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1. Synthesizing Stellar Populations in South Pole Telescope Galaxy Clusters. I. Ages of Quiescent Member Galaxies at 0.3 < z < 1.4

   https://doi.org/10.3847/1538-4357/ac7c0c  

   Khullar, Gourav; Bayliss, Matthew B.; Gladders, Michael D.; Kim, Keunho J.; Calzadilla, Michael S.; Strazzullo, Veronica; Bleem, Lindsey E.; Mahler, Guillaume; McDonald, Michael; Floyd, Benjamin; et al (August 2022, The Astrophysical Journal)

   Abstract Using stellar population synthesis models to infer star formation histories (SFHs), we analyze photometry and spectroscopy of a large sample of quiescent galaxies that are members of Sunyaev–Zel’dovich (SZ)-selected galaxy clusters across a wide range of redshifts. We calculate stellar masses and mass-weighted ages for 837 quiescent cluster members at 0.3 < z < 1.4 using rest-frame optical spectra and the Python-based Prospector framework, from 61 clusters in the SPT-GMOS Spectroscopic Survey (0.3 < z < 0.9) and three clusters in the SPT Hi-z cluster sample (1.25 < z < 1.4). We analyze spectra of subpopulations divided into bins of redshift, stellar mass, cluster mass, and velocity-radius phase-space location, as well as by creating composite spectra of quiescent member galaxies. We find that quiescent galaxies in our data set sample a diversity of SFHs, with a median formation redshift (corresponding to the lookback time from the redshift of observation to when a galaxy forms 50% of its mass, t 50 ) of z = 2.8 ± 0.5, which is similar to or marginally higher than that of massive quiescent field and cluster galaxy studies. We also report median age–stellar mass relations for the full sample (age of the universe at t 50 (Gyr) = 2.52 (±0.04)–1.66 (±0.12) log 10 ( M /10 11 M ⊙ )) and recover downsizing trends across stellar mass; we find that massive galaxies in our cluster sample form on aggregate ∼0.75 Gyr earlier than lower-mass galaxies. We also find marginally steeper age–mass relations at high redshifts, and report a bigger difference in formation redshifts across stellar mass for fixed environment, relative to formation redshifts across environment for fixed stellar mass. 

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2. A Comparison of Star-formation Histories Derived from UniverseMachine and LEGA-C at 0.6 < z < 1

   https://doi.org/10.3847/2515-5172/ad1be9  

   Steel, Cecilia; Pearl, Alan; Kaushal, Yasha; Bezanson, Rachel (January 2024, Research Notes of the AAS)

   Abstract In this work, we compare star formation histories of massive (10.5 $<\log \left(M_{*}/M_{\odot }\right)<$12) galaxies in the UniverseMachine model to those measured from the Large Early Galaxy Astrophysics Census (LEGA-C) at 0.6 < z < 1. Following the LEGA-C study, we investigate how 50% (t₅₀) and 90% (t₉₀) formation timescales depend on total stellar mass. We find good agreement between the observed and model timescales for the star-forming population Δ t_SF ≲ 1 Gyr across the full mass range. In contrast, the observed age-mass correlation is weaker for the quiescent population compared to UniverseMachine models (Δt_Q ≲ 2 Gyr), especially at the high-mass end. This indicates continued star formation or additional processes in the most massive quiescent galaxies, a behavior not accounted for in the UniverseMachine model. 

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3. DESI Survey Validation Spectra Reveal an Increasing Fraction of Recently Quenched Galaxies at z ∼ 1

   https://doi.org/10.3847/2041-8213/acc9b5  

   Setton, David J.; Dey, Biprateep; Khullar, Gourav; Bezanson, Rachel; Newman, Jeffrey A.; Aguilar, Jessica N.; Ahlen, Steven; Andrews, Brett H.; Brooks, David; de la Macorra, Axel; et al (April 2023, The Astrophysical Journal Letters)

   Abstract We utilize ∼17,000 bright luminous red galaxies (LRGs) from the novel Dark Energy Spectroscopic Instrument Survey Validation spectroscopic sample, leveraging its deep (∼2.5 hr galaxy⁻¹exposure time) spectra to characterize the contribution of recently quenched galaxies to the massive galaxy population at 0.4 <z< 1.3. We useProspectorto infer nonparametric star formation histories and identify a significant population of recently quenched galaxies that have joined the quiescent population within the past ∼1 Gyr. The highest-redshift subset (277 atz> 1) of our sample of recently quenched galaxies represents the largest spectroscopic sample of post-starburst galaxies at that epoch. At 0.4 <z< 0.8, we measure the number density of quiescent LRGs, finding that recently quenched galaxies constitute a growing fraction of the massive galaxy population with increasing look-back time. Finally, we quantify the importance of this population among massive ( $\log \left(M_{\star }/M_{\odot }\right)$> 11.2) LRGs by measuring the fraction of stellar mass each galaxy formed in the gigayear before observation,f_{1 Gyr}. Although galaxies withf_{1 Gyr}> 0.1 are rare atz∼ 0.4 (≲0.5% of the population), byz∼ 0.8, they constitute ∼3% of massive galaxies. Relaxing this threshold, we find that galaxies withf_{1 Gyr}> 5% constitute ∼10% of the massive galaxy population atz∼ 0.8. We also identify a small but significant sample of galaxies atz= 1.1–1.3 that formed withf_{1 Gyr}> 50%, implying that they may be analogs to high-redshift quiescent galaxies that formed on similar timescales. Future analysis of this unprecedented sample promises to illuminate the physical mechanisms that drive the quenching of massive galaxies after cosmic noon. 

