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1. 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.; Gomez, Percy; et al (February 2022, The Astrophysical Journal)

   Abstract We report the discovery of MAGAZ3NE J095924+022537, a spectroscopically confirmed protocluster at z = 3.3665 − 0.0012 + 0.0009 around a spectroscopically confirmed UVJ -quiescent ultramassive galaxy (UMG; M ⋆ = 2.34 − 0.34 + 0.23 × 10 11 M ⊙ ) 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 % versus 11.6 − 4.9 + 7.1 % for galaxies with stellar mass M ⋆ ≥ 10 11 M ⊙ ). This high quenched fraction provides a striking and important counterexample to the seeming ubiquitousness of star-forming galaxies in protoclusters at z > 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 ⋆ = 2.95 − 0.20 + 0.21 × 10 11 M ⊙ ). Protoclusters MAGAZ3NE J0959 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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2. Protoclusters as drivers of stellar mass growth in the early Universe, a case study: Taralay – a massive protocluster at z ∼ 4.57

   https://doi.org/10.1093/mnras/stae301  

   Staab, Priti; Lemaux, Brian C; Forrest, Ben; Shah, Ekta; Cucciati, Olga; Lubin, Lori; Gal, Roy R; Hung, Denise; Shen, Lu; Giddings, Finn; et al (February 2024, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Simulations predict that the galaxy populations inhabiting protoclusters may contribute considerably to the total amount of stellar mass growth of galaxies in the early universe. In this study, we test these predictions observationally, using the Taralay protocluster (formerly PCl J1001+0220) at z ∼ 4.57 in the COSMOS field. With the Charting Cluster Construction with VUDS and ORELSE (C3VO) survey, we spectroscopically confirmed 44 galaxies within the adopted redshift range of the protocluster (4.48 < z < 4.64) and incorporate an additional 18 galaxies from ancillary spectroscopic surveys. Using a density mapping technique, we estimate the total mass of Taralay to be ∼1.7 × 1015 M⊙, sufficient to form a massive cluster by the present day. By comparing the star formation rate density (SFRD) within the protocluster (SFRDpc) to that of the coeval field (SFRDfield), we find that SFRDpc surpasses the SFRDfield by Δlog (SFRD/M⊙yr−1 Mpc−3) = 1.08 ± 0.32 (or ∼12 ×). The observed contribution fraction of protoclusters to the cosmic SFRD adopting Taralay as a proxy for typical protoclusters is $$33.5~{{\ \rm per\ cent}}^{+8.0~{{\ \rm per\ cent}}}_{-4.3~{{\ \rm per\ cent}}}$$, a value ∼2σ higher than the predictions from simulations. Taralay contains three peaks that are 5σ above the average density at these redshifts. Their SFRD is ∼0.5 dex higher than the value derived for the overall protocluster. We show that 68 per cent of all star formation in the protocluster takes place within these peaks, and that the innermost regions of the peaks encase $$\sim 50~{{\ \rm per\ cent}}$$ of the total star formation in the protocluster. This study strongly suggests that protoclusters drive stellar mass growth in the early universe and that this growth may proceed in an inside-out manner. 

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3. The stellar mass function of quiescent galaxies in 2 < z < 2.5 protoclusters

   https://doi.org/10.1093/mnras/stad3751  

   Edward, Adit H; Balogh, Michael L; Bahé, Yannick M; Cooper, M C; Hatch, Nina A; Marchioni, Justin; Muzzin, Adam; Noble, Allison; Rudnick, Gregory H; Vulcani, Benedetta; et al (November 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present an analysis of the galaxy stellar mass function (SMF) of 14 known protoclusters between 2.0 < z < 2.5 in the COSMOS field, down to a mass limit of 109.5 M⊙. We use existing photometric redshifts with a statistical background subtraction, and consider star-forming and quiescent galaxies identified from (NUV − r) and (r − J) colours separately. Our fiducial sample includes galaxies within 1 Mpc of the cluster centres. The shape of the protocluster SMF of star-forming galaxies is indistinguishable from that of the general field at this redshift. Quiescent galaxies, however, show a flatter SMF than in the field, with an upturn at low mass, though this is only significant at ∼2σ. There is no strong evidence for a dominant population of quiescent galaxies at any mass, with a fraction <15 per cent at 1σ confidence for galaxies with log M*/M⊙ < 10.5. We compare our results with a sample of galaxy groups at 1 < z < 1.5, and demonstrate that a significant amount of environmental quenching must take place between these epochs, increasing the relative abundance of high-mass ($$\rm M_{\ast } \gt 10^{10.5} {\rm M}_{\odot }$$) quiescent galaxies by a factor ≳ 2. However, we find that at lower masses ($$\rm M_{\ast } \lt 10^{10.5} {\rm M}_{\odot }$$), no additional environmental quenching is required. 

