The HST-Hyperion Survey: Companion Fraction and Overdensity in a z ~ 2.5 Proto-supercluster

TL;DR

This study addresses how merger and interaction activity depends on environment in a $z\sim2.5$ protocluster by assembling a large COSMOS-based dataset and employing a redshift Monte Carlo framework tied to Voronoi-based local overdensity maps to identify close kinematic companions. The authors validate their $f_{\rm ckc}$ methodology with a simulated GAEA/Millennium lightcone to derive correction factors, then apply it to Hyperion versus the coeval field. They find a substantial enhancement in $f_{\rm ckc}$ for Hyperion ($59_{-10}^{+9}\%$) compared with the field ($23_{-1.8}^{+1.7}\%$), along with stronger tidal perturbations and shorter potential merger timescales in the dense environment. These results imply that mergers and interactions contribute significantly to stellar mass growth in protoclusters and demonstrate a scalable approach for studying environment-driven assembly across many high-redshift structures.

Abstract

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 build up of stellar mass in high density environments at higher-z. For this work, we utilize 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 measure 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 validate our $f_{ckc}$ MC methodology on a simulated lightcone built from the GAlaxy Evolution and Assembly semi-analytic model, and we determine correction factors that account for the underlying spectroscopic sampling rate of our dataset. We find that galaxies in Hyperion have close kinematic companions $\gtrsim 2.5\times$ more than galaxies in the field and measure a corrected $f_{ckc}=59_{-10}^{+9}$% for Hyperion and a corrected $f_{ckc}=23_{-1.8}^{+1.7}$% for the surrounding field; a $>3σ$ difference. This increase in $f_{ckc}$ indicates an enhancement in the merger and interaction activity within 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 z.

Figures (13)

• Figure 1: Source breakdown as a function of redshift for objects in our final catalog based on the cuts detailed in Section \ref{['sec:sampsel']}. Sources from different methods are shown in different panels: wide field spectroscopic surveys (top left), targeted spectra (top right), and HST Hyperion (bottom left). Rather than including a histogram of photometric redshift counts, we include the photometric redshift fraction (i.e the fraction of galaxies with only a photometric redshift measured) for reference on the bottom right. The location of Hyperion is included in each panel as a shaded gray region ($2.4 < z < 2.7$), and insets for this shaded region are included for the spectroscopic and grism redshift panels. Histograms with multiple survey sources are stacked.
• Figure 2: Galaxies in our final sample from $2 < z < 3$ plotted in stellar mass and IRAC ch1 magnitude space (lower left) along with individual normalized histograms of stellar mass (top) and IRAC ch1 magnitude (lower right). All values are drawn from COSMOS2020 aside from the redshifts where we employ the best spectroscopic or grism redshift when available. For reference, our imposed IRAC ch1 magnitude cut is plotted as the dotted black line (note: some galaxies in our final sample lie above this line as our IRAC cut is an or cut on both ch1 and ch2). As expected, our spectroscopic and grism redshift sources (spec-z and grism-z, respectively) are primarily at brighter magnitudes and higher stellar masses in comparison to the distribution of photometric redshift only sources. Our final sample has a rough stellar mass limit of $\log(M_*/M_{\odot}) \sim 9.3$ from $2 < z < 3$ based on our imposed IRAC cuts.
• Figure 3: Spectroscopic completeness (or the fraction of sources with a spectroscopic and/or grism redshift, $F_{\text{spec}}$) as a function of IRAC ch1 magnitude for all sources from $2<z<3$ in our final sample (purple) and a sub-sample of sources limited to the redshift range of Hyperion with tighter astrometric cuts (gold). Also included for reference are the underlying normalized IRAC ch1 magnitude distributions for each population along with the location of the imposed IRAC ch1 (or ch2) magnitude cut (vertical dashed black line). Spectroscopic completeness for both samples declines sharply as a function of magnitude. However, there is notably higher completeness for galaxies limited more closely to the bounds of Hyperion.
• Figure 4: Example of the distribution of galaxies in redshift and overdensity ($\sigma_{\rm \delta}$) space for one MC iteration used in this work (MC #82). Included are only those galaxies in the relevant redshift range for our fraction calculations (i.e., $2 < z < 3$). Galaxies associated with Hyperion for this iteration are marked in red and field galaxies for this iteration are marked in blue. Unused galaxies for this iteration are marked in gray (which includes galaxies in the redshift range of Hyperion that are not associated with overdensity and galaxies in the field sample redshift range that are associated with other large structure, see Section \ref{['sec:deter env']}).
• Figure 5: On-sky 2D view and 3D reconstruction of a potential galaxy companion for COSMOS2020 (C20) object #1006523 found during our MC process (MC iteration #87). The central galaxy (C20#1006523) is marked in gold with our 150 kpc search radius centered on that galaxy shown in green. The companion galaxy that meets both the projected separation and LoS velocity difference criteria is denoted in blue. Other nearby galaxies are shown in gray. While multiple galaxies fall within the projected separation criteria for this galaxy, the 3D LoS velocity difference cut helps isolate the single companion from other projected companions.