2013ApJ...769..147L

We have made a serendipitous discovery of a massive (∼5x10¹¹ 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 <10^–4.G/yr) stellar population. The cD galaxy lies on the same mass-size relation as the bulk of quiescent galaxies at similar redshifts. The cD galaxy has a higher stellar mass surface density (∼ M_*/R₅₀² but a similar velocity dispersion (∼sqrt{M_*/R₅₀}) to those of more massive, nearby cDs. If the cD galaxy is one of the progenitors of today's more massive cDs, its size (R₅₀) and stellar mass have had to increase on average by factors of 3.4±1.1 and 3.3±1.3 over the past ∼8 Gyr, respectively. Such increases in size and stellar mass without being accompanied by significant increases in velocity dispersion are consistent with evolutionary scenarios driven by both major and minor dissipationless (dry) mergers. If such cD envelopes originate from dry mergers, our discovery of even one example proves that some BCGs entered the dry merger phase at epochs earlier than z = 1. Our data match theoretical models which predict that the continuance of dry mergers at z < 1 can result in structures similar to those of massive cD galaxies seen today. Moreover, our discovery is a surprise given that the extreme depth of the HUDF is essential to reveal such an extended cD envelope at z > 1 and, yet, the HUDF covers only a minuscule region of sky (∼3.1x10^–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.