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1. Identification and characterization of six spectroscopically confirmed massive protostructures at 2.5 < z < 4.5

   https://doi.org/10.1093/mnras/stae519  

   Shah, Ekta A; Lemaux, Brian; Forrest, Benjamin; Cucciati, Olga; Hung, Denise; Staab, Priti; Hathi, Nimish; Lubin, Lori; Gal, Roy R; Shen, Lu; et al (March 2024, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present six spectroscopically confirmed massive protostructures, spanning a redshift range of 2.5 < z < 4.5 in the Extended Chandra Deep Field South (ECDFS) field discovered as part of the Charting Cluster Construction in VUDS and ORELSE (C3VO) survey. We identify and characterize these remarkable systems by applying an overdensity measurement technique on an extensive data compilation of public and proprietary spectroscopic and photometric observations in this highly studied extragalactic field. Each of these six protostructures, i.e. a large scale overdensity (volume >9000 cMpc3) of more than 2.5σδ above the field density levels at these redshifts, have a total mass Mtot ≥ 1014.8 M⊙ and one or more highly overdense (overdensity$$\, \gt 5\sigma _{\delta }$$) peaks. One of the most complex protostructures discovered is a massive (Mtot = 1015.1M⊙) system at z ∼ 3.47 that contains six peaks and 55 spectroscopic members. We also discover protostructures at z ∼ 3.30 and z ∼ 3.70 that appear to at least partially overlap on sky with the protostructure at z ∼ 3.47, suggesting a possible connection. We additionally report on the discovery of three massive protostructures at z = 2.67, 2.80, and 4.14 and discuss their properties. Finally, we discuss the relationship between star formation rate and environment in the richest of these protostructures, finding an enhancement of star formation activity in the densest regions. The diversity of the protostructures reported here provide an opportunity to study the complex effects of dense environments on galaxy evolution over a large redshift range in the early Universe. 

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2. LATIS: Galaxy–Environment Relations at Cosmic Noon and the Role of Sample Selection

   https://doi.org/10.3847/1538-4357/ae10b7  

   Chartab, Nima; Newman, Andrew B; Rudie, Gwen C; Blanc, Guillermo A; Kelson, Daniel D; Qezlou, Mahdi; Bird, Simeon; Lemaux, Brian C; Cucciati, Olga (November 2025, The Astrophysical Journal)

   Abstract We investigate the environmental dependence of galaxy properties atz ∼ 2.5 using the LyαTomography IMACS Survey (LATIS), which provides high-resolution three-dimensional maps of intergalactic medium (IGM) overdensity via Lyαforest tomography. Our analysis focuses on a UV-selected spectroscopic sample of 2185 galaxies from LATIS and a complementary set of 1157 galaxies from heterogeneous spectroscopic surveys in the COSMOS field. We compare these data sets to forward-modeled mock catalogs constructed from the IllustrisTNG300-1 simulation, incorporating realistic selection functions to match both LATIS and the literature sample. While the mass-complete simulation predicts strong environmental trends—more massive and quiescent galaxies preferentially occupy overdense regions—we find that such trends are significantly weaker or absent in the observed samples. The LATIS galaxies show no measurable correlation between specific star formation rate (sSFR) and IGM overdensity, a result reproduced by LATIS-like mock catalogs, confirming that UV selection systematically excludes passive and dusty galaxies in dense environments. The literature compilation, despite improved high-mass coverage, remains incomplete and affected by similar biases. We also analyze a mass-complete photometric sample from the COSMOS-Web catalog atz ∼ 2.5 and find no detectable sSFR–environment relation, a null result that our simulations indicate can be explained by photometric redshift uncertainties. In particular, we find no evidence for a reversal of the sSFR–density relation at cosmic noon. These results demonstrate that observed correlations can be heavily shaped by selection effects and caution against inferring physical trends from incomplete spectroscopic samples. Deeper, more representative spectroscopic surveys are needed to robustly characterize environmental effects at this epoch. 

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3. Environmental Effects on the Stellar Mass Function in a z ∼ 3.3 Overdensity of Galaxies in the COSMOS Field*

   https://doi.org/10.3847/1538-4357/ad5e78  

   Forrest, Ben; Lemaux, Brian C; Shah, Ekta A; Staab, Priti; Gal, Roy R; Lubin, Lori M; Cooper, M C; Cucciati, Olga; Hung, Denise; McConachie, Ian; et al (August 2024, The Astrophysical Journal)

