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

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 nonparametric star formation histories, emission lines, variable dust attenuation law, and dust and active galactic nucleus emission), together with an uncertainty and outlier model. We show that both spectroscopy and photometry are needed to break the dust–age–metallicity degeneracy. We find a large diversity of star formation histories: although the most massive (M⋆ > 2 × 1011 M⊙) galaxies formed the earliest (formation redshift of zf ≈ 5–10 with a short star formation 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 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 $\langle {\tau }_{\mathrm{quench}}\rangle ={1.0}_{-0.9}^{+0.8}\,\mathrm{Gyr}$ and of quenching epochs of zquench ≈ 0.8–5.0 ($\langle {z}_{\mathrm{quench}}\rangle ={1.3}_{-0.4}^{+0.7}$). 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-and-quench" framework and is consistent with a wide and continuously populated diversity of quenching timescales