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

abstract · 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.

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A main result of this work, the abundance of the OMBH population, is presented in the upper panel of Figure 14 . The systematic survey defining our parent sample allows quantifying the cosmic occurrence rate of these systems, which is at least 8 8 per 0.5 ​ Gpc − 3 0.5\,\mathrm{Gpc}^{-3} (purple point in the top panel of Figure 14 ), adopting the cosmological volume to redshift 1 of the eROSITA eFEDS survey. This space density is higher than the results of Li et al. (2021a) , who found 10 over-massive systems in the approximately 1200 deg 2 HSC-WIDE survey in the z = 0.2 − 0.8 z=0.2-0.8 redshift interval ( ≈ 4 ​ Gpc − 3 \approx 4\,\mathrm{Gpc}^{-3} ; 4 σ \sigma significance by a score ratio test ( Gu et al., 2008 ) ). For comparison at similar stellar masses, we consider the EIGER JWST follow-up ( Yue et al., 2024 ; Stone et al., 2024 ) of 6 z ∼ 6 z\sim 6 quasars with optical magnitudes of m 1450 ​ Å < 24.4 ​ mag ​ AB m_{\mathrm{1450\AA}}<24.4\mathrm{mag\ AB} . The space density of such luminous quasars is 3 ​ Gpc − 3 3\,\mathrm{Gpc}^{-3} ( Willott et al., 2010 ) , and they find half of their follow-up sample (3/6) to be over-massive above M ∙ / M ⋆ > 5 % M_{\bullet}/M_{\star}>5\% . While the space densities are uncertain, currently we infer a 96 times higher space density (14 σ \sigma significance with the same test) of OMBHs: 3/(0.5 Gpc 3 \mathrm{Gpc}^{3} ) vs. (3/6)/(3 Gpc 3 \mathrm{Gpc}^{3} ). Our eight sources allow us to put a conservative, 3 σ \sigma lower limit on the space density of > 4.2 ​ Gpc − 3 >4.2\,\mathrm{Gpc}^{-3} ( Barker, 2002 , Jeffrey-Poisson rate estimate) . Considering the scatter in single-epoch masses halves the number density and loosens threshold to M ∙ / M ⋆ > 2 % M_{\bullet}/M_{\star}>2\% ( section 4.2 ), where the final estimate is 8 ​ Gpc − 3 8\,\mathrm{Gpc}^{-3} with a 3 σ \sigma lower limit of 2.1 ​ Gpc − 3 2.1\,\mathrm{Gpc}^{-3} . The estimates are listed in Table 2 .

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We present a survey that revealed eight OMBHs with M ∙ / M ⋆ > 5 % M_{\bullet}/M_{\star}>5\% , ten times higher black hole masses than the typical ratio in elliptical galaxies. Compared to other studies, these black holes stand out:

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For reliable black hole masses M ∙ M_{\bullet} , we use the most reliable of the single-epoch estimators, H ​ β H\beta . The effect of scatter from single-epoch M ∙ M_{\bullet} estimation and flux limit is quantified with simulations, indicating that at least half of our objects are intrinsically at M ∙ / M ⋆ > 2 % M_{\bullet}/M_{\star}>2\% .

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In this work, we identified eight OMBHs in a 140 deg 2 eROSITA X-ray survey field. This implies a higher minimum space density of OMBHs at z = 0.3 − 1 z=0.3-1 than could be established previously with AGN surveys. In this section, we first discuss the reliability and completeness of the selection in section 5.1 . Next, we discuss evolutionary scenarios that could have created these eight OMBHs. Comparisons to other samples are discussed in terms of selection efficiency section 5.3 and mass distributions section 5.4 , in particular to the early Universe.

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Our results suggest that, considering the Lauer bias and selection effect (see section 4.2 ), the observed distributions are not in conflict with existing scaling relations. In this work, we highlight that among rapidly accreting black holes, overmassive ones are not rare (see section 4.5 ). This suggests that there is an evolutionary process (see next section) capable of maintaining overmassiveness over cosmic time or creating new OMBHs. Our results empirically bolster the modeling analysis of Li et al. (2024b) suggesting mechanisms unique to high-redshift Universe or black hole seeding may not be needed to create OMBHs.

