Is JWST observing impossibly huge supermassive black holes in the early universe? A new study suggests that observational biases can explain the recently discovered population of over-massive black holes.
Too Supermassive Black Holes
Most galaxies in the universe house a central supermassive black hole. Even galaxies that populate the early universe are home to black holes that somehow grew to millions of times the mass of the Sun within the first billion years after the Big Bang. Recent JWST observations have uncovered a seemingly “over-massive” population of central black holes with redshifts of z ≳ 4, which corresponds to galaxies from the first ~1.5 billion years of the universe.
Why do these black holes seem over-massive? Compared to galaxies in the local universe, these early universe black holes appear to be too massive given the masses of their host galaxies. In other words, these black holes fall above the local universe black hole mass–stellar mass relation. This suggests that most black holes within the early universe are over-massive and were probably born out of heavy seeds — likely clouds of material that directly collapsed into massive black holes. However, in the high-redshift observational regime, the impacts of observational biases and measurement uncertainties must be fully understood to confidently draw these conclusions.
The impacts of selection bias and measurement uncertainties on the black hole mass–stellar mass relation. Top left: Distribution of this study’s mock galaxies with active supermassive black holes (AGNs). The local black hole mass–stellar mass relation is overlaid. Top right: Distribution of mock AGN with the selection criteria (limiting luminosity and spectral line width) applied. Bottom left: Distribution of mock AGN including measurement uncertainties in black hole mass and host galaxy stellar mass. Bottom right: Distribution of mock AGN including both selection biases and mass uncertainties. Click to enlarge. [Li et al 2025]
Examining Observational Biases
Even with JWST’s advanced observational capabilities, there are selection effects and observational limitations at play when studying galaxies in the very distant past. In order to detect supermassive black holes at high redshifts, the galaxies must be bright enough to be seen from so far away and must have black holes that are actively accreting material and producing broad emission lines. To understand how these selection effects impact the interpretation of observations, Junyao Li (University of Illinois at Urbana-Champaign) and collaborators performed a rigorous statistical analysis to determine if the recently discovered over-massive black hole population is truly over-massive.
Assuming that galaxies at the mean redshift of current JWST observations follow the local black hole mass–stellar mass relation, the authors create a mock sample of galaxies with active central supermassive black holes to model the expected distribution of black hole mass and host galaxy stellar mass in the early universe. The authors then carefully apply the observational biases and inherent uncertainties in mass measurements to their mock sample to produce the relation that would be detected with JWST. With these limitations taken into account, the authors show that in practice, JWST is only sampling the brightest, most massive black holes — creating a seemingly over-massive black hole population when compared to the local universe. Their modeling suggests that a population of “under-massive” or lower-mass black holes is likely being missed in JWST observations, which if detected, would mean that the mass distribution of early universe black holes does not deviate from that in the local universe.
Small Seed Suggestions
The distribution of mock galaxies with active central supermassive black holes (AGN) with observations from JWST shown with blue stars. The evolutionary paths of black holes with light (circles) and heavy (triangles) seeds are plotted. The predicted depth of JWST survey JDEEP is outlined in blue, showing that the lowest mass black holes with light seeds would not be detected by this survey. Click to enlarge. [Li et al 2025]
This study highlights the need for careful consideration of selection effects and uncertainties, even with advanced and deep instruments like JWST. Obtaining measurements for low-mass black holes is imperative to gaining a further understanding of black hole seeding and the ways supermassive black holes evolve with their host galaxies. Using these measurements in tandem with improving theoretical predictions for the assembly history of black holes, the distant past will become a bit more familiar.
Citation
“Tip of the Iceberg: Overmassive Black Holes at 4 < z < 7 Found by JWST Are Not Inconsistent with the Local MBH–M* Relation,” Junyao Li et al 2025 ApJ 981 19. doi: 10.3847/1538-4357/ada603