Exploring the high-redshift black holes behind the RELICS clusters using gravitational lensing

Observations of high-redshift quasars suggest that many supermassive black holes (BHs) reached large masses within one billion years after the Big Bang. However, the origin of the first BHs remains a mystery. An exciting approach to probe the origin of the first BHs is to explore the average properties of BHs at the z>6. 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 high-redshift galaxies. I will present our study that used 155 gravitationally-lensed galaxies detected in the RELICS survey to search for X-ray signatures from accreting BHs. We searched for individually detected BHs and stacked the galaxy sample to increase the signal-to-noise ratios. We split our sample based on stellar mass, star-formation rate, and lensing magnification and stacked these sub-samples. We obtained a weak detection for massive galaxies and established that the observed X-ray luminosity corresponds to 3e5 M_⊙ BH accreting at its Eddington rate.