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Abstract: Atacama Large Millimeter/submillimeter Array observations have shown that candidate “post-starburst” galaxies (PSBs) at z~0.6 can retain significant molecular gas reservoirs. These results would imply that—unlike many model predictions—galaxies can shut down their star formation before their cold gas reservoirs are depleted. However, these studies inferred star formation rates (SFRs) either from [OII] line fluxes or from spectral energy distribution (SED) modeling and could have missed large dust-obscured contributions to the SFRs. In this study, we present Keck/NIRES observations of 13 massive (M_* >= 10^11M_⊙) PSBs, which allow us to estimate Hα SFRs in these gas-rich PSBs. We confirm the previously inferred low SFRs for the majority of the sample: 11/13 targets show clear Hα absorption, with minimal infilling indicating dust-corrected SFRs of <4.1Msun/yr. These SFRs are notably low given the large H2 reservoirs (∼(1–5) × 10^10Msun) present in 5/13 of these galaxies, placing them significantly offset from star-forming galaxies on the Kennicutt–Schmidt relation for star-forming galaxies. The [NII]/Hα ratios of all 13 PSBs imply contributions from non-star-forming ionization mechanisms (e.g., active galactic nuclei, shocks, or hot evolved stars) to their Hα emission, suggesting that even these low ongoing SFRs may be overestimated. These low Hα SFRs, dust corrected using Av estimates from SED fitting, confirm that these galaxies are very likely quiescent and, thus, that galaxies can quench before their cold gas reservoirs are fully depleted. 

Abstract Detecting the first generation of stars, Population III (Pop III), has been a long-standing goal in astrophysics, yet they remain elusive even in the JWST era. Here we present a novel NIRCam-based selection method for Pop III galaxies, and carefully validate it through completeness and contamination simulations. We systematically search ≃ 500 arcmin²across JWST legacy fields for Pop III candidates, including GLIMPSE, which, assisted by gravitational lensing, has produced JWST’s deepest NIRCam imaging thus far. We discover one promising Pop III galaxy candidate (GLIMPSE-16043) at $z=6.50_{-0.24}^{+0.03}$, a moderately lensed galaxy ( $\mu =2.9_{-0.2}^{+0.1}$) with an intrinsic UV magnitude of $M_{\mathrm{UV}}=-15.89_{-0.14}^{+0.12}$. It exhibits key Pop III features: strong Hαemission (rest-frame EW 2810 ± 550 Å); a Balmer jump; no dust (UV slopeβ = −2.34 ± 0.36); and undetectable metal lines (e.g., [Oiii]; [Oiii]/Hβ < 0.44), implying a gas-phase metallicity ofZ_gas/Z_⊙ < 0.5%. These properties indicate the presence of a nascent, metal-deficient young stellar population (<5 Myr) with a stellar mass of ≃10⁵M_⊙. Intriguingly, this source deviates significantly from the extrapolated UV–metallicity relation derived from recent JWST observations atz= 4–10, consistent with UV enhancement by a top-heavy Pop III initial mass function or the presence of an extremely metal-poor active galactic nucleus. We also derive the first observational constraints on the Pop III UV luminosity function atz ≃ 6–7. The volume density of GLIMPSE-16043 (≈10⁻⁴cMpc⁻³) is in excellent agreement with theoretical predictions, independently reinforcing its plausibility. This study demonstrates the power of our novel NIRCam method to finally reveal distant galaxies even more pristine than the Milky Way’s most metal-poor satellites, thereby promising to bring us closer to the first generation of stars than we have ever been before. 

Abstract We present the elemental abundances and ages of 19 massive quiescent galaxies atz∼ 1.4 andz∼ 2.1 from the Keck Heavy Metal Survey. The ultradeep LRIS and MOSFIRE spectra were modeled using a full-spectrum stellar population fitting code with variable abundance patterns. The galaxies have iron abundances between [Fe/H] = −0.5 and −0.1 dex, with typical values of −0.2 [−0.3] atz∼ 1.4 [z∼ 2.1]. We also find a tentative $\log {\sigma }_v$–[Fe/H] relation atz∼ 1.4. The magnesium-to-iron ratios span [Mg/Fe] = 0.1–0.6 dex, with typical values of 0.3 [0.5] dex atz∼ 1.4 [z∼ 2.1]. The ages imply formation redshifts ofz_form= 2–8. Compared to quiescent galaxies at lower redshifts, we find that [Fe/H] was ∼0.2 dex lower atz= 1.4–2.1. We find no evolution in [Mg/Fe] out toz∼ 1.4, though thez∼ 2.1 galaxies are 0.2 dex enhanced compared toz= 0–0.7. A comparison of these results to a chemical evolution model indicates that galaxies at higher redshift form at progressively earlier epochs and over shorter star formation timescales, with thez∼ 2.1 galaxies forming the bulk of their stars over 150 Myr atz_form∼ 4. This evolution cannot be solely attributed to an increased number of quiescent galaxies at later times; several Heavy Metal galaxies have extreme chemical properties not found in massive galaxies atz∼ 0.0–0.7. Thus, the chemical properties of individual galaxies must evolve over time. Minor mergers also cannot fully account for this evolution as they cannot increase [Fe/H], particularly in galaxy centers. Consequently, the buildup of massive quiescent galaxies sincez∼ 2.1 may require further mechanisms, such as major mergers and/or central star formation. 

