The eRosita X-ray telescope has discovered eight galaxies whose central black holes are significantly more massive than those in other galaxies
This is how researchers envision the center of an active galaxy, where matter swirls around a supermassive black hole before falling into its center of mass. In the process, the incoming matter heats up to such an extent that, when viewed from Earth, it is primarily the brightly glowing core that is visible.
© Animation: Johannes Buchner (MPE), 3D visualisation: Angel Ruiz, Maria Chira, Antonis Georgakakis (NOA, 4MOVE-U)
To the Point
- Black Holes and Galaxies: Eight black holes that were studied are extremely massive compared to their host galaxies and grew disproportional to the galaxies themselves.
- Unusual Mass Ratios: The ratio of black hole mass to stellar mass in the galaxies is at least 1 to 20, which is significantly higher than the typical value of about 1 to 200.
Black Hole Growth Phase: The black holes are currently accreting matter at a rate that could double their mass within about one billion years.
Future Research: Additional eROSITA data and high-resolution observations will help determine how common these systems are and how the host galaxies evolve.
An international team of astronomers has identified eight active black holes whose masses account for at least five per cent of the total stellar mass of their host galaxies. In the nearby Universe, the corresponding fraction is typically about half a per cent. The result challenges the widely held view that galaxies and the black holes at their centres always grow in close step.
The study, led by the Max Planck Institute for Extraterrestrial Physics (MPE), is based primarily on data from the eROSITA X-ray telescope and observations at ultraviolet, optical and infrared wavelengths.
The team searched a 140-square-degree region of the sky containing 22,079 quasars. Quasars are active galactic nuclei in which matter falls towards a supermassive black hole and releases large amounts of radiation. The intense X-ray emission detected by eROSITA reveals that the black holes in the eight selected systems are actively accreting matter.
"Astronomers assumed that central black holes always grow closely coupled to their host galaxies. Our results show that this is not always the case," says Johannes Buchner, Postdoc at MPE, who led the study with international collaborators.
An extreme mass ratio
In the eight systems, the mass ratio between the black hole and the total stellar mass of the host galaxy is at least about 1:20. The black hole therefore accounts for at least roughly five per cent of the host galaxy's stellar mass - more than ten times the typical value in the nearby Universe.
For a statistically well-defined subsample, the researchers also examined the 200 brightest quasars in a particularly deep part of the survey field. Three of the eight extreme objects were among the 200 brightest quasars in the survey. Simulations that account for the selection procedure and measurement uncertainties indicate that the systems are not simply isolated outliers, but represent a distinct population at the extreme end of the distribution.
The black holes have masses between approximately 800 million and four billion solar masses. They are therefore roughly a thousand times more massive than Sagittarius A*, the black hole at the centre of the Milky Way.
"Think of it like finding a Great Dane living in a studio apartment. The black hole has simply grown too large for its home.", says Prof. Kirpal Nandra.
Measuring the black holes
The researchers estimated the black-hole masses from optical spectra obtained by the Sloan Digital Sky Survey. The analysis relied in particular on the broad H-beta emission line, produced by ionised gas moving in the vicinity of the black hole.
The width of the line provides information about velocity, indicating that gas is circling the black hole with 40,000,000 km/h. Combined with an estimate of its luminosity, this allowed doctoral students Catarina Aydar and Qiaoya Wu to infer the black-hole mass. The method is calibrated for the mass range and cosmic epoch covered by the study. The uncertainty for individual black-hole masses is nevertheless approximately a factor of three. The conclusion therefore rests not on the exact mass of any one object, but on the statistical evidence for the population as a whole.
"The spectra reveal black holes with masses of roughly one to four billion Suns. What is particularly unusual, however, is their mass relative to the stellar mass of their host galaxies," says Qiaoya Wu, a doctoral researcher at the University of Illinois Urbana-Champaign.
Faint host galaxies
The black holes are clearly detected as active quasars through their X-ray emission. Their host galaxies, by contrast, are very faint or not unambiguously detected in the available images.
The team analysed observations across several wavelength ranges, including ultraviolet data from the GALEX satellite, optical images from the DESI Legacy Imaging Survey, near-infrared data from the VISTA Hemisphere Survey and infrared observations from the WISE satellite. The researchers modelled the quasar and galaxy light separately.
If the host galaxies had stellar masses comparable to that of the Milky Way, they would be substantially brighter in the available images. The observations therefore indicate that the galaxies contain considerably fewer stars. They do not yet establish whether the galaxies are small, compact or largely inactive, nor whether they are still forming stars.
"The faintness of the host galaxies shows that they cannot contain a stellar population comparable to that of a Milky-Way-sized galaxy. If they did, they would be much more clearly visible," says Johannes Buchner.
Higher-resolution observations will be needed to determine the morphology and stellar populations of the host galaxies.
The black holes are still growing
X-ray observations show that the black holes are currently accreting matter. To account for the currently measured luminosity of the galactic nucleus, the black hole must gain about 40 million solar masses per billion years. However, this growth rate generally increases as the black hole becomes more massive and, consequently, its gravitational pull increases. The eROSITA telescope is therefore observing the systems during growth spurts-that is, periods of exponential growth. Their mass could thus double within about one billion years.
If accretion continues, the mass ratio between the black hole and the stellar component of the host galaxy will become even more extreme.
"These black holes are already unusually massive compared with their host galaxies, yet they are still growing," says Catarina Aydar, a doctoral researcher at MPE.
A challenge for galaxy-evolution models
Results of measurements of stellar mass (from ultraviolet to infrared images) and black hole mass (from spectra). Optical galaxy image cutouts are positioned at the measured values. Most galaxies lie close to the orange line, with a mass ratio of 0.5%. The largest ratios of black hole to stellar mass are found in the upper left, above the dashed line marking 5%. Left-pointing triangles indicate cases where only an upper limit on the stellar mass could be determined, meaning the true ratio could be much higher than 5%.
© Buchner et al., 2026
The observations point to a possible growth channel in which black holes temporarily gain mass faster than the stellar populations of their host galaxies. Current cosmological simulations do not produce systems with such extreme mass ratios.
The researchers compared their results to the Illustris, TNG, Horizon-AGN, EAGLE, Simba, Magneticum and ASTRID computer simulations. These models predominantly produce stellar to black hole mass ratios near the local value of 1 to 200. Systems as extreme as the ones observed are not produced.
The results therefore expose a limitation in existing models of the co-evolution of galaxies and their central black holes. They also provide a possible link to the overmassive black holes observed in the early Universe, where similarly extreme systems have raised questions about how rapidly black holes can grow.
An important open question is: How could the black hole grow to be so massive, without the galaxy forming stars proportionally?
"These results are exciting and unexpected. We had assumed that galaxies and their central black holes grow closely connected to one another. These observations indicate that black holes can become extremely massive relative to their galaxies, apparently to a large extent independently of the evolution of the stellar population. This challenges us to rethink how black holes grow over cosmic history," says Roberto Maiolino, a professor at the University of Cambridge, who was not involved in the study.
The eight quasars are likely to represent only the active, observable part of a larger population. Inactive or heavily obscured black holes cannot be identified as easily using the method applied here. Further eROSITA data and spectroscopic surveys will help determine how common these systems are.
The key open question is how the black holes gained so much mass without a corresponding increase in the stellar mass of their host galaxies. High-resolution observations will show whether the galaxies are still forming stars, how they are structured and how gas reached their central black holes over extended periods.
