Exploring Gravitationally Lensed z ≳ 6 X-Ray Active Galactic Nuclei Behind the RELICS Clusters

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 greater than or similar to 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 greater than or similar to 6 galaxies that were detected in the Reionization Lensing Cluster Survey. We utilize Chandra X-ray observations to search for active galactic nuclei (AGNs) 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 subsamples. We obtain a 2.2 sigma detection for massive galaxies with an X-ray luminosity of (3.7 +/- 1.6) x 10(42) erg s(-1), which corresponds to a (3.0 +/- 1.3) x 10(5) M (circle dot) 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 greater than or similar to 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.

Automated summary

Observations of high-redshift galaxies provide insights into the properties of black holes formed after the Big Bang, but the origin of the first black holes remains unknown. This study uses gravitational lensing to study the X-ray emissions of 155 high-redshift galaxies and does not find a definitive association with X-ray sources. A statistically significant detection is found for massive galaxies, but no detection is found for other galaxy samples.