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 ∼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 ∼3500 quiescent galaxies shows that the average size of a quiescent galaxy increases by ≲25% from z ∼ 0.6 to z ∼ 0.1. This growth rate is a function of stellar mass: the most massive ( ) galaxies grow significantly more slowly in size than quiescent systems an order of magnitude less massive that grow by 70% in the 0.1 ≲ z ≲ 0.3 redshift interval. For 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 have mostly assembled their mass by z ∼ 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 ( ) grows predominantly as a result of progenitor bias. For more massive ( ) 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 ( ) galaxies experience the least rapid increase in size from z ∼ 0.6 to z ∼ 0.1.