   Abstract We present an analysis of the number density of galaxies as a function of stellar mass (i.e., the stellar mass function (SMF)) in the COSMOS field atz∼ 3.3, making a comparison between the SMF in overdense environments and the SMF in the coeval field. In particular, this region contains the Elentári proto-supercluster, a system of six extended overdensities spanning ∼70 cMpc on a side. A clear difference is seen in the high-mass slope of these SMFs, with overdense regions showing an increase in the ratio of high-mass galaxies to low-mass galaxies relative to the field, indicating a more rapid buildup of stellar mass in overdense environments. This result qualitatively agrees with analyses of clusters atz∼ 1, though the differences between protocluster and field SMFs atz∼ 3.3 are smaller. While this is consistent with overdensities enhancing the evolution of their member galaxies, potentially through increased merger rates, whether this enhancement begins in protocluster environments or even earlier in group environments is still unclear. Though the measured fractions of quiescent galaxies between the field and overdense environments do not vary significantly, implying that this stellar mass enhancement is ongoing and any starbursts triggered by merger activity have not yet quenched, we note that spectroscopic observations are biased toward star-forming populations, particularly for low-mass galaxies. If mergers are indeed responsible, high-resolution imaging of Elentári and similar structures at these early epochs should then reveal increased merger rates relative to the field. Larger samples of well-characterized overdensities are necessary to draw broader conclusions in these areas. 

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4. An ALMA survey of submillimetre galaxies in the Extended Chandra Deep Field South: an unbiased study of SMG environments measured with narrow-band imaging

   https://doi.org/10.1093/mnras/stae1868  

   Cornish, Thomas M; Wardlow, Julie; Wade, Heather; Sobral, David; Brandt, William N; Cox, Pierre; Dannerbauer, Helmut; Decarli, Roberto; Gullberg, Bitten; Knudsen, Kirsten; et al (August 2024, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Submillimetre galaxies (SMGs) are some of the most extreme star-forming systems in the Universe, whose place in the framework of galaxy evolution is as yet uncertain. It has been hypothesized that SMGs are progenitors of local early-type galaxies, requiring that SMGs generally reside in galaxy cluster progenitors at high redshift. We test this hypothesis and explore SMG environments using a narrow-band VLT/HAWK-I+GRAAL study of H $$\alpha$$ and [O iii] emitters around an unbiased sample of three ALMA-identified and spectroscopically confirmed SMGs at $$z \sim 2.3$$ and $$\sim 3.3$$, where these SMGs were selected solely on spectroscopic redshift. Comparing with blank-field observations at similar epochs, we find that one of the three SMGs lies in an overdensity of emission-line sources on the $$\sim 4$$ Mpc scale of the HAWK-I field of view, with overdensity parameter $$\delta _{g} = 2.6^{+1.4}_{-1.2}$$. A second SMG is significantly overdense only on $$\lesssim 1.6$$ Mpc scales and the final SMG is consistent with residing in a blank field environment. The total masses of the two overdensities are estimated to be $$\log (M_{h}/{\rm M}_{\odot }) =$$ 12.1–14.4, leading to present-day masses of $$\log (M_{h,z=0}/{\rm M}_{\odot }) =$$ 12.9–15.9. These results imply that SMGs occupy a range of environments, from overdense protoclusters or protogroups to the blank field, suggesting that while some SMGs are strong candidates for the progenitors of massive elliptical galaxies in clusters, this may not be their only possible evolutionary pathway. 

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5. A Quasar-anchored Protocluster at z = 6.6 in the ASPIRE Survey. I. Properties of [O iii ] Emitters in a 10 Mpc Overdensity Structure

   https://doi.org/10.3847/1538-4357/adb1bd  

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

   Abstract A SPectroscopic survey of bIased halos in the Reionization Era is a quasar legacy survey primarily using JWST to target a sample of 25z > 6 quasars with NIRCam slitless spectroscopy and imaging. The first study in this series found evidence of a strong overdensity of galaxies around J0305−3150, a luminous quasar atz= 6.61, within a single NIRCam pointing obtained in JWST Cycle 1. Here we present the first results of a JWST Cycle 2 mosaic that covers 35 arcmin²with NIRCam imaging/wide-field slitless spectroscopy of the same field to investigate the spatial extent of the putative protocluster. The F356W grism data target [Oiii]+Hβat 5.3 < z < 7 and reveal a population of 124 line emitters down to a flux limit of 1.2 × 10⁻¹⁸erg s⁻¹cm⁻². Fifty-three of these galaxies lie at 6.5 < z < 6.8 spanning 10 cMpc on the sky, corresponding to an overdensity within a 2500 cMpc³volume of 12.5 ± 2.6, anchored by the quasar. Comparing to the [Oiii] luminosity function from the Emission line galaxies and Intergalactic Gas in the Epoch of Reionization project, we find a dearth of faint [Oiii] emitters at log(L/erg s⁻¹) < 42.3, which we suggest is consistent with either bursty star formation causing galaxies to scatter around the grism detection limit or modest suppression from quasar feedback. While we find a strong filamentary overdensity of [Oiii] emitters consistent with a protocluster, we suggest that we could be insensitive to a population of older, more massive Lyman break galaxies with weak nebular emission on scales >​​​​​​10 cMpc. 

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