Astronomers discover hidden treasure of massive black holes

Astronomers discover hidden treasure of massive black holes
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Newly discovered massive black holes reside in dwarf galaxies, where their radiation competes with light from abundant young stars. Credit: NASA & ESA/Hubble, artistic conception of the black hole with jet by M. Polimera.

A team led by researchers from the University of North Carolina at Chapel Hill has discovered a previously overlooked treasure trove of massive black holes in dwarf galaxies. 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.

As a giant spiral galaxy, the Milky Way would have been built from the merger of many small dwarf galaxies. For example, the Magellanic Clouds seen in the southern sky are dwarf galaxies that will merge into 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 significant gap in our understanding of how black holes and galaxies grow together. New research published in the Astrophysical Journal contributes to fill this gap by revealing that massive black holes are much more common in dwarf galaxies than previously thought.

“This result really blew my mind because these black holes were previously hiding in plain sight,” said Mugdha Polimera, the study’s lead author and UNC-Chapel Hill Ph.D. student.

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, professor at UNC-Chapel Hill, Ph.D. of Polimera. advisor and co-author of the study, compared black holes to fireflies.

“Like fireflies, we only see black holes when they’re on – when they’re growing – and the ones that are on give us a clue to how many we can’t see.”

The problem is that while growing black holes shine with distinctive high-energy radiation, young newborn stars can too. Traditionally, astronomers have differentiated growing black holes from the formation of new stars using diagnostic tests which are based on detailed characteristics of each galaxy visible light when spread out in a spectrum like a rainbow.

The path to discovery began when undergraduate students working with Kannappan tried to apply these traditional tests to the 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 dwarf galaxy with primordial composition. and highly star-forming containing a growing massive black hole. “The fact that my simulations matched what the Kannappan group found made me want to explore the implications of galaxy evolution,” Richardson said.

A census of growing black holes

Polimera 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 conducted 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 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.”

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: Have 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 always pinch each other,” Kannappan said. “We’re excited to pursue a million 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 learn from them. topic.”

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More information:
Mugdha S. Polimera et al, RESOLVE and ECO: Finding z∼0 low metallicity dwarf AGN candidates using optimized emission line diagnostics, The Astrophysical Journal (2022). DOI: 10.3847/1538-4357/ac6595

Quote: Astronomers discover hidden trove of massive black holes (2022, May 24) Retrieved May 24, 2022 from

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