Discovery of the first heavily obscured QSO candidate at $z>6$ in a close galaxy pair (1906.04241v2)

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 $z>6$.

• The paper presents the first detection of a heavily obscured QSO candidate at z=6.515, providing new evidence of obscured SMBH accretion in the early universe.
• The study employs Chandra X-ray and ALMA [C II] data to overcome observational biases and identify significant X-ray emission in a close galaxy pair.
• The findings challenge conventional QSO selection methods and highlight the need for deep X-ray follow-ups to verify the source and refine SMBH growth models.

Discovery of a Heavily Obscured Quasar Candidate in the Early Universe

The research presented in this paper highlights a significant advancement in the paper of supermassive black holes (SMBHs), especially concerning their growth in the early universe. The authors report the discovery of a heavily obscured quasar (QSO) candidate at a redshift of $z=6.515$, a finding that adds substantial information to our understanding of the obscured phases of SMBH accretion when the universe was less than a billion years old.

Context and Methodology

The early growth of SMBHs is theorized to involve periods of obscured accretion, yet observational evidence has been limited due to biases in the selection methods for $z>6$ quasars, which traditionally exclude obscured QSOs. This paper employs data from the Chandra X-ray Observatory to identify what may be the first heavily obscured QSO candidate discovered in a pair of closely interacting galaxies, separated by approximately 5 kpc at $z=6.515$. The galaxies in question are PSO J167.6415--13.4960, an optically classified type-1 QSO, and its companion, initially detected as a [C II] emitter by the Atacama Large Millimeter/submillimeter Array (ALMA).

Findings

Significant X-ray emission was observed in the 2–5 keV band, suggesting heavy obscuration with a column density of $N_H > 2 \times 10^{24}\,\mathrm{cm^{-2}}$ and $N_H > 6 \times 10^{23}\,\mathrm{cm^{-2}}$ at 68% and 90% confidence levels, respectively. However, due to the positional uncertainty, the X-ray emission could not be definitively associated with either PSO167--13 or its companion. Interestingly, this emission presents the first candidate for a heavily obscured QSO at $z > 6$.

Implications and Discussion

The paper explores two main scenarios: If PSO167--13 is the X-ray source, it represents a case where a type-1 QSO demonstrates significant X-ray obscuration, potentially due to complex physical processes such as those observed in weak emission-line QSOs (WLQs) or broad absorption-line QSOs (BALQSOs). If the companion galaxy is the source, it posits the existence of a previously unobserved obscured QSO in a closely interacting pair, requiring PSO167--13 to be intrinsically X-ray weak.

The detection of such a heavily obscured QSO candidate is crucial as it expands the observational parameters for the paper of early SMBH growth, challenging the current models of QSO selection that favor optically luminous and unobscured systems. It advocates for more diverse observational strategies to fully unearth the population of obscured SMBHs in the early universe.

Conclusion and Future Directions

This discovery prompts further investigation into the nature and impact of obscured accretion phases in early QSO evolution. Future deep X-ray observations will be vital in improving positional accuracy and confirming the source of X-ray emission, thus clarifying the nature of this candidate and its implications for understanding SMBH growth during the universe’s formative years. Enhanced observational techniques targeting high-redshift obscured QSOs could prove invaluable in comprehensively mapping the early universe's SMBH landscape.