Tuesday, December 06, 2022

Astronomers find hidden pool of massive black holes

Illustration Of Dwarf Galaxy With Rising Black Hole And Jet

Image: Newly discovered giant black holes reside in dwarf galaxies, where their radiation competes with the light of abundant young stars.
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credit: Original image by NASA and ESA/Hubble, artistic conception of a black hole with a jet by M. Polymera.

A team led by researchers at the University of North Carolina at Chapel Hill has found a previously unseen treasure trove of massive black holes in dwarf galaxies. Newly discovered black holes offer a glimpse into the life story of the supermassive black hole at the center of our galaxy.

As a giant spiral galaxy, the Milky Way is believed to have formed from the merger of several smaller dwarf galaxies. For example, the Magellanic Clouds seen in the southern sky are dwarf galaxies that will merge into the Milky Way. Each dwarf can 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, leading to a significant gap in our understanding of how black holes and galaxies grow together. new research published in astrophysical journal It helps fill this gap by revealing that massive black holes are many times more common in dwarf galaxies than previously thought.

“This result really blew my mind because these black holes had previously been hidden in plain sight,” said study lead author and UNC-Chapel Hill Ph.D. Mugdha Polymera said. student.

send mixed messages

Black holes are usually detected when they are actively growing from the gas and stardust that surrounds them, causing them to glow intensely.

UNC-Chapel Hill Professor Sheela Kannappan, Ph.D. The study’s advisor and co-author compared black holes to fireflies.

“Like a firefly, we see black holes only when they are lit – when they are expanding – and lit up gives us an indication of how many we can’t see.”

The problem is that while growing black holes glow with typical high-energy radiation, so can young newborn stars. Traditionally, astronomers have distinguished growing black holes from new star formation using diagnostic tests that rely on the detailed characteristics of each galaxy’s visible light while spreading across a spectrum like a rainbow.

The road to discovery began when graduate students working with Kannappan attempted to apply these traditional tests to galaxy survey data. The team realized that some galaxies were sending mixed messages – two tests would indicate a growing black hole, but a third would only indicate star formation.

“Previous work had ruled out such ambiguous cases from statistical analysis, but I was shocked that they could be undiscovered black holes in dwarf galaxies,” Knappan said. He suspected that the third, sometimes contradictory, test was more sensitive than the other two distinguishing properties of dwarfs: their simple elemental composition (primarily the 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 match exactly what theory would predict for a primordial-composition, a highly star-forming dwarf galaxy in which There is a massive black hole. “The fact that my simulations combined with what the Knappen group found excited me to explore the implications for the evolution of galaxies,” Richardson said.

Growing Black Hole Census

Polymera took on the challenge of building a new census of growing black holes, with a focus on both conventional and mixed-message types. They obtained published measurements of visible light spectral features to test for black holes in thousands of galaxies found in two surveys, led by Knappan, 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 larger and brighter galaxies, RESOLVE and ECO are present. There are complete lists of giant volumes of the universe in which dwarf galaxies abound.

“It was important to me that we didn’t lead our black hole search to dwarf galaxies,” said Polymera. But looking through the entire census, I found that new types of growing black holes almost always appear in dwarfs. When I first saw them, I was amazed by the numbers. ,

Of all the growing black holes found in dwarf galaxies, more than 80 percent were of a new type.

The result looked great. “We all panicked,” said Polymera. “The first question on my mind was this: Have we missed a way that only extreme star formation can explain these galaxies?” He led a detailed search for alternative explanations involving star formation, modeling uncertainties, or exotic astrophysics. In the end, the team was forced to conclude that the newly identified black hole was real.

Knappan said, “We’re still pinching ourselves. We’re excited to put forward a billion follow-up ideas. The black holes we’ve found are the basic building blocks of supermassive black holes like our own.” In the Milky Way. We want to learn a lot about them.”

This research was funded in part by the National Science Foundation.

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