We have made a serendipitous discovery of a massive ( \sim 5 { \times } 10 ^ { 11 } M _ { \odot } ) cD galaxy at z=1.096 in a candidate rich cluster in the 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 \sim 10 { kpc } and reaching \sim 70 { kpc } in radius along the semi-major 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 } Gyr ^ { -1 } ) 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 ( \sim M _ { * } / R _ { 50 } ^ { 2 } ) but a similar velocity dispersion ( \sim \sqrt { M _ { * } / R _ { 50 } } ) to those of more-massive , nearby cDs .
If the cD galaxy is one of the progenitors of today ’ s more massive cDs , its size ( R _ { 50 } ) and stellar mass have had to increase on average by factors of 3.4 \pm 1.1 and 3.3 \pm 1.3 over the past \sim 8 Gyrs , 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 ( \sim 3.1 \times 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 .