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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 galaxiesmore »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.

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SPT0311-58, a system of two interacting galaxies in the Epoch of Reionization, exists in one of the rarest, most massive dark matter halos theoretically possible in that era. Studying the interstellar medium (ISM) in these galaxies can illuminate the process of galaxy formation in the early Universe. In this work, we explore the multiphase ISM in this system, using ALMA observations of the [Cii] 158, [Oi] 146, [Nii] 122, and [Oiii] 88 fine-structure lines and dust continuum. We find wide variations in line ratios between the eastern and western galaxies, as well as across the western galaxy. Continuum colors indicate that SPT0311-58 E has a higher ionization parameter ($\log U\approx -2.8$) than SPT0311-58 W ($\log U\approx -3.1$). The ratio of [Oiii] 88–[Nii] 122 and the ionization parameter constraints combine to demonstrate near-solar metallicity in these objects just 800 Myr after the Big Bang.

We explore how the presence of detectable molecular gas depends on the inferred star formation histories (SFHs) in eight massive, quiescent galaxies atz∼ 0.7. Half of the sample have clear detections of molecular gas, traced by CO(2–1). We find that the molecular gas content is unrelated to the rate of star formation decline prior to the most recent 1 Gyr, suggesting that the gas reservoirs are not left over from their primary star formation epoch. However, the recent SFHs of CO-detected galaxies demonstrate evidence for secondary bursts of star formation in their last Gyr. The fraction of stellar mass formed in these secondary bursts ranges fromf_burst≈ 0.3%–6% and ended betweent_end-burst≈ 0–330 Myr ago. The CO-detected galaxies form a higher fraction of mass in the last Gyr ($f_{M_{1\mathrm{Gyr}}}=2.6\%\pm 1.8\%$) compared to the CO-undetected galaxies ($f_{M_{1\mathrm{Gyr}}}=0.2\%\pm 0.1\%$). The galaxies with gas reservoirs have enhanced late-time star formation, highlighting this as a contributing factor to the observed heterogeneity in the gas reservoirs in high-redshift quiescent galaxies. We find that the amount of gas and star formation driven by these secondary bursts are inconsistent with that expected from dry minor mergers, and instead are likely driven bymore »recently accreted gas, i.e., gas-rich minor mergers. This conclusion would not have been made based on SFR_UV+IRmeasurements alone, highlighting the power of detailed SFH modeling in the interpretation of gas reservoirs. Larger samples are needed to understand the frequency of low-level rejuvenation among quiescent galaxies at intermediate redshifts, and to what extent this drives the diversity of molecular gas reservoirs.

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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 (burstmore »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« less

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 plausiblymore »suppress star formation directly, a qualitatively different physical mechanism than feedback-driven outflows.

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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 aremore »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.« less

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 quartermore »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.« less

We use ALMA observations of CO(2–1) in 13 massive (M_*≳ 10¹¹M_⊙) poststarburst galaxies atz∼ 0.6 to constrain the molecular gas content in galaxies shortly after they quench their major star-forming episode. The poststarburst galaxies in this study are selected from the Sloan Digital Sky Survey spectroscopic samples (Data Release 14) based on their spectral shapes, as part of the Studying QUenching at Intermediate-z Galaxies: Gas, angu$$\overrightarrow{L}\mathrm{ar}$$momentum, and Evolution ($\mathit{SQuIGG}\vec{L}E$) program. Early results showed that two poststarburst galaxies host large H₂reservoirs despite their low inferred star formation rates (SFRs). Here we expand this analysis to a larger statistical sample of 13 galaxies. Six of the primary targets (45%) are detected, with$M_{{\mathrm{H}}_2}\gtrsim {10}^9$M_⊙. Given their high stellar masses, this mass limit corresponds to an average gas fraction of$〈f_{{\mathrm{H}}_2}\equiv M_{{\mathrm{H}}_2}/M_{*}〉\sim 7\%$or ∼14% using lower stellar masses estimates derived from analytic, exponentially declining star formation histories. The gas fraction correlates with theD_n4000 spectral index, suggesting that the cold gas reservoirs decrease with time since burst, as found in local K+A galaxies. Star formation histories derived from flexible stellar population synthesis modeling support thismore »empirical finding: galaxies that quenched ≲150 Myr prior to observation host detectable CO(2–1) emission, while older poststarburst galaxies are undetected. The large H₂reservoirs and low SFRs in the sample imply that the quenching of star formation precedes the disappearance of the cold gas reservoirs. However, within the following 100–200 Myr, the$\mathit{SQuIGG}\vec{L}E$galaxies require the additional and efficient heating or removal of cold gas to bring their low SFRs in line with standard H₂scaling relations.

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Abstract Observations of cold molecular gas reservoirs are critical for understanding the shutdown of star formation in massive galaxies. While dust continuum is an efficient and affordable tracer, this method relies upon the assumption of a “normal” molecular-gas to dust mass ratio, δ GDR , typically of order 100. Recent null detections of quiescent galaxies in deep dust continuum observations support a picture where the cold gas and dust have been rapidly depleted or expelled. In this work, we present another viable explanation: a significant fraction of galaxies with low star formation per unit stellar mass are predicted to have extreme δ GDR ratios. We show that simulated massive quiescent galaxies at 0 < z < 3 in the simba cosmological simulations have δ GDR values that extend >4 orders of magnitude. The dust in most simulated quiescent galaxies is destroyed significantly more rapidly than the molecular gas depletes, and cannot be replenished. The transition from star-forming to quiescent halts dust formation via star formation processes, with dust subsequently destroyed by supernova shocks and thermal sputtering of dust grains embedded in hot plasma. After this point, the dust growth rate in the models is not sufficient to overcome the lossmore »of >3 orders of magnitude in dust mass to return to normal values of δ GDR despite having high metallicity. Our results indicate that it is not straight forward to use a single observational indicator to robustly preselect exotic versus normal ratios. These simulations make strong predictions that can be tested with millimeter facilities.« less

With Σ_SFR∼ 4200M_⊙yr⁻¹kpc⁻², SPT 0346–52 (z= 5.7) is the most intensely star-forming galaxy discovered by the South Pole Telescope. In this paper, we expand on previous spatially resolved studies, using ALMA observations of dust continuum, [Nii] 205μm, [Cii] 158μm, [Oi] 146μm, and undetected [Nii] 122μm and [Oi] 63μm emission to study the multiphase interstellar medium (ISM) in SPT 0346–52. We use pixelated, visibility-based lens modeling to reconstruct the source-plane emission. We also model the source-plane emission using the photoionization codecloudyand find a supersolar metallicity system. We calculateT_dust= 48.3 K andλ_peak= 80μm and see line deficits in all five lines. The ionized gas is less dense than comparable galaxies, withn_e< 32 cm⁻³, while ∼20% of the [Cii] 158μm emission originates from the ionized phase of the ISM. We also calculate the masses of several phases of the ISM. We find that molecular gas dominates the mass of the ISM in SPT 0346–52, with the molecular gas mass ∼4× higher than the neutral atomic gas mass and ∼100× higher than the ionized gas mass.