We investigate the stellar populations for a sample of 161 massive, mainly quiescent galaxies at 〈z _obs〉 = 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 z _f ≈ 5–10 with a short star formation timescale of τ _SF ≲ 1 Gyr), lower-mass galaxies have a wide range of formation redshifts, leading to only a weak trend of z _f with M _⋆. Interestingly, several low-mass galaxies have formation redshifts of z _f ≈ 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 (τ _quench ≈ 0–5 Gyr) with a median of 〈 τ quench 〉 = 1.0 − 0.9 + 0.8 Gyr and of quenching epochs of z _quench ≈ 0.8–5.0 ( 〈 z quench 〉 = 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.

We investigate the burstiness of star formation histories (SFHs) of galaxies at 0.4 < z < 1 by using the ratio of star formation rates (SFRs) measured from Hβ and FUV (1500 Å) (Hβ-to-FUV ratio). Our sample contains 164 galaxies down to stellar mass (M _*) of 10^8.5 M _⊙ in the CANDELS GOODS-N region, where Team Keck Redshift Survey Keck/DEIMOS spectroscopy and Hubble Space Telescope/WFC3 F275W images from CANDELS and Hubble Deep UV Legacy Survey are available. When the ratio of Hβ- and FUV-derived SFRs is measured, dust extinction correction is negligible (except for very dusty galaxies) with the Calzetti attenuation curve. The Hβ-to-FUV ratio of our sample increases with M _* and SFR. The median ratio is ∼0.7 at M _* ∼ 10^8.5 M _⊙ (or SFR ∼ 0.5 M _⊙ yr⁻¹) and increases to ∼1 at M _* ∼ 10¹⁰ M _⊙ (or SFR ∼ 10 M _⊙ yr⁻¹). At M _* < 10^9.5 M _⊙, our median Hβ-to-FUV ratio is lower than that of local galaxies at the same M _*, implying a redshift evolution. Bursty SFH on a timescale of a few tens of megayears on galactic scales provides a plausible explanation for our results, and the importance of the burstiness increases as M _* decreases. Due to sample selection effects, our Hβ-to-FUV ratio may be an upper limit of the true value of a complete sample, which strengthens our conclusions. Other models, e.g., non-universal initial mass function or stochastic star formation on star cluster scales, are unable to plausibly explain our results.

Existing models of galaxy formation have not yet explained striking correlations between structure and star formation activity in galaxies, notably the sloped and moving boundaries that divide star-forming from quenched galaxies in key structural diagrams. This paper uses these and other relations to “reverse engineer” the quenching process for central galaxies. The basic idea is that star-forming galaxies with larger radii (at a given stellar mass) have lower black hole (BH) masses due to lower central densities. Galaxies cross into the green valley when the cumulative effective energy radiated by their BH equals ∼4× their halo gas-binding energy. Because larger-radii galaxies have smaller BHs, one finds that they must evolve to higher stellar masses in order to meet this halo energy criterion, which explains the sloping boundaries. A possible cause of radii differences among star-forming galaxies is halo concentration. The evolutionary tracks of star-forming galaxies are nearly parallel to the green-valley boundaries, and it is mainly the sideways motions of these boundaries with cosmic time that cause galaxies to quench. BH scaling laws for star-forming, quenched, and green-valley galaxies are different, and most BH mass growth takes place in the green valley. Implications include the radii of star-forming galaxies are an important second parameter in shaping their BHs; BHs are connected to their halos but in different ways for star-forming, quenched, and green-valley galaxies; and the same BH–halo quenching mechanism has been in place since z ∼ 3. We conclude with a discussion of BH–galaxy coevolution and the origin and interpretation of BH scaling laws.

The relationship between quasars and their host galaxies provides clues on how supermassive black holes (SMBHs) and massive galaxies are jointly assembled. To elucidate this connection, we measure the structural and photometric properties of the host galaxies of ∼5000 Sloan Digital Sky Survey quasars at 0.2 < z < 1 using five-band (grizy) optical imaging from the Hyper Suprime-Cam Subaru Strategic Program. An automated analysis tool is used to forward-model the blended emission of the quasar as characterized by the point-spread function and the underlying host galaxy as a two-dimensional Sérsic profile. We find that quasars are preferentially hosted by massive star-forming galaxies with disklike light profiles. Furthermore, the size distribution of quasar hosts is broad at a given stellar mass and the average values exhibit a size–stellar mass relation as seen with inactive galaxies. In contrast, the sizes of quasar hosts are more compact than those of inactive star-forming galaxies on average, but not as compact as those of quiescent galaxies of similar stellar masses. This is true irrespective of quasar properties, including the bolometric luminosity, Eddington ratio, and black hole mass. These results are consistent with a scenario in which galaxies are concurrently fueling an SMBH and building their stellar bulge from a centrally concentrated gas reservoir. Alternatively, quasar hosts may be experiencing a compaction process in which stars from the disk and inflowing gas are responsible for growing the bulge. In addition, we confirm that the host galaxies of type 1 quasars have a bias of being closer to face-on systems, suggesting that galactic-scale dust can contribute to obscuring the broad-line region.