Abstract In this paper, we present the Heavy Metal Survey, which obtained ultradeep medium-resolution spectra of 21 massive quiescent galaxies at 1.3 <z< 2.3 with Keck/LRIS and MOSFIRE. With integration times of up to 16 hr per band per galaxy, we observe numerous Balmer and metal absorption lines in atmospheric windows. We successfully derive spectroscopic redshifts for all 21 galaxies, and for 19 we also measure stellar velocity dispersions (σ_v), ages, and elemental abundances, as detailed in an accompanying paper. Except for one emission-line active galactic nucleus, all galaxies are confirmed as quiescent through their faint or absent Hαemission and evolved stellar spectra. For most galaxies exhibiting faint Hα, elevated [Nii]/Hαsuggests a non-star-forming origin. We calculate dynamical masses (M_dyn) by combiningσ_vwith structural parameters obtained from the Hubble Space Telescope COSMOS(-DASH) survey and compare them with stellar masses (M_*) derived using spectrophotometric modeling, considering various assumptions. For a fixed initial mass function (IMF), we observe a strong correlation betweenM_dyn/M_*andσ_v. This correlation may suggest that a varying IMF, with high-σ_vgalaxies being more bottom heavy, was already in place atz∼ 2. When implementing theσ_v-dependent IMF found in the cores of nearby early-type galaxiesandcorrecting for biases in our stellar mass and size measurements, we find a low scatter inM_dyn/M_*of 0.14 dex. However, these assumptions result in unphysical stellar masses, which exceed the dynamical masses by 34%. This tension suggests that distant quiescent galaxies do not simply grow inside-out into today’s massive early-type galaxies and the evolution is more complicated. 

Abstract Accurate models of the star formation histories (SFHs) of recently quenched galaxies can provide constraints on when and how galaxies shut down their star formation. The recent development of nonparametric SFH models promises the flexibility required to make these measurements. However, model and prior choices significantly affect derived SFHs, particularly for post-starburst galaxies (PSBs), which have sharp changes in their recent SFH. In this paper, we create mock PSBs, then use the Prospector SED fitting software to test how well four different SFH models recover key properties. We find that a two-component parametric model performs well for our simple mock galaxies, but is sensitive to model mismatches. The fixed- and flexible-bin nonparametric models included in Prospector are able to rapidly quench a major burst of star formation, but systematically underestimate the post-burst age by up to 200 Myr. We develop a custom SFH model that allows for additional flexibility in the recent SFH. Our flexible nonparametric model is able to constrain post-burst ages with no significant offset and just ∼90 Myr of scatter. Our results suggest that while standard nonparametric models are able to recover first-order quantities of the SFH (mass, SFR, average age), accurately recovering higher-order quantities (burst fraction, quenching time) requires careful consideration of model flexibility. These mock recovery tests are a critical part of future SFH studies. Finally, we show that our new, public SFH model is able to accurately recover the properties of mock star-forming and quiescent galaxies and is suitable for broader use in the SED fitting community. https://github.com/bd-j/prospector 

Abstract We present visual classifications of merger-induced tidal disturbances in 143M_*∼ 10¹¹M_⊙post-starburst galaxies atz∼ 0.7 identified in the $\mathrm{SQuIGG}\vec{L}\mathrm{E}$Sample. This sample spectroscopically selects galaxies from the Sloan Digital Sky Survey that have stopped their primary epoch of star formation within the past ∼500 Myr. Visual classifications are performed on Hyper Suprime-Cam imaging. We compare to a control sample of mass- and redshift-matched star-forming and quiescent galaxies from the Large Early Galaxy Census and find that post-starburst galaxies are more likely to be classified as disturbed than either category. This corresponds to a factor of ${3.6}_{-1.3}^{+2.9}$times the disturbance rate of older quiescent galaxies and ${2.1}_{-.73}^{+1.9}$times the disturbance rate of star-forming galaxies. Assuming tidal features persist for ≲500 Myr, this suggests merging is coincident with quenching in a significant fraction of these post-starbursts. Galaxies with tidal disturbances are younger on average than undisturbed post-starburst galaxies in our sample, suggesting tidal features from a major merger may have faded over time. This may be exacerbated by the fact that, on average, the undisturbed subset is fainter, rendering low-surface-brightness tidal features harder to identify. However, the presence of 10 young (≲150 Myr since quenching) undisturbed galaxies suggests that major mergers are not the only fast physical mechanism that shut down the primary epoch of star formation in massive galaxies at intermediate redshift. 

