  We have made a serendipitous discovery of a massive cD galaxy at z=1.096 in a
candidate rich cluster in the HUDF area of GOODS-South. This brightest cluster
galaxy 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 semi-major axis. The spectral energy distributions indicate
that both its inner component and outer envelope are composed of an old,
passively-evolving 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 but a similar velocity dispersion to
those of more-massive, nearby cDs. If the cD galaxy is one of the progenitors
of today's more massive cDs, its size and stellar mass have had to increase on
average 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 accompanied
by significant increases in velocity dispersion are consistent with
evolutionary scenarios driven by both major and minor 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. Adding that cDs are rare, Our
serendipitous discovery hints that such cDs may be more common than expected.
[Abridged]