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4. The Demographics, Stellar Populations, and Star Formation Histories of Fast Radio Burst Host Galaxies: Implications for the Progenitors

   https://doi.org/10.3847/1538-4357/ace5aa  

   Gordon, Alexa C.; Fong, Wen-fai; Kilpatrick, Charles D.; Eftekhari, Tarraneh; Leja, Joel; Prochaska, J. Xavier; Nugent, Anya E.; Bhandari, Shivani; Blanchard, Peter K.; Caleb, Manisha; et al (August 2023, The Astrophysical Journal)

   Abstract We present a comprehensive catalog of observations and stellar population properties for 23 highly secure host galaxies of fast radio bursts (FRBs). Our sample comprises 6 repeating FRBs and 17 apparent nonrepeaters. We present 82 new photometric and 8 new spectroscopic observations of these hosts. Using stellar population synthesis modeling and employing nonparametric star formation histories (SFHs), we find that FRB hosts have a median stellar mass of ≈10^9.9M_⊙, mass-weighted age ≈5.1 Gyr, and ongoing star formation rate ≈1.3M_⊙yr⁻¹but span wide ranges in all properties. Classifying the hosts by degree of star formation, we find that 87% (20 of 23 hosts) are star-forming, two are transitioning, and one is quiescent. The majority trace the star-forming main sequence of galaxies, but at least three FRBs in our sample originate in less-active environments (two nonrepeaters and one repeater). Across all modeled properties, we find no statistically significant distinction between the hosts of repeaters and nonrepeaters. However, the hosts of repeating FRBs generally extend to lower stellar masses, and the hosts of nonrepeaters arise in more optically luminous galaxies. While four of the galaxies with the clearest and most prolonged rises in their SFHs all host repeating FRBs, demonstrating heightened star formation activity in the last ≲100 Myr, one nonrepeating host shows this SFH as well. Our results support progenitor models with short delay channels (i.e., magnetars formed via core-collapse supernova) for most FRBs, but the presence of some FRBs in less-active environments suggests a fraction form through more delayed channels. 

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5. The Hubble Space Telescope Survey of M31 Satellite Galaxies. IV. Survey Overview and Lifetime Star Formation Histories

   https://doi.org/10.3847/1538-4357/ada24f  

   Savino, Alessandro; Weisz, Daniel R; Dolphin, Andrew E; Durbin, Meredith J; Kallivayalil, Nitya; Wetzel, Andrew; Anderson, Jay; Besla, Gurtina; Boylan-Kolchin, Michael; Brown, Thomas M; et al (January 2025, The Astrophysical Journal)

   Abstract From >1000 orbits of HST imaging, we present deep homogeneous resolved star color–magnitude diagrams that reach the oldest main-sequence turnoff and uniformly measured star formation histories (SFHs) of 36 dwarf galaxies (−6 ≥M_V≥ −17) associated with the M31 halo, and for 10 additional fields in M31, M33, and the Giant Stellar Stream. From our SFHs, we find: (i) The median stellar age and quenching epoch of M31 satellites correlate with galaxy luminosity and galactocentric distance. Satellite luminosity and present-day distance from M31 predict the satellite quenching epoch to within 1.8 Gyr at all epochs. This tight relationship highlights the fundamental connection between satellite halo mass, environmental history, and star formation duration. (ii) There is no difference between the median SFH of galaxies on and off the great plane of Andromeda satellites. (iii) ~50% of our M31 satellites show prominent ancient star formation (>12 Gyr ago) followed by delayed quenching (8–10 Gyr ago), which is not commonly observed among the MW satellites. (iv) A comparison with TNG50 and FIRE-2 simulated satellite dwarfs around M31-like hosts shows that some of these trends (dependence of SFH on satellite luminosity) are reproduced in the simulations while others (dependence of SFH on galactocentric distance, presence of the delayed-quenching population) are weaker or absent. We provide all photometric catalogs and SFHs as High-Level Science Products on MAST. 

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