A Systematic Search for Galaxy Proto-Cluster Cores at $z\sim 2$

Abstract

A proto-cluster core is the most massive dark matter halo (DMH) in a given proto-cluster. To reveal the galaxy formation in core regions, we search for proto-cluster cores at $z\sim 2$ in $\sim 1.5{\mathrm{deg}}^2$ of the COSMOS field. Using pairs of massive galaxies ($\log (M_{\ast }/M_{\odot })\ge 11$) as tracers of cores, we find 75 candidate cores, among which 54\% are estimated to be real. A clustering analysis finds that these cores have an average DMH mass of $2.6_{-0.8}^{+0.9}\times 10^{13}{M}_{\odot }$, or $4.0_{-1.5}^{+1.8}\times 10^{13}{M}_{\odot }$ after contamination correction. The extended Press-Schechter model shows that their descendant mass at $z=0$ is consistent with Fornax-like or Virgo-like clusters. Moreover, using the IllustrisTNG simulation, we confirm that pairs of massive galaxies are good tracers of DMHs massive enough to be regarded as proto-cluster cores. We then derive the stellar mass function (SMF) and the quiescent fraction for member galaxies of the 75 candidate cores. We find that the core galaxies have a more top-heavy SMF than field galaxies at the same redshift, showing an excess at $\log (M_{\ast }/M_{\odot })\gtrsim 10.5$. The quiescent fraction, $0.17_{-0.04}^{+0.04}$ in the mass range $9.0\le \log (M_{\ast }/M_{\odot })\le 11.0$, is about three times higher than that of field counterparts, giving an environmental quenching efficiency of $0.13_{-0.04}^{+0.04}$. These results suggest that stellar mass assembly and quenching are accelerated as early as at $z\sim 2$ in proto-cluster cores.

14 pages, 11 figures, accepted for publication in MNRAS, 10th June 2020