No evidence of obscured, accreting black holes in most z=6 star-forming
  galaxies
  It has been claimed that there is a large population of obscured, accreting
black holes at high-redshift and that the integrated black hole density at z=6
as inferred from X-ray observations is ~100 times greater than inferred from
optical quasars. I have performed a stacking analysis of very deep Chandra
X-ray data at the positions of photometrically-selected z=6 galaxy candidates.
It is found that there is no evidence for a stacked X-ray signal in either the
soft (0.5-2 keV) or hard (2-8 keV) X-ray bands. Previous work which reported a
significant signal is affected by an incorrect method of background subtraction
which underestimates the true background within the target aperture. The puzzle
remains of why the z=6 black hole mass function has such a flat slope and a low
normalization compared to the stellar mass function.
