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1. Rapid build-up of the stellar content in the protocluster core SPT2349−56 at z = 4.3

   https://doi.org/10.1093/mnras/stab3539  

   Hill, Ryley; Chapman, Scott; Phadke, Kedar A.; Aravena, Manuel; Archipley, Melanie; Ashby, Matthew L. N.; Béthermin, Matthieu; Canning, Rebecca E. A.; Gonzalez, Anthony; Greve, Thomas R.; et al (December 2021, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The protocluster SPT2349−56 at $z = 4.3$ contains one of the most actively star-forming cores known, yet constraints on the total stellar mass of this system are highly uncertain. We have therefore carried out deep optical and infrared observations of this system, probing rest-frame ultraviolet to infrared wavelengths. Using the positions of the spectroscopically confirmed protocluster members, we identify counterparts and perform detailed source deblending, allowing us to fit spectral energy distributions in order to estimate stellar masses. We show that the galaxies in SPT2349−56 have stellar masses proportional to their high star formation rates, consistent with other protocluster galaxies and field submillimetre galaxies (SMGs) around redshift 4. The galaxies in SPT2349−56 have on average lower molecular gas-to-stellar mass fractions and depletion time-scales than field SMGs, although with considerable scatter. We construct the stellar-mass function for SPT2349−56 and compare it to the stellar-mass function of $z = 1$ galaxy clusters, finding consistent shapes between the two. We measure rest-frame galaxy ultraviolet half-light radii from our HST-F160W imaging, finding that on average the galaxies in our sample are similar in size to typical star-forming galaxies at these redshifts. However, the brightest HST-detected galaxy in our sample, found near the luminosity-weighted centre of the protocluster core, remains unresolved at this wavelength. Hydrodynamical simulations predict that the core galaxies will quickly merge into a brightest cluster galaxy, thus our observations provide a direct view of the early formation mechanisms of this class of object. 

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2. Spectroscopic Confirmation of an Ultra-Massive Galaxy in a Protocluster at $z\sim 4.9$

   https://doi.org/10.33232/001c.120087  

   Urbano_Stawinski, Stephanie M; Cooper, M C; Forrest, Ben; Muzzin, Adam; Marchesini, Danilo; Wilson, Gillian; Gomez, Percy; McConachie, Ian; Marsan, Z Cemile; Annuziatella, Marianna; et al (June 2024, The Open Journal of Astrophysics)

   We present spectroscopic confirmation of an ultra-massive galaxy (UMG) with $log\left(M_{\star }/M_{\odot }\right)=10.98\pm 0.07$at $z_{\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{c}}=4.8947$in the Extended Groth Strip (EGS), based on deep observations of Ly $\alpha$emission with Keck/DEIMOS. The ultra-massive galaxy (UMG-28740) is the most massive member in one of the most significant overdensities in the EGS, with four additional photometric members with $log\left(M_{\star }/M_{\odot }\right)>10.5$within $R_{\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{j}}\sim 1$cMpc. Spectral energy distribution (SED) fitting using a large suite of star formation histories and two sets of high-quality photometry from ground- and space-based facilities consistently estimates the mass of this object to be $log\left(M_{\star }/M_{\odot }\right)\sim 11$with a small standard deviation between measurements ( $\sigma =0.07$). While the best-fit SED models agree on stellar mass, we find discrepancies in the estimated star formation rate for UMG-28740, resulting in either a star-forming or quiescent system. $𝐽𝑊𝑆𝑇$/NIRCam photometry of UMG-28740 strongly favors a quiescent scenario, demonstrating the need for high-quality mid-IR observations. Assuming the galaxy to be quiescent, UMG-28740 formed the bulk of its stars at $z>10$and is quenching at $z\sim 8$, resulting in a high star formation efficiency at high redshift ( $ϵ\sim 0.2$at $z\sim 5$and $ϵ\gtrsim 1$at $z\gtrsim 8$). As the most massive galaxy in its protocluster environment, UMG-28740 is a unique example of the impossibly early galaxy problem. 

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3. A Quasar-anchored Protocluster at z = 6.6 in the ASPIRE Survey. II. An Environmental Analysis of Galaxy Properties in an Overdense Structure

   https://doi.org/10.3847/1538-4357/adb1bc  

   Champagne, Jaclyn B; Wang, Feige; Yang, Jinyi; Fan, Xiaohui; Hennawi, Joseph F; Sun, Fengwu; Bañados, Eduardo; Bosman, Sarah_E I; Costa, Tiago; Habouzit, Melanie; et al (March 2025, The Astrophysical Journal)

