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Anybody Home? Black Hole Occupation Fraction Helps Clarify Galactic History

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Visible and x-ray image of a galaxy indicating the possible presence of an accreting black hole offset from the galaxy's center.
This composite visible and X-ray image shows a galaxy that is thought to contain a "wandering" massive black hole, offset from the galaxy's center. [NASA/CXC/UNH/D.Lin et al; Optical: NASA/STScI]

Most galaxies are thought to contain at least one massive black hole, but how absolute is this association, really — and what can it tell us about galactic evolution?

Is There Anybody Home?

Massive black holes are everywhere. Well, not quite everywhere — but they are almost always found lurking deep within galaxies. The reason for this is somewhat straightforward: black holes can form in the early universe, and the repeated black hole mergers that occur as galaxies collide and grow over billions of years inevitably lead to some of these black holes ballooning to massive or supermassive status.

The ubiquity of massive black holes is quantified by the black hole occupation fraction (BHOF), a number that represents the fraction of all galaxies that contain at least one such object. For high-mass galaxies, determining the BHOF is relatively straightforward. In these systems, central black holes can have significant dynamical effects that enable direct detections, and observational evidence suggests that the BHOF is close to 1. When it comes to lower-mass galaxies, however, the BHOF is harder to measure. Black holes in lower-mass galaxies often cannot accrete enough material to form bright accretion disks or noticeably disrupt visible matter and stars, making it difficult to determine whether the BHOF stays close to unity across all galaxy masses. 

Galaxies are typically expected to contain at least one if not dozens of massive black holes (black and yellow circles). Their individual positions and electromagnetic signatures can vary greatly in different galaxies, however, as demonstrated by this simulated galaxy. [Ricarte et al. 2021]

Understanding the BHOF has implications for the study of galactic evolution. In high-mass galaxies, successive mergers and dense cores enable the formation of supermassive black holes — but also homogenize the central black holes, erasing clues to their origins. In low-mass galaxies, the position and masses of central black holes may retain some variation that can inform us about their origins. In the absence of robust observational data on the BHOF in low-mass galaxies, a team led by Emma Jane Weller (Yale University) decided to examine the next best thing: cosmological simulations.

Location, Location, Location!

Weller and collaborators utilized ASTRID, a high-resolution cosmological simulation that includes a sophisticated prescription for seeding and evolving black holes. Unlike in some other cosmological simulations, when massive black holes are formed they are not simply forced to sit at the centers of their host galaxies. Rather, perturbations from mergers and interactions with other black holes can displace them, creating two classes of massive black holes: central and wandering — where wandering black holes are any that sit in the outer half of their host galaxies. The simulation is also able to determine when a black hole undergoes significant accretion.

3-panel figure showing the total, central, and wandering BHOF across cosmic redshifts and a range of galaxy masses.

The total, central, and wandering BHOF (colored lines) for galaxies of various masses found in the ASTRID simulation. Recent constraints on the BHOF from Zou et al. 2025 are overlaid in gray. [Weller et al. 2026]

By examining the BHOF across a variety of systems, the authors identified a few notable trends. While the total BHOF remained very close to 1 across the entire mass range, the types of black holes varied significantly. For low-mass galaxies, the central BHOF fell over time, while the wandering BHOF rose. This trade-off can be explained by repeated mergers displacing central black holes. In small galaxies, shallow gravitational potentials make it difficult for these black holes to return to galactic centers after a merger. These off-center systems rarely accrete and are generally difficult to observe. This result has important implications for observational studies, because without understanding the prevalence of wandering black holes, observations of galaxies without an obvious black hole could be wrongly interpreted as evidence for inefficiency in the black hole seeding process.

Weller and collaborators additionally found that the BHOF for strongly accreting black holes was much smaller than the total BHOF, especially after cosmic noon where galactic growth slows. As with the trends in wandering versus central black holes, this finding highlights the need for nuanced interpretations of observational constraints on the BHOF, as the location, activity, and overall observability of massive black holes can vary significantly depending on their galactic environment, merger history, and age.

Citation

“The Black Hole Occupation Fraction as a Fossil Record of Seeding, Dynamics, and Galaxy Assembly,” Emma Jane Weller et al 2026 ApJL 1008 L54. https://doi.org/10.3847/2041-8213/ae9c38