No evidence of obscured, accreting black holes in most z=6 star-forming galaxies

TL;DR

This study uses deep X-ray data to investigate the presence of obscured, accreting black holes in z=6 galaxies and finds no evidence supporting their significant population, challenging previous claims.

Contribution

It provides a robust stacking analysis that refutes earlier reports of high black hole density at z=6 due to improved background subtraction methods.

Findings

No stacked X-ray signal detected in z=6 galaxy candidates

Previous claims of significant X-ray signals were due to background subtraction errors

Results suggest a lower black hole density than previously inferred at high redshift

Abstract

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.