Fast, Slow, Early, Late: Quenching Massive Galaxies at z ∼ 0.8

Abstract

We investigate the stellar populations for a sample of 161 massive, mainly quiescent galaxies at zobs = 0.8 with deep Keck/DEIMOS rest-frame optical spectroscopy (HALO7D survey). With the fully Bayesian framework Prospector, we simultaneously fit the spectroscopic and photometric data with an advanced physical model (including non-parametric star-formation histories, emission lines, variable dust attenuation law, and dust and AGN emission) together with an uncertainty and outlier model. We show that both spectroscopy and photometry are needed to break the dust-agemetallicity degeneracy. We find a large diversity of star-formation histories: although the most massive (M* > 2×1011 Msun) galaxies formed the earliest (formation redshift of zf ≍ 5‑10 with a short starformation timescale of τSF ≤ 1 Gyr), lower-mass galaxies have a wide range of formation redshifts, leading to only a weak trend of zf with M*. Interestingly, several low-mass galaxies with have formation redshifts of zf ≍ 5‑8. Star-forming galaxies evolve about the star-forming main sequence, crossing the ridgeline several times in their past. Quiescent galaxies show a wide range and continuous distribution of quenching timescales (τquench ≍ 0‑5 Gyr) with a median of τquench = 1.0+0.8 ‑0.9 Gyr and of quenching epochs of zquench ≍ 0.8‑5.0 (zquench = 1.3+0.7‑0.4). This large diversity of quenching timescales and epochs points toward a combination of internal and external quenching mechanisms. In our sample, rejuvenation and "late bloomers" are uncommon. In summary, our analysis supports the "grow & quench" framework and is consistent with a wide and continuously-populated diversity of quenching timescales.