Galaxy candidates at z ~ 10 in archival data from the Brightest of
  Reionizing Galaxies (BoRG[z8]) survey
  The Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) enabled
the search for the first galaxies observed at z ~ 8 - 11 (500 - 700 Myr after
the Big Bang). To continue quantifying the number density of the most luminous
galaxies (M_AB ~ -22.0) at the earliest epoch observable with HST, we search
for z ~ 10 galaxies (F125W-dropouts) in archival data from the Brightest of
Reionizing Galaxies (BoRG[z8]) survey, originally designed for detection of z ~
8 galaxies (F098M-dropouts). By focusing on the deepest 293 arcmin^2 of the
data along 62 independent lines of sight, we identify six z ~ 10 candidates
satisfying the color selection criteria, detected at S/N > 8 in F160W with M_AB
= -22.8 to -21.1 if at z = 10. Three of the six sources, including the two
brightest, are in a single WFC3 pointing (~ 4 arcmin^2), suggestive of
significant clustering, which is expected from bright galaxies at z ~ 10.
However, the two brightest galaxies are too extended to be likely at z ~ 10,
and one additional source is unresolved and possibly a brown dwarf. The
remaining three candidates have m_AB ~ 26, and given the area and completeness
of our search, our best estimate is a number density of sources that is
marginally higher but consistent at 2{\sigma} with searches in legacy fields.
Our study highlights that z ~ 10 searches can yield a small number of
candidates, making tailored follow-ups of HST pure-parallel observations viable
and effective.
