(ORDO NEWS) — A team led by researchers from the University of North Carolina at Chapel Hill has discovered a previously unseen treasure trove of massive black holes in dwarf galaxies. Newly discovered black holes provide a glimpse into the life history of the supermassive black hole at the center of our own Milky Way galaxy.
It is believed that the Milky Way, being a giant spiral galaxy, was formed as a result of the merger of many smaller dwarf galaxies. For example, the Magellanic Clouds that are visible in the southern sky are dwarf galaxies that will merge with the Milky Way in the future.
Each dwarf galaxy can bring with it a central massive black hole tens or hundreds of thousands times the mass of our Sun, which may be destined to be swallowed up by the Milky Way’s central supermassive black hole.
But how often dwarf galaxies host a massive black hole is unknown, leaving a key gap in our understanding of how black holes and galaxies grow together. A new study published in the Astrophysical Journal helps fill this gap by showing that massive black holes are many times more common in dwarf galaxies than previously thought.
“This result just blew me away because these black holes used to hide in plain sight,” said Mugdha Polymera, lead author of the study and a graduate student at UNC-Chapel Hill.
Black holes are usually discovered when they are actively growing, absorbing gas and stardust orbiting around them, causing them to glow intensely.
UNC-Chapel Hill professor Sheila Kannappan, Polymer’s adviser and co-author of the study, compared black holes to fireflies.
“Like fireflies, we only see black holes when they’re glowing – when they’re growing – and glowing holes give us a clue about how many of them are still missing.”
The problem is that while growing black holes glow with characteristic high-energy radiation, young newborn stars can also glow.
Traditionally, astronomers distinguish growing black holes from new star formations using diagnostic tests that rely on the detailed features of each galaxy’s visible light when it is spread out into a rainbow-like spectrum.
The road to discovery began when senior students working with Kannappan tried to apply these traditional tests to galaxy survey data. The team realized that some galaxies are sending mixed messages: two tests point to growing black holes, and a third only to star formation.
“In previous work, such ambiguous cases were simply discarded from the statistical analysis, but I had a hunch that these could be undiscovered black holes in dwarf galaxies,” Kannappan says.
She suspects that the third, sometimes controversial, test was more sensitive than the other two to the typical properties of dwarf galaxies: their simple elemental composition (mostly primordial hydrogen and helium from the Big Bang) and high rates of new star formation.
Study co-author Chris Richardson, an associate professor at Elon University, confirmed using theoretical simulations that the mixed message test results are exactly what theory predicts for a primordial dwarf galaxy with a high star formation rate containing a growing massive black hole.
“The fact that my simulations matched the results of the Kannappan group made me excited to explore the implications for galaxy evolution,” Richardson said.
Census of Growing Black Holes
Polymera has taken the trouble to build a new census of growing black holes, focusing on both traditional and mixed types. She obtained published measurements of the spectral characteristics of visible light to test for the presence of black holes in thousands of galaxies, discovered in two Kannappan-led surveys, RESOLVE and ECO.
These surveys include ultraviolet and radio data ideal for studying star formation: While most astronomical surveys sample large and bright galaxies, RESOLVE and ECO provide a complete inventory of the vast volumes of the modern universe, which is abundant in dwarf galaxies.
“It was important to me that we not shift our search for black holes towards dwarf galaxies,” says Polymera. “But after studying the entire census, I found that a new type of growing black holes is almost always found in dwarf galaxies. I was shocked by the numbers when I first saw them.”
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 were all nervous,” Polymera said. “The first question that came to my mind was: Have we missed the way that extreme star formation itself might explain these galaxies?”
She has spearheaded an exhaustive search for alternative explanations related to star formation, modeling errors, or exotic astrophysics. In the end, the team was forced to conclude that the newly discovered black holes were real.
“We’re still hurting ourselves,” Kannappan says. “We’re looking forward to new ideas. The black holes we’ve found are the basic building blocks of supermassive black holes like the one in our Milky Way. There’s so much we want to know about them.”
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