MINOR MERGERS OR PROGENITOR BIAS? THE STELLAR AGES OF SMALL AND LARGE QUENCHED GALAXIES

We investigate the origin of the evolution of the population-averaged size of quenched galaxies (QGs) through a spectroscopic analysis of their stellar ages. This evolution has been claimed to arise from either the size growth of individual galaxies through a sequence of dry minor mergers, or the addition of larger, newly quenched galaxies to the pre-existing population (i.e., a progenitor bias effect). We use the 20k zCOSMOS-bright spectroscopic survey to select bona fide QGs at 0.2 < z < 0.8. We stack their spectra in bins of redshift, stellar mass, and size to compute stellar population parameters through fits to the rest-frame optical spectra and Lick indices. The size–age relation differs below and above ∼1011 M⊙: at , at all redshifts the stellar populations of the largest galaxies are younger than those of the smaller counterparts, indicating progenitor bias as the main driver of the average size evolution. In contrast, at higher masses, there is no clear size–age trend, supporting a substantial role of dry mergers in increasing with cosmic time the sizes of these most massive QGs. The [α/Fe] abundance ratios of QGs are (i) above-solar over the entire redshift range of our analysis, hinting at universally short timescales for the buildup of the stellar populations of QGs, and (ii) similar at all masses and sizes, suggesting similar (short) timescales for the whole QG population—and strengthening the role of mergers in the buildup of the most massive QGs in the universe.