# Discovery of the first heavily obscured QSO candidate at $z>6$ in a close galaxy pair

## TL;DR

This paper reports the discovery of the first heavily obscured quasar candidate at redshift greater than 6, providing new insights into early supermassive black hole growth during the universe's first billion years.

## Contribution

It presents the first observational evidence of a heavily obscured QSO at z>6, identified through Chandra X-ray detection in a close galaxy pair, addressing biases in previous quasar selection methods.

## Key findings

- X-ray detection of a heavily obscured QSO candidate at z>6
- Estimated obscuring column density exceeds 2×10^{24} cm^{-2}
- First candidate linking obscured SMBH growth to early universe galaxy pairs

## Abstract

While theoretical arguments predict that most of the early growth of supermassive black holes (SMBHs) happened during heavily obscured phases of accretion, current methods used for selecting $z>6$ quasars (QSOs) are strongly biased against obscured QSOs, thus considerably limiting our understanding of accreting SMBHs during the first Gyr of the Universe from an observational point of view. We report the $Chandra$ discovery of the first heavily obscured QSO candidate in the early universe, hosted by a close ($\approx5$ kpc) galaxy pair at $z=6.515$. One of the members is an optically classified type 1 QSO, PSO167-13. The companion galaxy was first detected as a [C II] emitter by ALMA. An X-ray source is significantly ($P=0.9996$) detected by $Chandra$ in the 2-5 keV band, with $<1.14$ net counts in the 0.5-2 keV band, although the current positional uncertainty does not allow a conclusive association with either PSO167-13 or its companion galaxy. From X-ray photometry and hardness-ratio arguments, we estimated an obscuring column density of $N_H>2\times10^{24}\,\mathrm{cm^{-2}}$ and $N_H>6\times10^{23}\,\mathrm{cm^{-2}}$ at $68\%$ and $90\%$ confidence levels, respectively. Thus, regardless of which of the two galaxies is associated with the X-ray emission, this source is the first heavily obscured QSO candidate at $z>6$.