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

Abstract

In most galaxies, the central black hole accounts for no more than a percent of the total mass in stars. Recently, however, extremely over-massive black holes 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 super-massive black holes, such that most of the black hole mass was built at birth rather than through black hole accretion. Here we show that among evolved galaxies over-massive black holes are also present, indicating that overmassive BHs are not a signature unique to black hole formation channels. The first large-area sky survey of the eROSITA X-ray telescope on board SpectrRG identified 200 quasars by their luminous hard X-ray radiation. These 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 allow us unbiased estimates of cosmological distances, black hole masses and host galaxy stellar masses. We securely identify a sample of over-massive black holes 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 overmassive black holes near cosmic noon. This indicates an accretion channel disconnected from the stellar population that cause 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, can explain JWST AGN without requiring them to signify imprints of black hole seeding mechanism.

Figures (19)

• Figure 1: Optical spectral fits of ID 133 (top panel), 1136 (middle panel) and 32 (bottom panel), our three OMBH quasars from the eFEDS-hard sample. The title gives the SDSS ID, position and redshift assumed during fitting. After continuum modeling (upper inset), each line complex (inset) is modeled with up to three broad components, which are combined when measuring the total full width half maximum.
• Figure 2: As in \ref{['fig:optspec']}, but for two OMBH quasars from the eFEDS-main sample (top: eFEDS-main ID 583, bottom: eFEDS-main ID 33). Both show very massive black holes through broad H$\beta$ lines.
• Figure 3: Optical spectra of three OMBHs in the eFEDS main catalog (top: eFEDS-main ID 3276, middle: eFEDS-main ID 689, bottom: eFEDS-main ID 553). All show very massive black holes through broad H$\beta$ lines.
• Figure 4: Optical spectral fits (as in \ref{['fig:optspec']}) of eFEDS-hard ID 45 (top panel), the source with the fourth-largest $M_{\bullet}/M_{\star}$ ratio in that sample, and two under-massive AGN, eFEDS-hard IDs 54 (middle panel) and 39 (bottom panel).
• Figure 5: Main panel: Example GRAHSP SED fit with EFLL constraints on the host galaxy shown as purple up-wards pointing triangles. Top row: GALEX (UV), DECam (optical), and WISE (IR) images, each with a $30"$ field of view. The UV images has low significance, giving a upper limit in the SED data, the WISE image is blended, giving upper limits on the total, leaving only the optical and near-infrared measurements. The optical panel includes concentric apertures of 0.5, 0.75, 1.0, and $1.5"$ radius, the last is where extended flux beyond point source expectations was measured, which are constraints in the fit on the galaxy component (purple curve) alone.