UNIVERSITY PARK, Pa. — According to common convention, nearly every galaxy has a supermassive black hole at its center, but a new study using data from NASA’s Chandra X-ray Observatory has revealed that this idea does not hold true for smaller galaxies. An international team that includes a Penn State scientist found that most smaller galaxies may not have one of these giant black holes within their cores.
The study, which incorporates data from over 1,600 galaxies collected in more than two decades by NASA’s Chandra X-ray Observatory, was published in The Astrophysical Journal. These galaxies ranged from dwarf galaxies with stellar masses less than a few percent of the Milky Way to galaxies with more than 10 times the mass of the Milky Way. The team found that only about 30% of dwarf galaxies likely contain supermassive black holes.
“It’s important to get an accurate black hole head count in these smaller galaxies,” said Fan Zou, a postdoctoral researcher at the University of Michigan who earned a doctoral degree at Penn State in 2024 and led the study. “It’s more than just bookkeeping. Our study gives clues about how supermassive black holes are born. It also provides crucial hints about how often black hole signatures in dwarf galaxies can be found with new or future telescopes.”
As material falls onto black holes, it is heated by friction and produces X-rays. Many of the massive galaxies in the study contain bright X-ray sources in their centers, a clear signature of supermassive black holes in 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, including those with the mass of the Milky Way.
However, smaller galaxies in the study usually did not have these unambiguous black hole signals. Galaxies with masses less than three billion suns — about the mass of the Large Magellanic Cloud, a close neighbor to the Milky Way — usually do not contain bright X-ray sources in their centers.
“Robustly establishing these results relied upon a much larger galaxy sample than used in previous work — about five times larger — as well as some impressive statistical methods,” said Niel Brandt, Eberly Family Chair Professor of Astronomy and Astrophysics, professor of physics at Penn State and an author of the study. “The steady advances over the past few decades in both generating large X-ray observation samples and tackling gnarly statistical inference challenges have, slowly but surely, been transformational for high-energy astrophysical discovery.”
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 onto these black holes is so faint that Chandra cannot detect it.
“We think, based on our analysis of the Chandra data, that there really are fewer black holes in these smaller galaxies than in their larger counterparts,” said Elena Gallo, professor of astronomy at the University of Michigan and an author of the paper.
To reach their conclusion, the researchers considered both possibilities for the lack of X-ray sources in small galaxies in their large Chandra sample. The amount of gas falling onto a black hole determines how bright or faint they are in X-rays. Because smaller black holes are expected to pull in less gas than larger black holes, they should be fainter in X-rays and often not detectable. The researchers confirmed this expectation in their analysis.
However, they found that less massive galaxies show an additional deficit of X-ray sources beyond the expected decline from decreases in the amount of gas falling inwards. This additional deficit can be accounted for if many of the low mass galaxies simply don't have any black holes at their centers. The team’s conclusion was that the drop in X-ray detections in lower 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, the researchers said. There are two main ideas: a giant gas cloud directly collapses into a black hole, which contains thousands of times the sun’s mass from the start, or supermassive black holes instead come from the growth and/or merging of much smaller black holes, created when massive stars collapse.
“The formation of big black holes is expected to be rarer, in the sense that it occurs preferentially in the most massive galaxies being formed, so that would explain why we don't find black holes in all the smaller galaxies,” said Anil Seth, professor of physics and astronomy at the University of Utah and an author of the paper.
As such, the researchers said, this new study supports the theory that giant black holes are born already weighing several thousand times the Sun’s mass.. If the other idea were true, the researchers said they would have expected smaller galaxies to likely have the same fraction of black holes as larger ones.
This result also could have implications for the rates of black hole mergers from the collisions of dwarf galaxies. These mergers produce ripples in space time called gravitational waves that can be observed on Earth with specialized detectors, such as LIGO or the future Laser Interferometer Space Antenna. A much lower number of black holes would result in fewer sources of gravitational waves.
“Chandra is still working great for these kinds of observations and, if reliably funded, can keep building up the samples that underly such fundamental cosmic insights,” Brandt said
A full list of study authors is available in the paper. This work was funded by the National Aeronautics Space Administration and the U.S. National Science Foundation.
Editor’s Note: A version of this release originally appeared on the Chandra website.