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

We present an analysis of the galaxy population in XLSSC 122, an X-ray-selected, virialized cluster at redshift z = 1.98. We utilize Hubble Space Telescope (HST)Wide Field Camera 3 (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 0.5r500, significantly enhanced over the field value of 20⁺²₋₂ per cent at z ∼ 2. We find an excess of ‘bulge-like’ quiescent cluster members with Sérsic 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_ = 5× 10¹⁰ \, M_. 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.