A large population of overmassive black hole quasars at z=0.3-0.8 revealed by eROSITA

This is the authors' abstract. We don't add key points for this paper.

In most galaxies, the central black hole accounts for no more than a percent of the total mass in stars. Recently, however, extremely overmassive black holes (OMBHs) with ratios of 10% have been reported in dwarf galaxies at z < 1 and at cosmic dawn ( z > 5.5) by JWST. Both findings have been interpreted as signatures of the still mysterious origins of supermassive black holes. Accordingly, it is thought that most of the black hole mass was built at birth rather than through accretion. Here, we show that among evolved galaxies, OMBHs are also present, indicating that OMBHs are not a signature unique to black hole formation channels. The first large-area sky survey of the eROSITA X-ray telescope on board SRG identified 200 quasars by their luminous hard X-ray radiation. These quasars signpost rapidly growing black holes. Complementary optical spectroscopy from the Sloan Digital Sky Survey and archival UV to IR photometric data combined with galaxy-quasar decomposition techniques allowed us to make unbiased estimates of cosmological distances, black hole masses, and host galaxy stellar masses. We securely identified a sample of OMBHs with BH-to-host ratios of more than 5%, which may have undergone exponential accretion spurts lasting about a billion years. Our survey identified a high space density of at least 4/Gpc 3 of OMBHs near cosmic noon. This indicates an accretion channel disconnected from the stellar population that causes strong deviations from galaxy scaling relations. This channel is currently not part of galaxy evolution models. The identified channel, if applicable also for the first billion years of cosmic time, might explain JWST active galactic nuclei without requiring them to signify imprints of a black hole seeding mechanism.