X-ray Observations of a $z\sim 6.2$ 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 $72.3_{-8.6}^{+9.6}$ net counts, we find that PSO J308.0416$-$21.2339 is powerful ($L_X=2.31_{-0.76}^{+1.14}\times 10^{45}\text{erg}{\text{s}}^{-1}{\text{cm}}^{-2}$ in rest-frame $2.0-10.0$ keV) yet soft (spectral power-law index $\Gamma =2.39_{-0.36}^{+0.37}$ and optical-to-X-ray slope ${\alpha }_{\mathrm{OX}}=-1.41\pm 0.11$). In addition, we detect three hard-energy photons $2{.}^{\prime \prime }0$ 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 AGN.

8 pages, 4 figures, published in the Astrophysical Journal. Revised to reflect accepted version