   Abstract We present in this paper (Paper II of the series) a 35 arcmin²JWST/NIRCam imaging and wide-field slitless spectroscopy mosaic centered on J0305–3150, a luminous quasar atz= 6.61. The F356W grism data reveal 124 [Oiii]+Hβemitters at 5.3 < z < 7, 53 of which constitute a protocluster spanning (10 cMpc)²across 6.5 < z < 6.8. We find no evidence of any broad-line active galactic nucleus (AGN) in individual galaxies or stacking, reporting a median HβFWHM of 585 ± 152 km s⁻¹; however, the mass–excitation diagram and “little red dot” color and compactness criteria suggest that there are a few AGN candidates on the outskirts of the protocluster. We fit the spectral energy distributions (SEDs) of the [Oiii] emitters withProspectorandBagpipesand find that none of the SED-derived properties (stellar mass, age, or star formation rate) correlate with proximity to the quasar. While there is no correlation between galaxy age and local galaxy density, we find modest correlations of local galaxy density with increasing stellar mass, decreasing 10–100 Myr star formation rate ratios, and decreasing nebular line equivalent widths. We further find that the protocluster galaxies are consistent with being more massive, being older, and hosting higher star formation rates than the field sample at the 3σlevel, distributed in a filamentary structure that supports inside-out formation of the protocluster. There is modest evidence that galaxy evolution proceeds differently as a function of the density of local environment within protoclusters during the epoch of reionization, and the central quasar has little effect on the galaxy properties of the surrounding structure. 

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4. The Massive and Quiescent Elliptical Host Galaxy of the Repeating Fast Radio Burst FRB 20240209A

   https://doi.org/10.3847/2041-8213/ad9de2  

   Eftekhari, T.; Dong_董, Y_雨欣; Fong, W.; Shah, V.; Simha, S.; Andersen, B_C; Andrew, S.; Bhardwaj, M.; Cassanelli, T.; Chatterjee, S.; et al (January 2025, The Astrophysical Journal Letters)

   Abstract The discovery and localization of FRB 20240209A by the Canadian Hydrogen Intensity Mapping Fast Radio Burst (CHIME/FRB) experiment marks the first repeating FRB localized with the CHIME/FRB Outriggers and adds to the small sample of repeating FRBs with associated host galaxies. Here we present Keck and Gemini observations of the host that reveal a redshiftz = 0.1384 ± 0.0004. We perform stellar population modeling to jointly fit the optical through mid-IR data of the host and infer a median stellar mass log(M_*/M_⊙) = 11.35 ± 0.01 and a mass-weighted stellar population age  ~11 Gyr, corresponding to the most massive and oldest FRB host discovered to date. Coupled with a star formation rate  <0.31M_⊙yr⁻¹, the specific star formation rate  <10^−11.9yr⁻¹classifies the host as quiescent. Through surface brightness profile modeling, we determine an elliptical galaxy morphology, marking the host as the first confirmed elliptical FRB host. The discovery of a quiescent early-type host galaxy within a transient class predominantly characterized by late-type star-forming hosts is reminiscent of short-duration gamma-ray bursts, Type Ia supernovae, and ultraluminous X-ray sources. Based on these shared host demographics, coupled with a large offset as demonstrated in our companion Letter, we conclude that preferred sources for FRB 20240209A include magnetars formed through merging binary neutron stars/white dwarfs or the accretion-induced collapse of a white dwarf, or a luminous X-ray binary. Together with FRB 20200120E localized to a globular cluster in M81, our findings provide strong evidence that some fraction of FRBs may arise from a process distinct from the core collapse of massive stars. 

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5. The ALMA REBELS Survey: specific star formation rates in the reionization era

   https://doi.org/10.1093/mnras/stac2291  

   Topping, Michael W.; Stark, Daniel P.; Endsley, Ryan; Bouwens, Rychard J.; Schouws, Sander; Smit, Renske; Stefanon, Mauro; Inami, Hanae; Bowler, Rebecca A. A.; Oesch, Pascal; et al (August 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present specific star formation rates (sSFRs) for 40 ultraviolet (UV)-bright galaxies at z ∼ 7–8 observed as part of the Reionization Era Bright Emission Line Survey (REBELS) Atacama Large Millimeter/submillimeter Array (ALMA) large programme. The sSFRs are derived using improved star formation rate (SFR) calibrations and spectral energy distribution (SED)-based stellar masses, made possible by measurements of far-infrared (FIR) continuum emission and [C ii]-based spectroscopic redshifts. The median sSFR of the sample is $$18_{-5}^{+7}$$ Gyr−1, significantly larger than literature measurements lacking constraints in the FIR, reflecting the larger obscured SFRs derived from the dust continuum relative to that implied by the UV+optical SED. We suggest that such differences may reflect spatial variations in dust across these luminous galaxies, with the component dominating the FIR distinct from that dominating the UV. We demonstrate that the inferred stellar masses (and hence sSFRs) are strongly dependent on the assumed star formation history in reionization-era galaxies. When large sSFR galaxies (a population that is common at z > 6) are modelled with non-parametric star formation histories, the derived stellar masses can increase by an order of magnitude relative to constant star formation models, owing to the presence of a significant old stellar population that is outshined by the recent burst. The [C ii] line widths in the largest sSFR systems are often very broad, suggesting dynamical masses capable of accommodating an old stellar population suggested by non-parametric models. Regardless of these systematic uncertainties among derived parameters, we find that sSFRs increase rapidly toward higher redshifts for massive galaxies (9.6 < log (M*/M⊙) < 9.8), evolving as (1 + z)1.7 ± 0.3, broadly consistent with expectations from the evolving baryon accretion rates. 

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