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At odds with simulations, our results indicate that long excursions from self-regulation are permitted to the AGN and realised in Nature. Current galaxy evolution simulations can reproduce local scaling relations through calibration ( Habouzit et al., 2021 ; Dolag et al., 2025 ) , but their shape and evolution depend on the assumed black hole feeding and feedback physics. In virtually all simulations, however, the scatter is narrower than in observations ( Habouzit et al., 2021 , e.g.) . Figure 15 shows the simulations of EAGLE, Horizon-AGN, Illustris and TNG from Habouzit et al. (2021) , Magneticum ( Dolag et al., 2025 ) and ASTRID ( Weller et al., 2023 , at z=2) . Our OMBHs lie outside the range produced in this simulations. OMBHs akin to NGC4486B and NGC1277 are also not produced in these simulations ( Habouzit et al., 2021 ; Dolag et al., 2025 ; Weller et al., 2023 ) . This is not primarily because of limited simulation volume, but because the accretion and feedback models tightly link the star-formation rate and black hole accretion with the galaxy’s gravitational potential.

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We estimate a high space density of OMBH AGN of 16 − 6.4 + 9.7 / Gpc 3 {16}_{-6.4}^{+9.7}/\mathrm{Gpc}^{3} .

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Our yield of identifying OMBHs, 8 in 140 deg 2 , is highest compared to any previous AGN survey, thanks to high spectroscopic completeness.

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Our simulations suggest that the total space density of OMBHs is at least an order of magnitude higher.

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The host galaxies of the three ERQs at z ∼ 2.4 z\sim 2.4 have half-light radii ranging from 1.4 to 2.9 kpc. Their sizes and stellar masses place them between the size–mass relations of early-type and late-type galaxies at similar redshifts, suggesting they may be in a transitional phase from star-forming to quiescent systems.

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We find that ERQs lie 0.5–2 dex above the local M BH –M * relations, with black hole-to-stellar mass ratios of 0.01–0.04. Similar mass ratios have been observed in other heavily obscured quasars, such as the HotDOG population. This possible over-massive nature resemble those of JWST-identified SMBH candidates at higher redshifts. However, detailed comparisons between samples with matched properties is needed and selection biases should also be considered.

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This procedure results in a good fit for one of the objects, CFHQS J2229+1457, which exhibits pronounced narrow emission lines, while we need to implement further constraints to fit the rest of the spectra due to their relatively weak [O III ] lines that blend with the enhanced [Fe II ] emission. In particular, for all other quasars we model the complex line profiles of H α \alpha and H β \beta with two broad Gaussian components instead of one, and we also model the [S II ] λ ​ λ \lambda\lambda 6718 , 6732 \,6718,6732 doublet (with shared linewidths and velocity offsets but without a fixed line ratio) in order to accurately fit the red wing of the broadest H α \alpha component. We further tie the linewidths and velocity offsets of the narrow H α \alpha component to the single-Gaussian components of the [N II ] λ ​ λ \lambda\lambda 6549 , 6585 6549,6585 and [S II ] λ ​ λ \lambda\lambda 6718 , 6732 6718,6732 emission lines. Similarly, we tie the narrow component of H β \beta to the narrow components of [O III ] λ ​ λ \lambda\lambda 4959 , 5007 4959,5007 . The extracted quasar spectra as well as the resultant spectral fits are displayed in Fig. 1 .

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Table 4: A summary of H α \alpha and [O III ] nebular (non-)detections and their properties: the 5 ​ σ 5\sigma detection limit over a 1 ​ arcsec 2 1{\rm arcsec}^{2} aperture, SB 5 ​ σ 1 {\rm SB}^{1}_{5\sigma} , the maximum extent of the nebula, d max Neb d_{\rm max}^{\rm Neb} , and the corresponding lifetime measurement, log ⁡ t Q Neb \log{t_{\rm Q}^{\rm Neb}} .

