Chapel Hill: A team led by researchers at the University of North Carolina at Chapel Hill has discovered a previously overlooked treasure trove of massive black holes in a dwarf galaxy.
The newly discovered black hole offers a glimpse into the life story of the supermassive black hole at the center of our Milky Way.
The study was published in The Astrophysical Journal.
As a giant spiral galaxy, the Milky Way is believed to have formed from the merger of many smaller dwarf galaxies. For example, the Magellanic Clouds visible in the southern sky are dwarf galaxies that will merge to form the Milky Way. Any fallen dwarf could carry with it a central supermassive black hole, tens or hundreds of thousands of times the mass of our sun, which could potentially be swallowed up by the Milky Way’s central supermassive black hole.
But how often dwarf galaxies contain supermassive black holes is unknown, leaving a huge gap in our understanding of how black holes and galaxies grow together.
“This result surprised me because this black hole was previously hiding in plain sight,” said Mujda Polymera, the study’s lead author and doctoral student at the University of North Carolina at Chapel Hill.
Send mixed message
Black holes are usually found when they are actively growing by eating the gas and dust of the stars that revolve around them, causing them to glow intensely.
Professor Sheila Kanaban, doctoral advisor at Polimera and co-author of the study, at UNC-Chapel Hill compared black holes and fireflies.
He added: “Just like fireflies, we only see black holes when they light up – as they grow – and the glowing holes give us evidence of how much we can’t see.”
The problem is that when a growing black hole glows with a different kind of high-energy radiation, a newborn star can too. Traditionally, astronomers have distinguished growing black holes from new star formation using diagnostic tests that rely on the detailed features of each galaxy’s visible light as it spreads out in a rainbow-like spectrum.
The road to discovery began when undergraduate students working with Kanaban tried to apply this traditional test to galactic survey data. The team noticed that some galaxies were sending mixed messages – two tests showed a growing black hole, and a third test only showed star formation.
“Previous work has only recently dismissed mysterious cases like this from statistical analysis, but I have a hunch that it may be an undetected black hole in a dwarf galaxy,” Kanaban said. I suspect that the third test, which is sometimes contradictory, is more sensitive than the other two for the typical properties of dwarfs: their simple elemental composition (mainly hydrogen and helium from the Big Bang) and their high rate of new star formation.
Study co-author Chris Richardson, professor at Elon University, confirmed through theoretical simulations that the mixed message test results matched exactly what theory could predict for primordial dwarf galaxies that are highly formed from stars containing increasingly massive black holes. “The fact that my simulations are in line with those found by the Kanaban group makes me excited to explore the implications of how galaxies evolve,” Richardson said.
The growing black hole count
Polymers have taken up the challenge of building a new, growing population of black holes, taking into account both traditional and mixed message types. It obtained published measurements of the spectroscopic properties of visible light to test black holes in thousands of galaxies in two surveys led by Kannappan, RESOLVE and ECO. These surveys include ultraviolet and radio data that are ideal for studying star formation, and they have an unusual design: While most astronomical surveys select samples that favor large, bright galaxies, RESOLVE and ECO are the complete collections of large volumes of the universe currently in the universe. where dwarf galaxies abound.
“It’s important to me not to bias our search for black holes to dwarf galaxies,” said Polymera.
“But looking at the entire population, I found that a new kind of black hole growing up was always appearing in the dwarfs. I was surprised by the number when I first saw it.”
More than 80 percent of all developing black holes that I find in dwarf galaxies are new types.
The result looks very good. “We’re all nervous,” said Polymera.
“The first question that pops into my mind is, have we missed any extreme star formation that could explain these galaxies?” He leads an exhaustive search for alternative explanations involving star formation, uncertainty modeling, or exotic astrophysics. In the end, the team had to conclude that the newly identified black hole was real.
“We are still struggling with ourselves,” Kanaban said.
“We’re excited to have over a million follow-up ideas. The black holes we found are the building blocks of supermassive black holes like those in our Milky Way galaxy. There’s a lot we want to learn about them,” Kanaban added. .
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