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

Abstract

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}<SUB>-8.6</SUB><SUP>+9.6</SUP> net counts, we find that PSO J308.0416-21.2339 is powerful ({L}<SUB>X</SUB>={2.31}<SUB>-0.76</SUB><SUP>+1.14</SUP>× {10}<SUP>45</SUP> {erg} {{{s}}}<SUP>-1</SUP> {cm}}<SUP>-2</SUP> in rest-frame 2.0-10.0 keV) yet soft (spectral power-law index {{Γ }}={2.39}<SUB>-0.36</SUB><SUP>+0.37</SUP> and optical-to-X-ray slope α <SUB>OX</SUB> = -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.