A team led by researchers at the University of North Carolina at Chapel Hill has found a previously overlooked treasure trove of massive black holes in dwarf galaxies. The newly discovered black holes offer a glimpse into the life story of the supermassive black hole at the center of our own Milky Way galaxy.
As a giant spiral galaxy, the Milky Way is believed to be made up of mergers of many smaller dwarf galaxies. For example, the Magellanic Clouds we see in the southern sky are dwarf galaxies that will merge into the Milky Way. Any dwarf that falls into it could bring with it a central massive black hole, tens or hundreds of thousands of times the mass of our sun, possibly destined to be swallowed by the Milky Way’s central supermassive black hole.
But how often dwarf galaxies contain a massive black hole is unknown, leaving an important gap in our understanding of how black holes and galaxies grow together. New research published in the Astrophysical Journal helps fill this gap by revealing that massive black holes are many times more common in dwarf galaxies than previously thought.
“This result really surprised me because these black holes were previously hidden in plain sight,” he said Mugdha Polimera, lead author of the study and a UNC-Chapel Hill Ph.D. student.
Send mixed messages
Black holes are usually detected when they are actively growing by taking in gas and stardust swirling around them, causing them to glow intensely.
UNC Chapel Hill Professor Sheila Kannappan, Polimera’s Ph.D. consultant and co-author of the study, compared black holes to fireflies.
“Like fireflies, we only see black holes when they’re illuminated — as they grow — and the illuminated ones give us an idea of how many we can’t see.”
The problem is that while growing black holes glow with signature high-energy radiation, so can young newborn stars. Traditionally, astronomers have distinguished between growing black holes and new star formation using diagnostic tests based on detailed features of each galaxy’s visible light as it is scattered in a spectrum such as a rainbow.
The path to discovery began when undergraduate students working with Kannappan tried to apply these traditional tests to galaxy survey data. The team realized that some galaxies were sending mixed messages — two tests would indicate growing black holes, but a third would indicate star formation only.
“Earlier work had just dismissed ambiguous cases like this from statistical analysis, but I had a hunch that they might be undiscovered black holes in dwarf galaxies,” Kannappan said. She suspected that the third, sometimes contradictory, test was more sensitive than the other two to typical properties of dwarfs: their simple elemental composition (primarily primordial hydrogen and helium from the Big Bang) and their high speed to form new stars.
Study co-author Chris Richardson, an associate professor at Elon University, confirmed with theoretical simulations that the mixed-message test results matched exactly what the theory would predict for a primordial composite, highly star-forming dwarf galaxy with an expanding massive black hole. “The fact that my simulations matched what the Kannappan group found made me excited to explore the implications for galaxy evolution,” Richardson said.
A count of growing black holes
Polimera took on the challenge of setting up a new count of growing black holes, focusing on both traditional and mixed messages. She obtained published measurements of visible light spectral features to test for black holes in thousands of galaxies found in two studies led by Kannappan, SOLVE and ECO† These studies include ultraviolet and radio data ideal for studying star formation, and they are of unusual design: while most astronomical studies select samples that prefer large and bright galaxies, RESOLVE and ECO are complete inventories of vast volumes of the current universe in which dwarf galaxies are abundant.
“It was important to me that we don’t bias our search for a black hole toward dwarf galaxies,” Polimera said. “But when I looked at the whole census, I found that the new type of growing black holes almost always appeared in dwarfs. When I first saw them, the numbers shocked me.”
More than 80 percent of all the growing black holes she found in dwarf galaxies belonged to the new type.
The result seemed too good. “We all got nervous,” said Polimera. “The first question that came to my mind was, did we miss a way that only extreme star formation could explain these galaxies?” She led an exhaustive search for alternative explanations for star formation, uncertainty modeling or exotic astrophysics. Ultimately, the team had to conclude that the newly identified black holes were real.
“We’re still pinching ourselves,” Kannappan said. “We are excited to pursue a countless number of follow-up ideas. The black holes we have found are the basic building blocks of supermassive black holes like those in our own Milky Way. There’s so much we want to learn about them.”
This research was funded in part by the National Science Foundation.
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