X-Ray Observations of a z ∼ 6.2 Quasar/Galaxy Merger

Thomas Connor, Eduardo Bañados, Daniel Stern, Roberto Decarli, Jan Torge Schindler, Xiaohui Fan, Emanuele Paolo Farina, Chiara Mazzucchelli, John S. Mulchaey, Fabian Walter

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23 Scopus citations


Quasars at early redshifts (z > 6) with companion galaxies offer unique insights into the growth and evolution of the first supermassive black holes. Here, we report on a 150 ks Chandra observation of PSO J308.0416-21.2339, a z = 6.23 quasar with a merging companion galaxy identified in [C ii] and rest-frame UV emission. With 72.3 +9.6 -8.6 net counts, we find that PSO J308.0416-21.2339 is powerful (LX = 2.31 +1.14 -0.76 × 1045 erg s-1 cm-2 in rest-frame 2.0-10.0 keV) yet soft (spectral power-law index Γ = 2.39+0.37 -0.36 and optical-to-X-ray slope α OX = -1.41 ± 0.11). In addition, we detect three hard-energy photons 2.″0 to the west of the main quasar, cospatial with the brightest UV emission of the merging companion. As no soft-energy photons are detected in the same area, this is potentially indicative of a highly obscured source. With conservative assumptions, and accounting for both background fluctuations and the extended wings of the quasar's emission, these photons only have a probability P = 0.021 of happening by chance. If confirmed by deeper observations, this system is the first high-redshift quasar and companion individually detected in X-rays and is likely a dual active galactic nucleus.

Original languageEnglish (US)
Article number171
JournalAstrophysical Journal
Issue number2
StatePublished - Dec 20 2019

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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