NASA’s Chandra Finds Small Galaxies May Buck the Black Hole Trend
Most small 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 idea that almost all galaxies have one of these giant black holes in their core, while NASA leads the world in exploring how our universe works.
A team of astronomers used data from more than 1,600 galaxies collected over more than two decades of the Chandra mission. Researchers have studied galaxies ranging from more than ten times the mass of the Milky Way to dwarf galaxies, whose stellar mass is a few percent less than that 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 small galaxies,” said Fan Zou of the University of Michigan in Ann Arbor, who led the study. “It’s more than just accounting. Our study provides clues about how supermassive black holes are born. It also provides crucial insights into how often black hole signatures in dwarf galaxies can be found with new or future telescopes.”
When matter falls on black holes, it is heated by friction and produces X-rays. Many massive galaxies studied contain bright X-ray sources at their centers, a clear signature of supermassive black holes at their centers. The team concluded that more than 90% of massive galaxies – including those with the mass of the Milky Way – contain supermassive black holes.
However, the smaller galaxies studied generally lacked these unambiguous black hole signals. Galaxies with a mass of less than three billion Suns – about the mass of the Large Magellanic Cloud, a close neighbor of the Milky Way – generally do not contain bright X-ray sources at their centers.
The researchers considered two possible explanations for this lack of X-ray sources. The first is that the fraction of galaxies containing massive black holes is much lower for these less massive galaxies. The second is that the amount of X-rays produced by matter falling on these black holes is so small that Chandra cannot detect it.
“We think, based on our analysis of the Chandra data, that there are actually fewer black holes in these smaller galaxies than in their larger counterparts,” said co-author Elena Gallo, also from the University of Michigan.
To reach their conclusion, Zou and his colleagues examined two possibilities related to the lack of X-ray sources in the small galaxies in their large Chandra sample. The amount of gas falling on a black hole determines how bright or dim it is in X-rays. Since smaller black holes are thought to suck in less gas than larger black holes, they are expected to be fainter in X-rays and often undetectable. The researchers confirmed this expectation.
However, they found that an additional deficit of X-ray sources is observed in less massive galaxies, beyond the expected decline due to the decrease in the amount of gas falling inward. This additional deficit can be explained if many low-mass galaxies simply do not have a black hole at their center. The team’s conclusion is that the decline in X-ray detections in low-mass galaxies reflects a true decrease in the number of black holes located in these galaxies.
This result could have important implications for understanding how supermassive black holes form. There are two main ideas: In the first, a giant gas cloud collapses directly into a black hole, which contains thousands of times the mass of the Sun from the start. The other idea is that supermassive black holes instead come from much smaller black holes, created when massive stars collapse.
“The formation of large black holes should be rarer, in the sense that it occurs preferentially in the most massive galaxies forming, which would explain why we don’t find black holes in all small galaxies,” said co-author Anil Seth of the University of Utah.
This study supports the theory according to which giant black holes are born, already weighing several thousand times the mass of the Sun. If the other idea were true, the researchers said they would have expected that smaller galaxies would likely have the same fraction of black holes as larger ones.
This result could also have important implications for black hole merger rates resulting from dwarf galaxy collisions. A much lower number of black holes would result in fewer sources of gravitational waves detected in the future by the Laser Interferometer’s space antenna. The number of black holes tearing apart stars in dwarf galaxies will also be smaller.
NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.
To learn more about Chandra, visit:
https://science.nasa.gov/chandra
Learn more about NASA’s Chandra X-ray Observatory
Learn more about the Chandra X-ray Observatory and its mission here:
https://chandra.si.edu
Megan Watzke
Chandra X-ray Center
Cambridge, Mass.
617-496-7998
mwatzke@cfa.harvard.edu
Corinne Beckinger
Marshall Space Flight Center, Huntsville, Alabama
256-544-0034
corinne.m.beckinger@nasa.gov
