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1. Galaxy populations in the most distant SPT-SZ clusters: II. Galaxy structural properties in massive clusters at 1.4 ≲ z ≲ 1.7

   https://doi.org/10.1051/0004-6361/202245268  

   Strazzullo, V.; Pannella, M.; Mohr, J. J.; Saro, A.; Ashby, M. L.; Bayliss, M. B.; Canning, R. E.; Floyd, B.; Gonzalez, A. H.; Khullar, G.; et al (January 2023, Astronomy & Astrophysics)

   We investigate structural properties of massive galaxy populations in the central regions (< 0.7  r 500 ) of five very massive ( M 200  > 4 × 10 14   M ⊙ ), high-redshift (1.4 ≲  z  ≲ 1.7) galaxy clusters from the 2500 deg 2 South Pole Telescope Sunyaev Zel’dovich effect (SPT-SZ) survey. We probe the connection between galaxy structure and broad stellar population properties at stellar masses of log( M / M ⊙ ) > 10.85. We find that quiescent and star-forming cluster galaxy populations are largely dominated by bulge- and disk-dominated sources, respectively, with relative contributions being fully consistent with those of field counterparts. At the same time, the enhanced quiescent galaxy fraction observed in these clusters with respect to the coeval field is reflected in a significant morphology-density relation, with bulge-dominated galaxies already clearly dominating the massive galaxy population in these clusters at z  ∼ 1.5. At face value, these observations show no significant environmental signatures in the correlation between broad structural and stellar population properties. In particular, the Sersic index and axis ratio distribution of massive, quiescent sources are consistent with field counterparts, in spite of the enhanced quiescent galaxy fraction in clusters. This consistency suggests a tight connection between quenching and structural evolution towards a bulge-dominated morphology, at least in the probed cluster regions and galaxy stellar mass range, irrespective of environment-related processes affecting star formation in cluster galaxies. We also probe the stellar mass–size relation of cluster galaxies, and find that star-forming and quiescent sources populate the mass–size plane in a manner largely similar to their field counterparts, with no evidence of a significant size difference for any probed sub-population. In particular, both quiescent and bulge-dominated cluster galaxies have average sizes at fixed stellar mass consistent with their counterparts in the field. 

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2. Spitzer -selected z > 1.3 protocluster candidates in the LSST Deep Drilling Fields

   https://doi.org/10.1093/mnras/stad3885  

   Gully, Harry; Hatch, Nina; Bahé, Yannick; Balogh, Michael; Bolzonella, Micol; Cooper, M. C.; Muzzin, Adam; Pozzetti, Lucia; Rudnick, Gregory; Vulcani, Benedetta; et al (December 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We have identified 189 candidate z > 1.3 protoclusters and clusters in the LSST Deep Drilling Fields. This sample will enable the measurement of the metal enrichment and star formation history of clusters during their early assembly period through the direct measurement of the rate of supernovae identified through the LSST. The protocluster sample was selected from galaxy overdensities in a Spitzer/IRAC colour-selected sample using criteria that were optimized for protocluster purity using a realistic light-cone. Our tests reveal that $$60\!-\!80~{{\ \rm per\ cent}}$$ of the identified candidates are likely to be genuine protoclusters or clusters, which is corroborated by a ∼4σ stacked X-ray signal from these structures. We provide photometric redshift estimates for 47 candidates which exhibit strong peaks in the photo-z distribution of their candidate members. However, the lack of a photo-z peak does not mean a candidate is not genuine, since we find a stacked X-ray signal of similar significance from both the candidates that exhibit photo-z peaks and those that do not. Tests on the light-cone reveal that our pursuit of a pure sample of protoclusters results in that sample being highly incomplete ($$\sim 4~{{\ \rm per\ cent}}$$) and heavily biased towards larger, richer, more massive, and more centrally concentrated protoclusters than the total protocluster population. Most ($$\sim 75~{{\ \rm per\ cent}}$$) of the selected protoclusters are likely to have a maximum collapsed halo mass of between 1013 and 1014 M⊙, with only $$\sim 25~{{\ \rm per\ cent}}$$ likely to be collapsed clusters above 1014 M⊙. However, the aforementioned bias ensures our sample is $$\sim 50~{{\ \rm per\ cent}}$$ complete for structures that have already collapsed into clusters more massive than 1014 M⊙. 

