Joel Diamond | Search arXiv

arXiv2026

The HST-Hyperion Survey: X-ray AGN Enhancement in the Hyperion Proto-supercluster

Dense environments can influence both galaxy evolution and supermassive black hole growth, but whether they enhance or suppress (or both) active galactic nuclei (AGN) activity at high redshift ($z\gtrsim2$) remains uncertain. As one of the most massive structures identified in the early universe, the Hyperion proto-supercluster at $z\approx2.5$ provides a unique opportunity to investigate this connection during an epoch near the peak of both cosmic star formation rate and AGN activity density. To examine the impact of galaxy environment on AGN activity, we measure the fraction of X-ray-selected AGN in Hyperion as a whole, its highly overdense peaks, and its outskirts, comparing these measurements to a coeval field sample. We identify X-ray AGN using the deep Chandra COSMOS-Legacy Survey and combine optical/near-infrared (NIR) photometry and photometric redshift information from COSMOS2020 with extensive ground-based spectroscopy and HST-grism observations. Redshift information is incorporated statistically using a Monte Carlo framework. We find a higher X-ray AGN fraction in Hyperion ($7.4^{+3.2}_{-1.8}\%$) than in the coeval field ($3.2^{+0.6}_{-0.5}\%$), corresponding to a $\sim2.3σ$ difference. Strikingly, this enhancement is concentrated in the overdense peaks, where the X-ray AGN fraction reaches $12.3^{+5.5}_{-3.4}\%$ ($15.0^{+6.9}_{-4.4}\%$ in the most massive peaks versus $6.2^{+11.9}_{-2.1}\%$ in the less massive peaks), while the lower-density Hyperion outskirts ($2.1^{+3.6}_{-1.1}\%$) show no evidence of enhancement relative to the field. These results add to the limited but growing evidence for enhanced AGN activity in high-redshift protostructures, suggesting that the dense environments at these epochs may preferentially trigger AGN during the early stages of cluster assembly.

astro-ph.GA↗

arXiv2026

The HST-Hyperion Survey: Environmental Imprints on the Stellar-Mass Function at z~2.5

Not all galaxies at Cosmic Noon evolve in the same way. It remains unclear how the local environment -- especially the extreme overdensities of protoclusters -- affects stellar mass assembly at high redshift. The stellar mass function (SMF) encodes these processes; comparing SMFs across environments reveals differences in evolutionary history. We present the SMF of the Hyperion proto-supercluster at $z\sim2.5$, one of the largest and most massive protostructures known. This dataset provides the most statistically robust SMF of a single protostructure at $z>2$. By comparing the SMF of overdense peaks within Hyperion to the coeval field, we ask: how early, and how strongly, does a dense environment favor massive galaxies? Using COSMOS2020 photometry with ground-based and new HST grism spectroscopy, we construct a 3D overdensity map that assigns galaxies to peaks, outskirts, or the field. We perform 100 Monte Carlo realizations to propagate redshift and mass uncertainties, and derive SMFs normalized to the field. The peaks show a clear excess of massive galaxies: number densities at $\log(M_*/M_\odot)\sim 11$ are ~10x higher than the field, while those at $\log(M_*/M_\odot)\sim 9.5$ are enhanced by only ~3.5x. By contrast, the outskirts and Hyperion as a whole mirror the field. Environmental effects on stellar mass growth are thus evident by $z\sim 2.5$. The densest regions already host galaxies with accelerated growth, while the global SMF masks this signal. Protostructures therefore begin shaping the high-mass end of the SMF well before cluster quenching, and may drive the elevated star formation at Cosmic Noon.

astro-ph.GA↗