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4. The FENIKS Survey: Spectroscopic Confirmation of Massive Quiescent Galaxies at z~3-5

   https://doi.org/10.3847/1538-4357/ad8b30  

   Antwi-Danso, Jacqueline; Papovich, Casey; Esdaile, James; Nanayakkara, Themiya; Glazebrook, Karl; Hutchison, Taylor A; Whitaker, Katherine E; Marsan, Z Cemile; Diaz, Ruben J; Marchesini, Danilo; et al (December 2024, The Astrophysical Journal)

   The measured ages of massive, quiescent galaxies at z∼3-4 imply that massive galaxies quench as early as z~6. While the number of spectroscopic confirmations of quiescent galaxies at z<3 has increased over the years, there are only a handful at z> 3.5. We report spectroscopic redshifts of one secure (z=3.757) and two tentative (z=3.336 and z=4.673) massive quiescent galaxies with 11 hr of Keck/MOSFIRE K-band observations. Our candidates were selected from the FLAMINGOS-2 Extragalactic Near-Infrared K-band Split (FENIKS) survey, which uses deep Gemini/Flamingos-2 Kb/Kr imaging optimized for increased sensitivity to the characteristic red colors of galaxies at z>3 with a strong Balmer/4000 Å break. The rest-frame UVJ and ugi colors of three out of four quiescent candidates are consistent with 1-2 Gyr old stellar populations. This places these galaxies as the oldest objects at these redshifts, and challenges the notion that quiescent galaxies at z>3 are all recently quenched, post-starburst galaxies. Our spectroscopy shows that the other quiescent-galaxy candidate is a broad-line active galactic nucleus (z=3.594) with strong, redshifted Hbeta+[OIII] emission with a velocity offset >1000 km/s, indicative of a powerful outflow. The star formation history of our highest redshift candidate suggests that its progenitor was already in place by z~7-11, reaching ∼10^11 M⊙ by z~8. These observations reveal the limit of what is possible with deep near-infrared photometry and targeted spectroscopy from the ground and demonstrate that secure spectroscopic confirmation of quiescent galaxies at z>4 is feasible only with JWST. 

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5. Beyond UVJ: Color Selection of Galaxies in the JWST Era

   https://doi.org/10.3847/1538-4357/aca294  

   Antwi-Danso, Jacqueline; Papovich, Casey; Leja, Joel; Marchesini, Danilo; Marsan, Z C; Martis, Nicholas S.; Labbe, Ivo; Muzzin, Adam; Glazebrook, Karl; Straatman, Caroline M.; et al (February 2023, The Astrophysical journal)

   We present a new rest-frame color–color selection method using synthetic us − gs and gs − is, (ugi)s colors to identify star-forming and quiescent galaxies. Our method is similar to the widely used U − V versus V − J (UVJ) diagram. However, UVJ suffers known systematics. Spectroscopic campaigns have shown that UVJ-selected quiescent samples at z ≳ 3 include ∼10%–30% contamination from galaxies with dust-obscured star formation and strong emission lines. Moreover, at z > 3, UVJ colors are extrapolated because the rest-frame band shifts beyond the coverage of the deepest bandpasses at <5 μm (typically Spitzer/IRAC 4.5 μm or future JWST/NIRCam observations). We demonstrate that (ugi)s offers improvements to UVJ at z > 3, and can be applied to galaxies in the JWST era. We apply (ugi)s selection to galaxies at 0.5 < z < 6 from the (observed) 3D-HST and UltraVISTA catalogs, and to the (simulated) JAGUAR catalogs. We show that extrapolation can affect (V − J)0 color by up to 1 mag, but changes $${({g}_{s}-{i}_{s})}_{0}$$ color by ≤0.2 mag, even at z ≃ 6. While (ugi)s-selected quiescent samples are comparable to UVJ in completeness (both achieve ∼85%–90% at z = 3–3.5), (ugi)s reduces contamination in quiescent samples by nearly a factor of 2, from ≃35% to ≃17% at z = 3, and from ≃60% to ≃33% at z = 6. This leads to improvements in the true-to-false-positive ratio (TP/FP), where we find TP/FP ≳2.2 for (ugi)s at z ≃ 3.5 − 6, compared to TP/FP < 1 for UVJ-selected samples. This indicates that contaminants will outnumber true quiescent galaxies in UVJ at these redshifts, while (ugi)s will provide higher-fidelity samples. 

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