Quiescent galaxies in a virialized cluster at redshift 2: Evidence for accelerated size-growth

Abstract: We present an analysis of the galaxy population in XLSSC 122, an X-ray selected, virialized cluster at redshift $z=1.98$. We utilize HST WFC3 photometry to characterize the activity and morphology of spectroscopically confirmed cluster members. The quiescent fraction is found to be $88^{+4}_{-20}$ per cent within 0.5$r_{500}$, significantly enhanced over the field value of $20^{+2}_{-2}$ per cent at $z\sim2$. We find an excess of "bulge-like" quiescent cluster members with Sersic index $n>2$ relative to the field. These galaxies are found to be larger than their field counterparts at 99.6 per cent confidence, being on average $63^{+31}_{-24}$ per cent larger at a fixed mass of $M_\star = 5\times10^{10} M_\odot$. This suggests that these cluster member galaxies have experienced an accelerated size evolution relative to the field at $z>2$. We discuss minor mergers as a possible mechanism underlying this disproportionate size growth.