Most smaller galaxies may not have supermassive black holes at their centers, according to a recent study using NASA's Chandra X-ray Observatory. This contrasts with the common belief that almost every galaxy has one of these giant black holes at its core, as NASA leads the world in studying how our universe works.
The team of astronomers used data from more than 1,600 galaxies collected over more than two decades by the Chandra mission. The researchers studied galaxies ranging in size from more than ten times the mass of the Milky Way to dwarf galaxies whose stellar mass is less than a few percent of the mass of our home galaxy. A paper describing these results was published in The Astrophysical Journal and is available here. https://arxiv.org/abs/2510.05252.
The team reported that only about 30% of dwarf galaxies likely contain supermassive black holes.
“It's important to get an accurate count of the number of black holes in these smaller galaxies,” said Fang Zou of the University of Michigan in Ann Arbor, who led the study. “This is more than just bookkeeping. Our study provides clues to how supermassive black holes are born. It also provides important clues about how often the signatures of black holes in dwarf galaxies can be detected by new or future telescopes.”
When material falls into black holes, it heats up due to friction and produces X-rays. Many of the massive galaxies examined in the study contain bright X-ray sources at their centers, a clear sign of supermassive black holes at their centers. The team concluded that more than 90% of massive galaxies, including galaxies with the mass of the Milky Way, contain supermassive black holes.
However, the smaller galaxies in the study generally did not have these unambiguous black hole signals. Galaxies with masses of less than three billion Suns—about the mass of the Large Magellanic Cloud, a close neighbor of the Milky Way—tend to contain no bright X-ray sources at their centers.
The researchers considered two possible explanations for the lack of X-ray sources. First, the fraction of galaxies containing massive black holes is much lower for these less massive galaxies. Second, the amount of X-rays produced by the matter falling into these black holes is so weak that Chandra cannot detect them.
“Based on our analysis of the Chandra data, we think that these smaller galaxies actually have fewer black holes than their larger counterparts,” said Elena Gallo, co-author also from the University of Michigan.
To reach this conclusion, Zou and his colleagues considered both possibilities for the absence of X-ray sources in the small galaxies in their large Chandra sample. The amount of gas falling into a black hole determines how bright or dim they appear in X-rays. Because smaller black holes are expected to absorb less gas than larger black holes, they should be fainter in X-rays and often undetectable. The researchers confirmed this expectation.
However, they found that less massive galaxies exhibit an additional shortage of X-ray sources, beyond the expected decrease due to the reduced amount of gas falling inward. This additional deficit can be explained by the fact that many low-mass galaxies simply do not have black holes at their centers. The team's conclusion was that the decline in X-ray detections in lower-mass galaxies reflects a true decline in the number of black holes located in those galaxies.
This result could have important implications for understanding how supermassive black holes form. There are two main ideas: first, a giant gas cloud directly collapses into a black hole, which from the very beginning contains thousands of times the mass of the Sun. Another idea is that supermassive black holes arise from much smaller black holes formed when massive stars collapse.
“The formation of large black holes is expected to be more rare, in the sense that it occurs predominantly in the most massive galaxies being formed, and this explains why we do not find black holes in all smaller galaxies,” said co-author Anil Seth of the University of Utah.
This study confirms the theory that giant black holes are being born, already weighing several thousand times the mass of the Sun. If the other idea were true, the researchers said they would expect smaller galaxies to likely have the same proportion of black holes as larger ones.
This result could also have important implications for the rate of black hole mergers resulting from dwarf galaxy collisions. A much smaller number of black holes will mean that in the future the spaceborne laser interferometer antenna will be able to detect fewer sources of gravitational waves. The number of black holes tearing apart stars in dwarf galaxies will also be fewer.
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center monitors science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.
To learn more about Chandra, visit:
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Megan Watzke
Chandra X-ray Center
Cambridge, Massachusetts.
617-496-7998
[email protected]
Corinne Beckinger
Marshall Space Flight Center, Huntsville, Alabama
256-544-0034
[email protected]
