Primus: Constraints on star formation quenching and galaxy merging, and the evolution of the stellar mass function from z = 0-1

We measure the evolution of the stellar mass function (SMF) from z = 0-1 using multi-wavelength imaging and spectroscopic redshifts from the PRism MUlti-object Survey (PRIMUS) and the Sloan Digital Sky Survey (SDSS). From PRIMUS we construct an i < 23 flux-limited sample of ∼40, 000 galaxies at z = 0.2-1.0 over five fields totaling ≈5.5 deg², and from the SDSS we select ∼170, 000 galaxies at z = 0.01-0.2 that we analyze consistently with respect to PRIMUS to minimize systematic errors in our evolutionary measurements. We find that the SMF of all galaxies evolves relatively little since z = 1, although we do find evidence for mass assembly downsizing; we measure a ≈30% increase in the number density of ∼10 ¹⁰M_⊙ galaxies since z ≈ 0.6, and a ≲ 10% change in the number density of all ≳ 10¹¹M_⊙ galaxies since z ≈ 1. Dividing the sample into star-forming and quiescent using an evolving cut in specific star formation rate, we find that the number density of ∼10¹⁰M_⊙ star-forming galaxies stays relatively constant since z ≈ 0.6, whereas the space density of ≳ 10¹¹M_⊙ star-forming galaxies decreases by ≈50% between z ≈ 1 and z ≈ 0. Meanwhile, the number density of ∼10 ¹⁰M_⊙ quiescent galaxies increases steeply toward low redshift, by a factor of ∼2-3 since z ≈ 0.6, while the number of massive quiescent galaxies remains approximately constant since z ≈ 1. These results suggest that the rate at which star-forming galaxies are quenched increases with decreasing stellar mass, but that the bulk of the stellar mass buildup within the quiescent population occurs around ∼10^10.8 . In addition, we conclude that mergers do not appear to be a dominant channel for the stellar mass buildup of galaxies at z < 1, even among massive (≳ 10 ¹¹M_⊙ ) quiescent galaxies.