Letter to the Editor

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

¹ Instituto de Astrofísica and Centro de Astroingenieria, Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
e-mail: fabio.vito@uc.cl
² Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100012, PR China
³ Department of Astronomy & Astrophysics, 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA
⁴ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
⁵ Department of Physics, The Pennsylvania State University, University Park, PA 16802, USA
⁶ Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sótero Sanz 100, Providencia, Santiago, Chile
⁷ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA
⁸ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁹ School of Astronomy and Space Science, Nanjing University, Nanjing 210093, PR China
¹⁰ Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Ministry of Education, Nanjing, Jiangsu 210093, PR China
¹¹ Collaborative Innovation Center of Modern Astronomy and Space Exploration, Nanjing 210093, PR China
¹² European Southern Observatory, Alonso de Córdova 3107, Vitacura, Región Metropolitana, Chile
¹³ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
¹⁴ Department of Physics, University of North Texas, Denton, TX 76203, USA
¹⁵ Physics Department, University of Miami, Coral Gables, FL 33124, USA
¹⁶ 15 Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
¹⁷ Sorbonne Universités, UPMC Université Paris 06 et CNRS, UMR7095, Institut d’Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France

Received: 20 May 2019
Accepted: 12 June 2019

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 gigayear 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 (≈5 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 Atacama large millimeter array (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 × 10²⁴ cm⁻² and N_H >  6 × 10²³ 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.