Chapel Hill: A previously overlooked treasure trove of massive black holes in dwarf galaxies has been discovered by a team led by researchers at the University of North Carolina at Chapel Hill.
The newly discovered black holes offer a glimpse into the life history of the supermassive black hole at the center of our own galaxy, the Milky Way.
The study was published in the journal “The Astrophysical Journal”.
As a giant spiral galaxy, the Milky Way would have been built from the merger of many smaller dwarf galaxies. For example, the Magellanic Clouds seen in the southern sky are dwarf galaxies that will merge with the Milky Way. Each dwarf that falls may bring with it a central massive black hole, tens or hundreds of thousands of times the mass of our sun, potentially destined to be swallowed up by the Milky Way’s central supermassive black hole.
But how often dwarf galaxies contain a massive black hole is unknown, leaving a major gap in our understanding of how black holes and galaxies grow together.
“This result blew my mind because these black holes were previously hiding in plain sight,” said Mugdha Polimera, the study’s lead author and a doctoral student at UNC-Chapel Hill.
Send mixed messages
Black holes are usually detected when they are actively growing by ingesting gas and stardust swirling around them, causing them to glow brightly.
Sheila Kannappan, a UNC-Chapel Hill professor, Polimera thesis supervisor and co-author of the study, compared black holes to fireflies.
“Like fireflies, we only see black holes when they’re lit – when they grow – and lit ones give us a clue to how many we can’t see,” he added.
The problem is that if growing black holes shine with distinctive high-energy radiation, so can young, newborn stars. Traditionally, astronomers have differentiated growing black holes from the formation of new stars using diagnostic tests that rely on detailed characteristics of each galaxy’s visible light as it is spread out in a spectrum like an arc- in sky.
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 of the galaxies were sending mixed messages – two tests would indicate black hole growth, but a third would only indicate star formation.
“Previous work had just dismissed ambiguous cases like these from statistical analysis, but I had a hunch 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 the typical properties of dwarfs: their simple elemental composition (mainly primordial hydrogen and helium from the Big Bang) and their high rate of new star formation. .
Study co-author Chris Richardson, an associate professor at Elon University, confirmed with theoretical simulations that the mixed-message test results matched exactly what theory would predict for a compositional dwarf galaxy. primordial and highly star-forming planet containing a growing massive black hole. “The fact that my simulations matched what the Kannappan group found made me want to explore the implications for galaxy evolution,” Richardson said.
A census of growing black holes
Polymer took on the challenge of building a new census of growing black holes, paying attention to traditional and mixed message types. She obtained published measurements of the spectral characteristics of visible light to test black holes in thousands of galaxies found in two surveys by Kannappan, RESOLVE and ECO. These surveys include ultraviolet and radio data ideal for studying star formation, and they have an unusual design: while most astronomical surveys select samples that favor large and bright galaxies, RESOLVE and ECO are comprehensive inventories of huge volumes of the current universe in which dwarf galaxies are abundant.
“It was important to me that we didn’t skew our search for black holes towards dwarf galaxies,” Polimera said.
“But when looking at the whole census, I found that the new type of growing black holes almost always appeared in dwarfs. I was surprised by the numbers when I first saw them,” said he added.
More than 80% 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,” Polimera said.
“The first question that came to mind was, had we missed a way that extreme star formation alone could explain these galaxies?” She conducted an exhaustive search for alternative explanations involving star formation, modeling uncertainties, or exotic astrophysics. Ultimately, the team was forced to conclude that the newly identified black holes were real.
“We still pinch each other,” Kannappan said.
“We’re excited to pursue a zillion follow-up ideas. The black holes we’ve found are the building blocks of supermassive black holes like the one in our own Milky Way. There’s so much we want to teach them about. topic,” Kannappan added.
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