Abstract We present structural measurements of 145 spectroscopically selected intermediate-redshift ( z ∼ 0.7), massive ( M ⋆ ∼ 10 11 M ⊙ ) post-starburst galaxies from the SQuIGG L ⃗ E sample measured using wide-depth Hyper Suprime-Cam i -band imaging. This deep imaging allows us to probe the sizes and structures of these galaxies, which we compare to a control sample of star-forming and quiescent galaxies drawn from the LEGA-C Survey. We find that post-starburst galaxies systematically lie ∼0.1 dex below the quiescent mass–size (half-light radius) relation, with a scatter of ∼0.2 dex. This finding is bolstered by nonparametric measures, such as the Gini coefficient and the concentration, which also reveal these galaxies to have more compact light profiles than both quiescent and star-forming populations at similar mass and redshift. The sizes of post-starburst galaxies show either negative or no correlation with the time since quenching, such that more recently quenched galaxies are larger or similarly sized. This empirical finding disfavors the formation of post-starburst galaxies via a purely central burst of star formation that simultaneously shrinks the galaxy and shuts off star formation. We show that the central densities of post-starburst and quiescent galaxies at this epoch are very similar, in contrast with their effective radii. The structural properties of z ∼ 0.7 post-starburst galaxies match those of quiescent galaxies that formed in the early universe, suggesting that rapid quenching in the present epoch is driven by a similar mechanism to the one at high redshift. 

Abstract Observations and simulations have demonstrated that star formation in galaxies must be actively suppressed to prevent the formation of overly massive galaxies. Galactic outflows driven by stellar feedback or supermassive black hole accretion are often invoked to regulate the amount of cold molecular gas available for future star formation but may not be the only relevant quenching processes in all galaxies. We present the discovery of vast molecular tidal features extending up to 64 kpc outside of a massivez= 0.646 post-starburst galaxy that recently concluded its primary star-forming episode. The tidal tails contain (1.2 ± 0.1) × 10¹⁰M_⊙of molecular gas, 47% ± 5% of the total cold gas reservoir of the system. Both the scale and magnitude of the molecular tidal features are unprecedented compared to all known nearby or high-redshift merging systems. We infer that the cold gas was stripped from the host galaxies during the merger, which is most likely responsible for triggering the initial burst phase and the subsequent suppression of star formation. While only a single example, this result shows that galaxy mergers can regulate the cold gas contents in distant galaxies by directly removing a large fraction of the molecular gas fuel, and plausibly suppress star formation directly, a qualitatively different physical mechanism than feedback-driven outflows. 

Abstract We describe the Studying Quenching in Intermediate- z Galaxies: Gas, angu L → ar momentum, and Evolution ( SQuIGG L ⃗ E ) survey of intermediate-redshift post-starburst galaxies. We leverage the large sky coverage of the Sloan Digital Sky Survey to select ∼ 1300 recently quenched galaxies at 0.5 < z ≤ 0.9 based on their unique spectral shapes. These bright, intermediate-redshift galaxies are ideal laboratories to study the physics responsible for the rapid quenching of star formation: they are distant enough to be useful analogs for high-redshift quenching galaxies, but low enough redshift that multiwavelength follow-up observations are feasible with modest telescope investments. We use the Prospector code to infer the stellar population properties and nonparametric star formation histories (SFHs) of all galaxies in the sample. We find that SQuIGG L ⃗ E galaxies are both very massive ( M * ∼ 10 11.25 M ⊙ ) and quenched, with inferred star formation rates ≲1 M ⊙ yr −1 , more than an order of magnitude below the star-forming main sequence. The best-fit SFHs confirm that these galaxies recently quenched a major burst of star formation: >75% of SQuIGG L ⃗ E galaxies formed at least a quarter of their total stellar mass in the recent burst, which ended just ∼200 Myr before observation. We find that SQuIGG L ⃗ E galaxies are on average younger and more burst-dominated than most other z ≲ 1 post-starburst galaxy samples. This large sample of bright post-starburst galaxies at intermediate redshift opens a wide range of studies into the quenching process. In particular, the full SQuIGG L ⃗ E survey will investigate the molecular gas reservoirs, morphologies, kinematics, resolved stellar populations, active galactic nucleus incidence, and infrared properties of this unique sample of galaxies in order to place definitive constraints on the quenching process.