Stellar populations from spectroscopy of a large sample of quiescent galaxies at z > 1: Measuring the contribution of progenitor bias to early size growth

Abstract

We analyze the stellar populations of a sample of 62 massive (log M_∗/M_⊙ > 10.7) galaxies in the redshift range 1 < z < 1.6, with the main goal of investigating the role of recent quenching in the size growth of quiescent galaxies over the redshift range 1.25 < z < 2 where the apparent growth is particularly rapid. We demonstrate that our sample is not biased toward bright, compact, or young galaxies, and thus is representative of the overall quiescent population. Our high signal-to-noise ratio Keck LRIS spectra probe the rest-frame Balmer break region which contains important absorption line diagnostics of recent star formation activity. We show that improved measures of the various stellar population parameters, including the star-formation timescale τ, age and dust extinction, can be determined by fitting templates jointly to both our spectroscopic and broad-band photometric data. These parameter fits allow us to backtrack the evolving trajectory of individual galaxies on the UV J color-color plane. In addition to identifying which quiescent galaxies were recently quenched, an effect often termed progenitor bias in studies of the evolution of this population, we discover important variations in the star formation history of quiescent galaxies indicating the likelihood of more than one quenching route. By using sizes measured in the previous paper of this series, we confirm that the largest galaxies are indeed among the youngest at a given redshift, an effect consistent with some contribution to the apparent growth from recent arrivals. However, we calculate that recently-quenched objects can only be responsible for about half of the increase in the average size of quiescent galaxies over a 1.5 Gyr period corresponding to the redshift interval 1.25 < z < 2. The remainder of the observed size evolution arises from a genuine growth of long-standing quiescent galaxies. We discuss the prospects of determining whether the combination of minor merging and progenitor bias can explain the remarkably fast apparent growth rate during this short period of cosmic history.

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