of our samples and to begin to characterize the distribution of stellar populations in our co-added spectra. The light-weighted ages of compact early-type galaxies at redshifts 0.5 < z < 1.4 are compared to those of a control sample of larger galaxies at similar redshifts. We find that massive compact early-type galaxies selected on the basis of red color and high bulge-to-total ratio are younger than similarly selected larger galaxies, suggesting that size growth in these objects is not driven mainly by progenitor bias, and that individual galaxies grow as their stellar populations age. However, compact early-type galaxies selected on the basis of image smoothness and high bulge-to-total ratio are older than a control sample of larger galaxies. Progenitor bias will play a significant role in defining the apparent size changes of early-type galaxies if they are selected on the basis of the smoothness of their light distributions.