Serendipitous discovery of a massive cD galaxy at z = 1.096: implications for the early formation and late evolution of cD galaxies

We have made a serendipitous discovery of a massive (~5 × 1011 M ☉) cD galaxy at z = 1.096 in a candidate-rich cluster in the Hubble Ultra Deep Field (HUDF) area of GOODS-South. This brightest cluster galaxy (BCG) is the most distant cD galaxy confirmed to date. Ultra-deep HST/WFC3 images reveal an extended envelope starting from ~10 kpc and reaching ~70 kpc in radius along the semimajor axis. The spectral energy distributions indicate that both its inner component and outer envelope are composed of an old, passively evolving (specific star formation rate 1 and, yet, the HUDF covers only a minuscule region of sky (~3.1 × 10–8). Adding that cDs are rare, our serendipitous discovery hints that such cDs may be more common than expected, perhaps even ubiquitous. Images reaching HUDF depths of more area (especially with cluster BCGs at z > 1) are needed to confirm this conjecture.