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1. Short GRB Host Galaxies. II. A Legacy Sample of Redshifts, Stellar Population Properties, and Implications for Their Neutron Star Merger Origins

   https://doi.org/10.3847/1538-4357/ac91d1  

   Nugent, Anya E. ; Fong, Wen-Fai ; Dong, Yuxin ; Leja, Joel ; Berger, Edo ; Zevin, Michael ; Chornock, Ryan ; Cobb, Bethany E. ; Kelley, Luke Zoltan ; Kilpatrick, Charles D. ; et al ( November 2022 , The Astrophysical Journal)

   We present the stellar population properties of 69 short gamma-ray burst (GRB) host galaxies, representing the largest uniformly modeled sample to date. Using theProspectorstellar population inference code, we jointly fit photometry and/or spectroscopy of each host galaxy. We find a population median redshift of$z={0.64}_{-0.32}^{+0.83}$(68% confidence), including nine photometric redshifts atz≳ 1. We further find a median mass-weighted age oft_m=${0.8}_{-0.53}^{+2.71}$Gyr, stellar mass of log(M_*/M_⊙) =${9.69}_{-0.65}^{+0.75}$, star formation rate of SFR =${1.44}_{-1.35}^{+9.37}$M_⊙yr⁻¹, stellar metallicity of log(Z_*/Z_⊙) =$-{0.38}_{-0.42}^{+0.44}$, and dust attenuation of$A_V={0.43}_{-0.36}^{+0.85}$mag (68% confidence). Overall, the majority of short GRB hosts are star-forming (≈84%), with small fractions that are either transitioning (≈6%) or quiescent (≈10%); however, we observe a much larger fraction (≈40%) of quiescent and transitioning hosts atz≲ 0.25, commensurate with galaxy evolution. We find that short GRB hosts populate the star-forming main sequence of normal field galaxies, but do not include as many high-mass galaxies as the general galaxy population, implying that their binary neutron star (BNS) merger progenitors are dependent on a combination of host star formation and stellar mass. The distribution of ages and redshifts implies a broad delay-time distribution,more »with a fast-merging channel atz> 1 and a decreased neutron star binary formation efficiency from high to low redshifts. If short GRB hosts are representative of BNS merger hosts within the horizon of current gravitational wave detectors, these results can inform future searches for electromagnetic counterparts. All of the data and modeling products are available on the Broadband Repository for Investigating Gamma-ray burst Host Traits website.

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2. Spectroscopic Confirmation of a Protocluster at z = 3.37 with a High Fraction of Quiescent Galaxies

   https://doi.org/10.3847/1538-4357/ac2b9f  

   McConachie, Ian ; Wilson, Gillian ; Forrest, Ben ; Marsan, Z. Cemile ; Muzzin, Adam ; Cooper, M. C. ; Annunziatella, Marianna ; Marchesini, Danilo ; Chan, Jeffrey C. C. ; Gomez, Percy ; et al ( February 2022 , The Astrophysical Journal)

   We report the discovery of MAGAZ3NE J095924+022537, a spectroscopically confirmed protocluster at$z={3.3665}_{-0.0012}^{+0.0009}$around a spectroscopically confirmedUVJ-quiescent ultramassive galaxy (UMG;$M_{\star }={2.34}_{-0.34}^{+0.23}\times {10}^{11}{M}_{\odot }$) in the COSMOS UltraVISTA field. We present a total of 38 protocluster members (14 spectroscopic and 24 photometric), including the UMG. Notably, and in marked contrast to protoclusters previously reported at this epoch that have been found to contain predominantly star-forming members, we measure an elevated fraction of quiescent galaxies relative to the coeval field (${73.3}_{-16.9}^{+26.7}\mathrm{\%}$versus${11.6}_{-4.9}^{+7.1}\mathrm{\%}$for galaxies with stellar massM_⋆≥ 10¹¹M_⊙). This high quenched fraction provides a striking and important counterexample to the seeming ubiquitousness of star-forming galaxies in protoclusters atz> 2 and suggests, rather, that protoclusters exist in a diversity of evolutionary states in the early universe. We discuss the possibility that we might be observing either “early mass quenching” or nonclassical “environmental quenching.” We also present the discovery of MAGAZ3NE J100028+023349, a second spectroscopically confirmed protocluster, at a very similar redshift of$z={3.3801}_{-0.0281}^{+0.0213}$. We present a total of 20 protocluster members, 12 of which are photometric and eight spectroscopic including a poststarburst UMG ($M_{\star }={2.95}_{-0.20}^{+0.21}\times {10}^{11}{M}_{\odot }$). Protoclusters MAGAZ3NE J0959more »and MAGAZ3NE J1000 are separated by 18′ on the sky (35 comoving Mpc), in good agreement with predictions from simulations for the size of “Coma”-type cluster progenitors at this epoch. It is highly likely that the two UMGs are the progenitors of Brightest Cluster Galaxies seen in massive virialized clusters at lower redshift.

