Study: Central Massive Black Holes Are Not Ubiquitous in Local Low-Mass Galaxies (DOI: 10.3847/1538-4357/ae06a1)
A new study challenges convention and suggests that there are a large number of galaxies, especially small ones, without black holes at their cores. This could shed more light on how big black holes came to exist.
This finding, led by researchers at the University of Michigan using NASA's Chandra X-ray Observatory, is at odds with the prevailing astronomical notion that nearly every massive galaxy has one of these giant black holes at its core.
The team, made up of researchers from more than a dozen institutions around the world, used data from more than 1,600 galaxies collected over 20-plus years of the federally-funded Chandra mission. The size of the galaxies ranged from more than 10 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's analysis showed that only about 30% of dwarf galaxies likely contain supermassive black holes. Black holes were far more common in massive galaxies, such as the Milky Way, being present in more than 90%. The team published its findings in The Astrophysical Journal.
"It's more than just bookkeeping," said Fan Zou, a postdoctoral researcher in the U-M Department of Astronomy who led the study. "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."
What's going on in small galaxies is of particular interest because they're more reflective of what the universe was like longer ago, Zou said.
"It's important to get an accurate black hole head count in these smaller galaxies," Zou said. "With low mass galaxies, we expect that they haven't changed much since they were born, so they're kind of like fossils from the early universe. By looking at low-mass galaxies and black holes, we can learn more about what was happening in the early universe."
"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, a coauthor and a professor at University of Utah.
There are currently two main theories about how supermassive black holes form. One is that they grow from smaller black holes, created when giant stars run out of fuel and collapse. The second idea is that the giant black holes are born big from the collapse of enormous gas clouds so that they have the mass of thousands of suns to begin with. The team's findings suggest the latter is more likely.
The case of the missing X-rays
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. But the study's smaller galaxies-galaxies with masses less than 3 billion suns-usually lacked these unambiguous black hole signals (for comparison, the Milky Way has a mass of around 60 billion suns).
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, a coauthor and U-M professor of astronomy.
Gallo and her colleagues were able to consider 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.
They also found, however, that this explanation alone could not account for the entire deficit of X-ray sources. That is, there was an additional deficit beyond what was expected. And this additional deficit could be accounted for if many of the low mass galaxies simply don't have any black holes at their centers.
"Our analysis, statistically, is able to tell us that the likelihood is much higher that the black holes aren't there," Gallo said.
Gallo and Zou said the Laser Interferometer Space Antenna, a joint project between NASA and the European Space Agency that's being developed to launch in 2035, will be able to further test their conclusions. With NASA's funding outlook murky at the moment, though, it remains to be seen how the agency will continue the legacy of large, flagship missions like Chandra, the Hubble Space Telescope and the JWST.
"These machines, these products of human ingenuity, have really given us an understanding of the universe and our place in it," Gallo said. "These great missions have delivered enormous knowledge and this study is one tiny piece of that."
