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We have made a serendipitous discovery of a massive cD galaxy at z=1.096 in acandidate rich cluster in the HUDF area of GOODS-South. This brightest clustergalaxy is the most distant cD galaxy confirmed to date. Ultra-deep HST/WFC3images reveal an extended envelope starting from ~10 kpc and reaching ~70 kpcin radius along the semi-major axis. The spectral energy distributions indicatethat both its inner component and outer envelope are composed of an old,passively-evolving stellar population. The cD galaxy lies on the same mass-sizerelation as the bulk of quiescent galaxies at similar redshifts. The cD galaxyhas a higher stellar mass surface density but a similar velocity dispersion tothose of more-massive, nearby cDs. If the cD galaxy is one of the progenitorsof today's more massive cDs, its size and stellar mass have had to increase onaverage by factors of $3.4\pm1.1$ and $3.3\pm1.3$ over the past ~8 Gyrs,respectively. Such increases in size and stellar mass without being accompaniedby significant increases in velocity dispersion are consistent withevolutionary scenarios driven by both major and minor dry mergers. If such cDenvelopes originate from dry mergers, our discovery of even one example provesthat some BCGs entered the dry merger phase at epochs earlier than z=1. Ourdata match theoretical models which predict that the continuance of dry mergersat z<1 can result in structures similar to those of massive cD galaxies seentoday. Moreover, our discovery is a surprise given that the extreme depth ofthe HUDF is essential to reveal such an extended cD envelope at z>1 and, yet,the HUDF covers only a minuscule region of sky. Adding that cDs are rare, Ourserendipitous discovery hints that such cDs may be more common than expected.[Abridged]

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