eROSITA Finds Eight Black Holes 10 Times More Dominant Than Seven Simulations Predict

Eight actively feeding supermassive black holes are far too large for the faint galaxies around them under the usual local relationship. Each black hole accounts for at least 5% of its host galaxy’s stellar mass, compared with a typical proportion of roughly 0.5%. Yet none of seven major cosmological simulations produced systems with comparably extreme ratios.

That mismatch does not overturn every theory of black-hole evolution. It does identify a shared blind spot: black holes can apparently outgrow their galaxies much more dramatically than current large-scale models commonly reproduce. The Max Planck Institute for Extraterrestrial Physics-led study, published in Astronomy & Astrophysics, reached that conclusion using X-ray selection, optical spectroscopy and ultraviolet-to-infrared measurements rather than relying on one unusually bright object.

Eight systems emerged from 22,079 quasars

The researchers started with 22,079 quasars in the 140-square-degree eROSITA Final Equatorial-Depth Survey field. The eROSITA space-based X-ray telescope detected radiation associated with active accretion, confirming that the eight selected black holes were still drawing in matter rather than representing dormant objects inferred only from their gravitational effects.

The black holes have estimated masses ranging from about 800 million to four billion times the Sun’s mass. They sit at redshifts between 0.3 and 0.8, corresponding to roughly three billion to seven billion years after the Big Bang under the study’s chronology.

To estimate each mass, the team used Sloan Digital Sky Survey spectra and measured the broad H-beta emission line. Gas producing that line was moving at roughly 40 million kilometers per hour. Combining its velocity with a luminosity-based estimate of the emitting region’s size gave the researchers a mass estimate for the central black hole.

This method carries substantial uncertainty. An individual estimate can be off by approximately a factor of three, which would make a single extreme object a weak basis for rewriting galaxy-growth models. The stronger result is the repeated pattern across eight independently measured systems: all occupy the unusually black-hole-heavy end of the distribution.

Faint host galaxies are the other half of the measurement

A massive black hole alone does not establish an abnormal mass ratio. Researchers also had to determine whether the surrounding galaxy contained enough stars to restore the familiar relationship.

That is difficult because an actively feeding black hole can produce a quasar bright enough to overwhelm the host galaxy’s light. The team addressed the problem by combining ultraviolet, optical and infrared observations from GALEX, the DESI Legacy Imaging Survey, the VISTA Hemisphere Survey and WISE. Galaxy-quasar decomposition then modeled the two light sources separately.

The resulting host galaxies remained too faint to contain the stellar populations required by the usual ratio. As study leader Dr. Johannes Buchner explained, a stellar population comparable to that of a Milky Way-sized galaxy would have been substantially more visible. The measured ratios were at least 1:20, meaning the black hole represented 5% or more of the host’s stellar mass. The typical local comparison used by the study is about 1:200.

Seven simulations miss the same extreme population

The researchers compared the observations with Illustris, TNG, Horizon-AGN, EAGLE, SIMBA, Magneticum and ASTRID. None generated systems with similarly extreme black-hole-to-stellar-mass ratios.

That common miss matters because these simulations do not all implement galaxy and black-hole growth identically. Finding the same limitation across seven models points toward a broader modeling constraint rather than an isolated quirk in one software framework. The observations challenge the assumption that central black-hole growth remains tightly coupled to the buildup of a galaxy’s stars under all conditions.

Several physical explanations remain possible. Star formation might have been suppressed early while the central black hole continued growing. The systems could have begun with unusually massive black-hole seeds, reducing the amount of later growth needed. Sustained super-Eddington accretion growth exceeding the conventional rate associated with outward radiation pressure could also help produce the imbalance. The study does not confirm any of those scenarios.

There is another important boundary: eROSITA selected black holes bright in X-rays because they were actively accreting. The observations therefore establish a population of active, overmassive black holes, not the size of any unseen dormant population. Determining whether these eight are rare active outliers or the visible fraction of a much larger population will require additional surveys and measurements. For now, the confirmed result remains unusually stark: eight observed systems occupy a region that seven leading galaxy-growth simulations fail to reproduce.

By David Whitaker — Associate editor for AMI’s aerospace and drone systems desk, translating flight systems, aircraft programs, spaceflight, and UAV developments into accessible technical stories.