We measure the evolution of the  BH –  ⋆ relation using 584 uniformly selected Sloan Digital Sky Survey quasars at 0.2 < z < 0.8. The black hole masses (  BH ) are derived from the single-epoch virial mass estimator using the Hβ emission line and span the range 7.0 < log  BH / M ⊙ < 9.5 . The host-galaxy stellar masses (  ⋆ ), which cover the interval 10.0 < log  ⋆ / M ⊙ < 11.5 , are determined by performing two-dimensional quasar-host decomposition of the Hyper Suprime-Cam images and spectral energy distribution fitting. To quantify sample selection biases and measurement uncertainties on the mass terms, a mock quasar sample is constructed to jointly constrain the redshift evolution of the  BH –  ⋆ relation and its intrinsic scatter (σ _μ ) through forward modeling. We find that the level of evolution is degenerate with σ _μ , such that both a positive mild evolution (i.e.,  BH /  ⋆ increases with redshift) with a small σ _μ and a negative mild evolution with a larger σ _μ are consistent with our data. The posterior distribution of σ _μ enables us to put a strong constraint on the intrinsic scatter of the  BH –  ⋆ relation, which has a best inference of 0.25 − 0.04 + 0.03 dex, consistent with the local value. The redshift evolution of the  BH –  ⋆ relation relative to the local relation is constrained to be 1 + z 0.12 − 0.27 + 0.28 , in agreement with no significant evolution since z ∼ 0.8. The tight and unevolving  BH –  ⋆ relation is suggestive of a coupling through active galactic nuclei feedback or/and a common gas supply at work, thus restricting the mass ratio of galaxies and their black holes to a limited range. Given the considerable stellar disk component, the  BH –  bulge relation may evolve as previously seen at higher redshifts.

We present the stellar mass ( M * )–gas-phase metallicity relation (MZR) and its scatter at intermediate redshifts ( 0.5 ≤ z ≤ 0.7 ) for 1381 field galaxies collected from deep spectroscopic surveys. The star formation rate (SFR) and color at a given M * of this magnitude-limited ( R ≲ 24 AB) sample are representative of normal star-forming galaxies. For masses below 10⁹ M ⊙ , our sample of 237 galaxies is ∼10 times larger than those in previous studies beyond the local universe. This huge gain in sample size enables superior constraints on the MZR and its scatter in the low-mass regime. We find a power-law MZR at 10⁸ M ⊙ < M * < 10 11 M ⊙ : 12 + log ( O / H ) = ( 5.83 ± 0.19 ) + ( 0.30 ± 0.02 ) log ( M * / M ⊙ ) . At 10⁹ M ⊙ < M * < 10 10.5 M ⊙ , our MZR shows agreement with others measured at similar redshifts in the literature. Our power-law slope is, however, shallower than the extrapolation of the MZRs of others to masses below 10⁹ M ⊙ . The SFR dependence of the MZR in our sample is weaker than that found for local galaxies (known as the fundamental metallicity relation). Compared to a variety of theoretical models, the slope of our MZR for low-mass galaxies agrees well with predictions incorporating supernova energy-driven winds. Being robust against currently uncertain metallicity calibrations, the scatter of the MZR serves as a powerful diagnostic of the stochastic history of gas accretion, gas recycling, and star formation of low-mass galaxies. Our major result is that the scatter of our MZR increases as M * decreases. Our result implies that either the scatter of the baryonic accretion rate ( σ M ˙ ) or the scatter of the M * – M halo relation ( σ SHMR ) increases as M * decreases. Moreover, our measure of scatter at z = 0.7 appears consistent with that found for local galaxies. This lack of redshift evolution constrains models of galaxy evolution to have both σ M ˙ and σ SHMR remain unchanged from z = 0.7 to z = 0.