Publication details

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


BOGDÁN Ákos KOVÁCS Orsolya Eszter JONES Christine FORMAN William R. KRAFT Ralph P. STRAIT Victoria COE Dan BRADAČ Maruša

Year of publication 2022
Type Article in Periodical
Magazine / Source Astrophysical Journal
MU Faculty or unit

Faculty of Science

Keywords High-redshift galaxies; Active galactic nuclei; X-ray active galactic nuclei; Galaxy clusters; Gravitational lensing
Description 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 ? 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 ? 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? detection for massive galaxies with an X-ray luminosity of (3.7 ± 1.6) × 1042 erg s-1, which corresponds to a (3.0 ± 1.3) × 105 M? 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 ? 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.
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