Exploring Gravitationally-Lensed $z\gtrsim 6$ X-ray AGN Behind the RELICS clusters

Abstract

Although observations of high-redshift quasars demonstrate that many supermassive black holes (BHs) reached large masses within one billion years after the Big Bang, the origin of the first BHs is still a mystery. A promising way to constrain the origin of the first BHs is to explore the average properties of $z\gtrsim 6$ BHs. However, typical BHs remain hidden from X-ray surveys, which is due to their relatively faint nature and the limited sensitivity of X-ray telescopes. Gravitational lensing provides an attractive way to study this unique galaxy population as it magnifies the faint light from these high-redshift galaxies. Here, we study the X-ray emission originating from 155 gravitationally-lensed $z\gtrsim 6$ galaxies that were detected in the RELICS survey. We utilize Chandra X-ray observations to search for AGN in the individual galaxies and in the stacked galaxy samples. We did not identify an individual X-ray source that was undoubtedly associated with a high-redshift galaxy. We stack the signal from all galaxies and do not find a statistically significant detection. We split our sample based on stellar mass, star-formation rate, and lensing magnification and stack these sub-samples. We obtain a $2.2\sigma$ detection for massive galaxies with an X-ray luminosity of $(3.7\pm 1.6)\times 10^{42}\mathrm{erg}{\mathrm{s}}^{-1}$, which corresponds to a $(3.0\pm 1.3)\times 10^5{\mathrm{M}}_{\odot }$ BH accreting at its Eddington rate. Other stacks remain undetected and we place upper limits on the AGN emission. These limits imply that the bulk of BHs at $z\gtrsim 6$ either accrete at a few percent of their Eddington rate and/or are $1-2$ orders of magnitude less massive than expected based on the stellar mass of their host galaxy.

14 pages, 5 figures, accepted for publication in The Astrophysical Journal

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