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3. Fast supermassive black hole growth in the SPT2349–56 protocluster at z = 4.3

   https://doi.org/10.1051/0004-6361/202450225  

   Vito, F; Brandt, W N; Comastri, A; Gilli, R; Ivison, R J; Lanzuisi, G; Lehmer, B D; Lopez, I E; Tozzi, P; Vignali, C (September 2024, Astronomy & Astrophysics)

   Context.Large-scale environment is one of the main physical drivers of galaxy evolution. The densest regions at high redshifts (i.e.z > 2 protoclusters) are gas-rich regions characterised by high star formation activity. The same physical properties that enhance star formation in protoclusters are also thought to boost the growth of supermassive black holes (SMBHs), most likely in heavily obscured conditions. Aims.We aim to test this scenario by probing the active galactic nucleus (AGN) content of SPT2349–56: a massive, gas-rich, and highly star-forming protocluster core atz = 4.3 discovered as an overdensity of dusty star-forming galaxies (DSFGs). We compare our results with data on the field environment and other protoclusters. Methods.We observed SPT2349–56 withChandra(200 ks) and searched for X-ray emission from the known galaxy members. We also performed a spectral energy distribution fitting procedure to derive the physical properties of the discovered AGNs. Results.In the X-ray band, we detected two protocluster members: C1 and C6, corresponding to an AGN fraction among DSFGs in the structure of ≈10%. This value is consistent with other protoclusters atz  =  2 − 4, but higher than the AGN incidence among DSFGs in the field environment. Both AGNs are heavily obscured sources, hosted in star-forming galaxies with ≈3 × 10¹⁰ M_⊙stellar masses. We estimate that the intergalactic medium in the host galaxies contributes to a significant fraction (or even entirely) to the nuclear obscuration. In particular, C1 is a highly luminous (L_X = 2 × 10⁴⁵ erg s⁻¹) and Compton-thick (N_H = 2 × 10²⁴ cm⁻²) AGN, likely powered by aM_BH > 6 × 10⁸ M_⊙SMBH, assuming Eddington-limited accretion. Its high accretion rate suggests that it is in the phase of efficient growth that is generally required to explain the presence of extremely massive SMBHs in the centres of local galaxy clusters. Considering SPT2349–56 and DRC, a similar protocuster atz = 4, and under different assumptions on their volumes, we find that gas-rich protocluster cores atz ≈ 4 enhance the triggering of luminous (logL_X/erg s⁻¹ = 45 − 46) AGNs by three to five orders of magnitude with respect to the predictions from the AGN X-ray luminosity function at a similar redshift in the field environment. We note that this result is not solely driven by the overdensity of the galaxy population in the structures. Conclusions.Our results indicate that gas-rich protoclusters at high redshift boost the growth of SMBHs, which will likely impact the subsequent evolution of the structures. Therefore, they stand as key science targets to obtain a complete understanding of the relation between the environment and galaxy evolution. Dedicated investigations of similar protoclusters are required to definitively confirm this conclusion with a higher statistical significance. 

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4. 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 (January 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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5. Distinct origins of environmentally quenched galaxies in the core and outer virialized regions of massive clusters at 0.8 < z < 1.5

   https://doi.org/10.1093/mnras/staf932  

   Hewitt, Guillaume; Sarron, Florian; Balogh, Michael_L; Rudnick, Gregory; Bahé, Yannick; Baxter, Devontae_C; Castignani, Gianluca; Cerulo, Pierluigi; Cooper, M_C; Demarco, Ricardo; et al (June 2025, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT High-redshift ($$z\sim 1$$) galaxy clusters are the domain where environmental quenching mechanisms are expected to emerge as important factors in the evolution of the quiescent galaxy population. Uncovering these initially subtle effects requires exploring multiple dependencies of quenching across the cluster environment, and through time. We analyse the stellar mass functions (SMFs) of 17 galaxy clusters within the GOGREEN and GCLASS surveys in the range $0.8< z<1.5$, and with $$\log {(M/{\rm {M_\odot }})}>9.5$$. The data are fit simultaneously with a Bayesian model that allows the Schechter function parameters of the quiescent and star-forming populations to vary smoothly with cluster-centric radius and redshift. The model also fits the radial galaxy number density profile of each population, allowing the global quenched fraction to be parametrized as a function of redshift and cluster velocity dispersion. We find the star-forming SMF to not depend on radius or redshift. For the quiescent population however, there is $$\sim 2\sigma$$ evidence for a radial dependence. Outside the cluster core ($$R>0.3\, R_{\rm 200}$$), the quenched fraction above $$\log {(M/{\rm {M_\odot }})}=9.5$$ is $$\sim 40{\rm\,\,per\, cent}$$, and the quiescent SMF is similar in shape to the star-forming field. In contrast, the cluster core has an elevated quenched fraction ($$\sim 70{\rm \,\,per\, cent}$$), and a quiescent SMF similar in shape to the quiescent field population. We explore contributions of ‘early mass-quenching’ and mass-independent ‘environmental-quenching’ models in each of these radial regimes. The core is well described primarily by early mass-quenching, which we interpret as accelerated quenching of massive galaxies in protoclusters, possibly through merger-driven feedback mechanisms. The non-core is better described through mass-independent environmental-quenching of the infalling field population. 

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