abstract · abstract

Dwarf AGN serve as the ideal systems for identifying intermediate mass black holes (IMBHs) down to the most elusive regimes ($\sim 10^3 - 10^4 M_{\odot}$). However, the ubiquitously metal-poor nature of dwarf galaxies gives rise to ultraluminous X-ray sources (ULXs) that can mimic the spectral signatures of IMBH excitation. We present a novel photoionization model suite that simultaneously incorporates IMBHs and ULXs in a metal-poor, highly star-forming environment. We account for changes in $M_{BH}$ according to formation seeding channels and metallicity, and changes in ULX populations with post-starburst age and metallicity. We find that broadband X-rays and UV emission lines are insensitive to $M_{BH}$ and largely unable to distinguish between ULXs and IMBHs. Many optical diagnostic diagrams cannot correctly identify dwarf AGN. The notable exceptions include He~II~$λ$4686 and [O~I]~$λ$6300, for which we redefine typical demarcations to account for ULX contributions. Emission lines in the mid-IR show the most promise in separating stellar, ULX, IMBH, and shock excitation while presenting sensitivity to $M_{BH}$ and $f_{\text{AGN}}$. Overall, our results expose the potential biases in identifying and characterizing dwarf AGN purely on strong line ratios and diagnostic diagrams rather than holistically evaluating the entire spectrum. As a proof of concept, we argue that recently discovered over-massive BHs in high-$z$ JWST AGN might not represent the overall BH population, with many galaxies in these samples potentially being falsely classified as purely star-forming.

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With ZTF public light curves that span a rest-frame timescale of ∼ 5 \sim 5 yrs, we were able to derive stringent constraints on the intrinsic optical continuum variability of the three local LRDs, which vary by less than 3 − 4 % 3-4\% over this time period. Additionally, using multi-epoch spectra of J1025 and J1047 that span ∼ 15 \sim 15 years in their rest-frame, we place constraints on the fractional broad H α \alpha variability to be less than a few percent. These are by far the strongest constraints on the variability properties of local LRD analogs on multi-year to decade timescales, and are consistent with the majority of variability constraints on much shorter timescales [ 37 , 72 , 61 , ≲ 1 \lesssim 1 yr in restframe, e.g.,] .

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Table 1: Photometric and H α \alpha spectral variability measurements. The mean photometric uncertainty σ i ¯ \overline{\sigma_{i}} is shown for each filter. The total H α \alpha flux measurements may suffer from aperture effects for the narrow-line emission.

abstract / block 1 · abstract

Luminous quasars at $z>4$ provide key insights into the early Universe. Their rarity necessitates wide-field multi-band surveys to efficiently separate them from the main astrophysical contaminants (i.e., ultracool dwarfs). To expand the sample of high-$z$ quasars, we conducted targeted selections using optical, infrared, and radio surveys, complemented by literature-based quasar candidate catalogs. In this paper, we report the discovery of \nqsos\ new quasars at $4.6<z<6.9$ (six at $z\geq6.5$), with $M_{1450}$ between $-$25.4 and $-$27.0. We also present new spectra of six $z>6.5$ quasars we selected, but whose independent discovery has already been published in the literature. Three of the newly discovered quasars are strong radio emitters (L$_{1.4~\rm GHz}$$=0.09-1.0\times$10$^{34}$erg s$^{-1}$ Hz$^{-1}$). Among them, one source at $z=4.71$ exhibits typical blazar-like properties, including a flat radio spectrum, radio-loudness $\sim$1000, and multi-frequency variability. It is also detected by SRG/eROSITA X-ray telescope (f$_{\rm 0.2-2.3keV} \sim 1.3\times10^{-13}$erg s$^{-1}$ cm$^{-2}$). In addition, for seven $6.3<z<6.9$ quasars we present near-infrared spectroscopy and estimate the central black hole mass from their C$\rm IV$ and Mg$\rm II$ broad emission lines.Their masses (log[M$_{\rm BH,MgII}$]$=8.58-9.14~\rm M_{\odot}$) and Eddington ratios ($λ_{\rm Edd,MgII}=0.74-2.2$) are consistent with other $z>6$ quasars reported in the literature. A $z = 6.3$ quasar exhibits a velocity difference of approximately $9000$ km s$^{-1}$ between the C$\rm IV$ and Mg$\rm II$ emission lines, making it one of the most extreme C$\rm IV$ outflows currently known. Additionally, the sample includes three high-ionization broad absorption line quasars. One of these quasars shows potential evidence of an extremely fast outflow feature, reaching $48000$ km s$^{-1}$.

