The <i>HST</i> -Hyperion Survey: Companion fraction and overdensity in a <i>z</i> ∼ 2.5 proto-supercluster

We present a study of the galaxy merger and interaction activity within the Hyperion Proto-supercluster at z ∼ 2.5 in an effort to assess the occurrence of galaxy mergers and interactions in contrast to the coeval field and their impact on the buildup of stellar mass in high-density environments at higher redshifts. For this work, we utilized data from the Charting Cluster Construction with VUDS and ORELSE Survey (C3VO) along with extensive spectroscopic and photometric datasets available for the COSMOS field – including the HST -Hyperion Survey. To evaluate potential merger and interaction activity, we measured the fraction of galaxies with close kinematic companions ( f ckc ) both within Hyperion and the coeval field by means of a Monte Carlo (MC) methodology developed in this work that probabilistically employs our entire combined spectroscopic and photometric dataset. We validated our f ckc MC methodology on a simulated lightcone built from the GAlaxy Evolution and Assembly (GAEA) semi-analytic model, and we determined correction factors that account for the underlying spectroscopic sampling rate of our dataset. We find that galaxies in Hyperion have close kinematic companions ≳2.5× more than galaxies in the field and measure a corrected f ckc = 59 +9 −10 % for Hyperion and a corrected f ckc = 23 +1.7 −1.8 % for the surrounding field; a ≳3 σ difference. The enhancement in f ckc likely correlates to an enhancement in the merger and interaction activity within the high-density environment of Hyperion and matches the trend seen in other structures. The rate of merger and interactions within the field implied from our field f ckc measurement is well aligned with values measured from other observations in similar redshift ranges. The enhanced f ckc measured within Hyperion suggests that merger and interaction activity play an important role in the mass growth of galaxies in denser environments at higher redshifts.