Scientists Find More Active Black Holes in Dwarf and Milky Way-Sized Galaxies by Cutting Through the Glare of Star Formation

Astronomers have just completed what is being described as the most comprehensive census of active galactic nuclei (AGN) ever carried out in the nearby universe, and the results are reshaping how scientists think about black holes in galaxies of all sizes. By analyzing more than 8,000 nearby galaxies, researchers have found that active black holes are significantly more common in dwarf galaxies than previously believed, while also confirming a sharp rise in black hole activity in galaxies similar in size to the Milky Way.

This new census provides the clearest statistical picture yet of how frequently galaxies host actively feeding black holes, and it highlights how improved detection techniques are helping scientists see what was once hidden behind the intense glow of star formation.

What the New Census Looked At

The research team, made up of scientists from the Center for Astrophysics | Harvard & Smithsonian and the University of North Carolina at Chapel Hill, revisited thousands of galaxies that had already been observed in earlier surveys. Instead of relying on a single method, they used a combination of optical, infrared, and X-ray data to reassess whether these galaxies showed signs of AGN activity.

AGN are regions at the centers of galaxies where supermassive or massive black holes are actively accreting material, releasing enormous amounts of energy in the process. Detecting them is not always straightforward, especially in galaxies undergoing heavy star formation, where bright starlight can overwhelm the much subtler signals of a feeding black hole.

The galaxies were grouped by stellar mass, allowing the researchers to compare dwarf galaxies, Milky Way-sized galaxies, and larger systems using a consistent framework.

Dwarf Galaxies Host More Active Black Holes Than Expected

One of the most striking findings involves dwarf galaxies, which are small, low-mass galaxies often thought to be simple systems. Previous studies typically suggested that only about 1% of dwarf galaxies hosted active black holes. In practical terms, that meant roughly 10 active black holes per 1,000 dwarf galaxies.

The new census paints a very different picture. Using updated detection methods and mass-based selection techniques, the researchers found that between 2% and 5% of dwarf galaxies show evidence of AGN activity. That translates to 20 to 50 active black holes per 1,000 dwarf galaxies, a substantial increase over earlier estimates.

While this number is still relatively small compared to larger galaxies, it strongly suggests that massive black holes in dwarf galaxies are far more common than astronomers once assumed.

A Sharp Jump in Milky Way-Sized Galaxies

The study also revealed a dramatic increase in AGN activity in galaxies with masses similar to the Milky Way. In these mid-sized or transitional galaxies, the fraction hosting active black holes rises sharply to between 16% and 27%.

Even larger galaxies show even higher levels of activity, with approximately 20% to 48% displaying signs of an active galactic nucleus. This steep rise between dwarf galaxies and Milky Way-sized systems is one of the most intriguing results of the census.

Scientists are still working to understand why this transition is so pronounced. It could reflect fundamental changes in galaxy structure, gas availability, or black hole fueling mechanisms, or it may indicate that AGN in smaller galaxies remain harder to detect despite improved techniques.

Cutting Through the Glare of Star Formation

A key reason this census differs from earlier efforts is its ability to separate black hole activity from star formation. Young, rapidly forming stars emit intense light, particularly in optical and infrared wavelengths, which can easily hide the weaker signals produced by low-luminosity AGN.

By combining data across multiple wavelengths and applying new analysis methods, the researchers were able to identify faint accreting black holes that had slipped under the radar in previous surveys. This approach allowed them to detect AGN even in galaxies where star formation is vigorous and visually dominant.

Because of this, the authors emphasize that the reported percentages are still approximate. As observations improve and future surveys expand, the census is expected to evolve further.

Why These Results Matter

Understanding how often galaxies host active black holes is essential for answering some of the biggest questions in astrophysics. Black holes are thought to play a central role in galaxy evolution, influencing star formation, gas dynamics, and the overall growth of galaxies across cosmic time.

The higher-than-expected number of AGN in dwarf galaxies is especially important for testing models of black hole formation. Many theories suggest that the first black holes formed early in the universe and later grew through mergers and accretion. If dwarf galaxies commonly host massive black holes, they could represent relics of these early formation processes.

The findings also support the idea that galaxies like the Milky Way were built through the merging of smaller galaxies. In such a scenario, the black holes from these dwarf systems would eventually merge as well, contributing to the growth of the Milky Way’s central supermassive black hole.

The Role of Galaxy Mass in Black Hole Growth

One clear takeaway from the census is that galaxy mass matters. As galaxies grow larger and more massive, their likelihood of hosting an active black hole increases significantly. This suggests that more massive galaxies may provide better conditions for sustained black hole feeding, such as larger gas reservoirs or more efficient mechanisms for funneling material toward their centers.

However, the abrupt rise in AGN activity around Milky Way-sized galaxies hints that the relationship between galaxy mass and black hole growth is not smooth or simple. Instead, there may be critical thresholds where the internal physics of galaxies changes in ways that strongly favor black hole activity.

Data Transparency and Future Research

The research team is in the process of releasing the processed measurements used in the census, allowing other scientists to reproduce the results and build upon them. This open-data approach is expected to accelerate further studies into black hole demographics and galaxy evolution.

Future surveys, particularly those using next-generation telescopes and deeper X-ray and infrared observations, are likely to uncover even more hidden black holes, especially in the smallest and faintest galaxies.

As astronomers continue to refine their tools, this census serves as a strong foundation for understanding how black holes form, grow, and influence the galaxies around them.

Research paper (preprint):
https://arxiv.org/abs/2601.0XXXX