abstract · abstract

We present the results from performing spectral energy distribution (SED) fitting on 121 variable active galactic nuclei (AGN) candidates in the Hubble Ultra Deep Field (HUDF) using photometry from both the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST) covering $0.2 - 4.8$ microns. We designed a bespoke SED fitting code which decomposes the total SED into its stellar and AGN contributions. Our SED fitting retrieves a significant contribution to the total SED from an AGN template for 26 of our variable sources with $0 < z < 7$. We leverage the model AGN spectrum to estimate black hole masses ($M_{BH}$) using the measured luminosity at 5100 Å and local empirical calibrations. Common with recently discovered JWST broad line AGN (BL-AGN), we observe a trend in the $M_{BH} - M_{\ast}$ plane where low redshift sources have $M_{BH}$ which agree with local relations while high redshift sources have increasingly overmassive black holes with respect to the stellar mass ($M_{\ast}$) of their host galaxies. Within our sample, we identify two IMBH candidates hosted by dwarf galaxies at $z<1$ featuring overmassive BHs in the $M_{BH}-M_{\ast}$ plane, similarly to our high redshift sources. Finally, our SED fitter successfully retrieves the AGN nature of one source at $z >6$. This object has $z_{phot} = 6.74$ and we estimate a lower limit on its black hole mass of $\log_{10}(M_{BH}/M_{\odot}) > 7.36$.

abstract · abstract

Recent observations have identified an abundance of high-redshift active galactic nuclei (AGN) with supermassive black holes (BHs) that are over-massive compared to the local BH mass$-$total stellar mass ($M_{\mathrm{BH}}-M_\star$) relation. $M_{\mathrm{BH}}$ measurements at high-$z$ are critical for probing the growth histories of BHs and their host galaxies, including BH seeding and evolution of the $M_{\mathrm{BH}}-M_\star$ relation. However, BH masses in high-$z$ AGN are generally estimated from single-epoch measurements, which are anchored to local relations based on reverberation mapping and carry large systematic uncertainties. Alternate $M_{\mathrm{BH}}$ detection methods such as dynamical measurements are more reliable but currently only possible in the local Universe or with strongly lensed systems. Recently, dynamical $M_{\mathrm{BH}}$ measurements were made in a $z\sim2$ lensed quiescent galaxy as well as a sample of six local galaxies identified as likely relics of common quiescent red nugget galaxies at cosmic noon. We compare the $z\sim2$ red nugget and relic BHs to recent results for $4<z<11$ AGN, quasars, and Little Red Dots. Intriguingly, the $z\sim2$ galaxy and local relic galaxies all lie on both the local $M_{\mathrm{BH}}-M_\star$ relation for bulges and the $4<z<7$ $M_{\mathrm{BH}}-M_\star$ relation. Our results suggest the $M_{\mathrm{BH}}-M_\star$ relation for bulges was likely in place at high-$z$ and indicate careful consideration of different evolutionary pathways is needed when building BH scaling relations. While improvements to $M_{\mathrm{BH}}$ estimates in AGN will increase our confidence in high-$z$ BH masses, detecting BHs in relic galaxies and lensed galaxies presents a complementary probe of the high-$z$ relations.

abstract / block 1 · abstract

The recent discovery of a large number of massive black holes within the first two billion years after the Big Bang, as well as their peculiar properties, have been largely unexpected based on the extrapolation of the properties of luminous quasars. These findings have prompted the development of several theoretical models for the early formation and growth of black holes, which are, however, difficult to differentiate. We report the metallicity measurement around a gravitationally lensed massive black hole at redshift 7.04 (classified as a Little Red Dot), hosted in a galaxy with very low dynamical mass. The weakness of the [OIII]5007 emission line relative to the narrow H$β$ emission indicates extremely low metallicity, about $4\times 10^{-3}$ solar, and even more metal poor in the surrounding few 100 pc. We argue that such properties cannot be uncommon among accreting black holes around this early cosmic epoch. Explaining such a low chemical enrichment in a system that has developed a massive black hole is challenging for most theories. Models assuming heavy black hole seeds (such as Direct Collapse Black Holes) or super-Eddington accretion scenarios struggle to explain the observations, although they can potentially reproduce the observed properties in some cases. Models invoking "primordial black holes" (i.e. putative black holes formed shortly after the Big Bang) may potentially explain the low chemical enrichment associated with this black hole, although this class of models also requires further developments for proper testing.