Quiescent Galaxy Size and Spectroscopic Evolution: Combining HSC Imaging and Hectospec Spectroscopy

Abstract

We explore the relationships between size, stellar mass, and average stellar population age (indicated by D(n)4000 indices) for a sample of similar to 11,000 intermediate-redshift galaxies from the SHELS spectroscopic survey (Geller et al. 2014) augmented by high-resolution Subaru Telescope Hyper Suprime-Cam imaging. In the redshift interval 0.1 < z < 0.6, star-forming galaxies are on average larger than their quiescent counterparts. The mass-complete sample of similar to 3500 M* >10(10) M-circle dot quiescent galaxies shows that the average size of a 10(11) M-circle dot quiescent galaxy increases by less than or similar to 25% from z similar to 0.6 to z similar to 0.1. This growth rate is a function of stellar mass: the most massive (M* > 10(11) M-circle dot) galaxies grow significantly more slowly in size than quiescent systems an order of magnitude less massive that grow by 70% in the 0.1 less than or similar to z less than or similar to 0.3 redshift interval. For M* < 10(11) M-circle dot galaxies, age and size are anticorrelated at fixed mass; more massive quiescent systems show no significant trend in size with average stellar population age. The evolution in absolute and fractional abundances of quiescent systems at intermediate redshift are also a function of galaxy stellar mass. The suite of evolutionary trends suggests that galaxies more massive than similar to 10(11) M-circle dot have mostly assembled their mass by z similar to 0.6. Quiescent galaxies with lower stellar masses show more complex evolution that is characterized by a combination of individual quiescent galaxy size growth (through mergers) and an increase in the size of newly quenched galaxies joining the population at later times (progenitor bias). The low-mass population (M* similar to 10(10) M-circle dot) grows predominantly as a result of progenitor bias. For more massive (M* similar to 5 x 10(10) M-circle dot ) quiescent galaxies, (predominantly minor) mergers and progenitor bias make more comparable contributions to the size growth. At intermediate redshift, quiescent size growth is mass-dependent; the most massive (M* > 10(11) M-circle dot ) galaxies experience the least rapid increase in size from z similar to 0.6 to z similar to 0.1.