Most smaller galaxies may not have supermassive black holes in their centers, according to a new study using NASA's Chandra X-ray Observatory. This contrasts with other studies that suggested nearly every galaxy has one of these giant black holes within their cores.
"This result is a little surprising to me, because a lot of my other work has found that small galaxies do, at least sometimes, have central supermassive black holes," said Anil Seth, University of Utah professor in the Department of Physics & Astronomy and co-author on the study.
The team of astronomers used data from over 1,600 galaxies collected in more than two decades of the Chandra mission. David Ohlson, doctoral student at the U and co-author of the study, created the massive catalogue of galaxy information.
"I'm thrilled to see our galaxy catalog used to better understand the likely origin of massive black holes," he said.
The researchers looked at galaxies ranging in heft from over ten times the mass of the Milky Way down to dwarf galaxies, which have masses less than a few percent of our home galaxy. The team has reported 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 of the University of Michigan in Ann Arbor, who 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 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 detectable X-ray sources in 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 number 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, a co-author also of the University of Michigan.
To reach their conclusion, Zou and his colleagues 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.
However, they found that an additional deficit of X-ray sources is seen in less massive galaxies 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. There are two main ideas: in the first a giant gas cloud directly collapses into a black hole, which contains thousands of times the sun's mass 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 big black holes is expected to be rarer, so that would explain why we don't find black holes in all the smaller galaxies," said the U's Anil Seth.
This new study supports the theory where giant black holes are born large. If the other idea were true, the researchers said they would have expected smaller galaxies to have the same fraction of black holes as larger ones.
This result also could have important implications for the rates of black hole mergers from the collisions of dwarf galaxies. A much lower number of black holes would result in fewer sources of gravitational waves to be detected in the future by the Laser Interferometer Space Antenna. The number of black holes tearing stars apart in dwarf galaxies will also be smaller.
A paper describing these results has been published in The Astrophysical Journal.
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.
Adapted from a release from NASA CHANDRA X-ray Observatory.
