A Large Number of z > 6 Galaxies around a QSO at z = 6.43: Evidence for
  a Protocluster?
  QSOs have been thought to be important for tracing highly biased regions in
the early universe, from which the present-day massive galaxies and galaxy
clusters formed. While overdensities of star-forming galaxies have been found
around QSOs at 2<z<5, the case for excess galaxy clustering around QSOs at z>6
is less clear. Previous studies with HST have reported the detection of small
excesses of faint dropout galaxies in some QSO fields, but these surveys probed
a relatively small region surrounding the QSOs. To overcome this problem, we
have observed the most distant QSO at z=6.4 using the large field of view of
the Suprime-Cam (34' x 27'). Newly-installed CCDs allowed us to select Lyman
break galaxies (LBG) at z~6.4 more efficiently. We found seven LBGs in the QSO
field, whereas only one exists in a comparison field. The significance of this
apparent excess is difficult to quantify without spectroscopic confirmation and
additional control fields. The Poisson probability to find seven objects when
one expects four is ~10%, while the probability to find seven objects in one
field and only one in the other is less than 0.4%, suggesting that the QSO
field is significantly overdense relative to the control field. We find some
evidence that the LBGs are distributed in a ring-like shape centered on the QSO
with a radius of ~3 Mpc. There are no candidate LBGs within 2 Mpc from the QSO,
i.e., galaxies are clustered around the QSO but appear to avoid the very
center. These results suggest that the QSO is embedded in an overdense region
when defined on a sufficiently large scale. This suggests that the QSO was
indeed born in a massive halo. The central deficit of galaxies may indicate
that (1) the strong UV radiation from the QSO suppressed galaxy formation in
its vicinity, or (2) that star-formation closest to the QSO occurs mostly in an
obscured mode that is missed by our UV selection.