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3. Now You See It, Now You Don’t: Star Formation Truncation Precedes the Loss of Molecular Gas by ∼100 Myr in Massive Poststarburst Galaxies at z ∼ 0.6

   https://doi.org/10.3847/1538-4357/ac3dfa  

   Bezanson, Rachel ; Spilker, Justin S. ; Suess, Katherine A. ; Setton, David J. ; Feldmann, Robert ; Greene, Jenny E. ; Kriek, Mariska ; Narayanan, Desika ; Verrico, Margaret ( February 2022 , The Astrophysical Journal)

   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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4. Evidence for AGN-regulated Cooling in Clusters at z ∼ 1.4: A Multiwavelength View of SPT-CL J0607-4448

   https://doi.org/10.3847/1538-4357/acae9e  

   Masterson, Megan ; McDonald, Michael ; Ansarinejad, Behzad ; Bayliss, Matthew ; Benson, Bradford A. ; Bleem, Lindsey E. ; Calzadilla, Michael S. ; Edge, Alastair C. ; Floyd, Benjamin ; Kim, Keunho J. ; et al ( February 2023 , The Astrophysical Journal)

   We present a multiwavelength analysis of the galaxy cluster SPT-CL J0607-4448 (SPT0607), which is one of the most distant clusters discovered by the South Pole Telescope atz= 1.4010 ± 0.0028. The high-redshift cluster shows clear signs of being relaxed with well-regulated feedback from the active galactic nucleus (AGN) in the brightest cluster galaxy (BCG). Using Chandra X-ray data, we construct thermodynamic profiles and determine the properties of the intracluster medium. The cool-core nature of the cluster is supported by a centrally peaked density profile and low central entropy ($K_0={18}_{-9}^{+11}$keV cm²), which we estimate assuming an isothermal temperature profile due to the limited spectral information given the distance to the cluster. Using the density profile and gas cooling time inferred from the X-ray data, we find a mass-cooling rate${\dot{M}}_{\mathrm{cool}}={100}_{-60}^{+90}{M}_{\odot }$yr⁻¹. From optical spectroscopy and photometry around the [Oii] emission line, we estimate that the BCG star formation rate is${\mathrm{SFR}}_{[\mathrm{O}\mathrm{II}]}={1.7}_{-0.6}^{+1.0}{M}_{\odot }$yr⁻¹, roughly two orders of magnitude lower than the predicted mass-cooling rate. In addition, using ATCA radio data at 2.1 GHz, we measure a radio jet power$P_{\mathrm{cav}}={3.2}_{-1.3}^{+2.1}\times {10}^{44}$erg s⁻¹, which is consistent withmore »the X-ray cooling luminosity ($L_{\mathrm{cool}}={1.9}_{-0.5}^{+0.2}\times {10}^{44}$erg s⁻¹withinr_cool= 43 kpc). These findings suggest that SPT0607 is a relaxed, cool-core cluster with AGN-regulated cooling at an epoch shortly after cluster formation, implying that the balance between cooling and feedback can be reached quickly. We discuss the implications for these findings on the evolution of AGN feedback in galaxy clusters.

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5. 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)

   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 ($\log (M_{\star }/M_{\odot })$> 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∼more »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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