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, virializedcluster at redshift 𝑧 = 1.98. We utilize Hubble Space Telescope (HST)Wide Field Camera 3 (WFC3) photometry to characterize the activity andmorphology 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+2−2per cent at 𝑧 ∼ 2. We find an excess of 'bulge-like' quiescent cluster members with Sérsic index 𝑛 > 2relative to the field. These galaxies are found to be larger than their field counterparts at 99.6per 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.