Black holes previously hiding in plain sight have been discovered by researchers at the University of North Carolina at Chapel Hill, who have found a treasure trove of massive black holes in dwarf galaxies.
As a giant spiral galaxy, the Milky Way is believed to have been built up from mergers of many 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 that falls in 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 by the Milky Way’s central supermassive black hole.
Black holes this large can’t easily grow from the black holes that form when ordinary individual stars die”
Primordial “Seed” Black Holes of the Early Universe
“Conventionally, ‘massive’ black holes refers to BHs in the range of 10s to 100s of thousands of solar masses’ ‘ University of North Carolina -Chapel Hill astronomer Sheila Kannappan, wrote in an email to The Daily Galaxy. “The consensus view (not yet observationally verified),” she wrote, “is that black holes this large can’t easily grow from the black holes that form when ordinary individual stars die, so most theories suggest that ‘seed’ black holes in the range of 100s to 1000s of solar masses must form in the early universe and grow into massive black holes like the ones we find in dwarf galaxies today. Theoretical models suggest that the first structures that formed in the Universe — whether gas clouds, stars, or star clusters — were likely more dense and/or massive than we see in today’s Universe and therefore collapsed under self-gravity to form seed black holes.”
How often dwarf galaxies contain a massive black hole is unknown”
But how often dwarf galaxies contain a massive black hole is unknown, leaving a key gap in our understanding of how black holes and galaxies grow together. New research published in the Astrophysical Journal helps to fill in 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 were previously hiding in plain sight,” said Mugdha Polimera, lead author of the study and a UNC-Chapel Hill Ph.D. student.
Black holes are typically detected when they are actively growing by ingesting gas and stardust swirling around them, which makes them glow intensely.
“Just like fireflies, we see black holes only when they’re lit up—when they’re growing—and the lit-up ones give us a clue to how many we can’t see,” said UNC-Chapel Hill Professor Sheila Kannappan, Polimera’s Ph.D. advisor and coauthor of the study,
The problem is, while growing black holes glow with distinctive high-energy radiation, young newborn stars can too. Traditionally, astronomers have differentiated growing black holes from new star formation using diagnostic tests that rely on detailed features of each galaxy’s visible light when spread out into a spectrum like a rainbow.
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 growing black holes, but a third would indicate only star formation.
“Previous work had just rejected 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 typical properties of dwarfs: their simple elemental composition (mainly primordial hydrogen and helium from the Big Bang) and their high rate of forming new stars.
Study coauthor Chris Richardson, an associate professor at Elon University, confirmed with theoretical simulations that the mixed-message test results exactly matched what theory would predict for a primordial-composition, highly star-forming dwarf galaxy containing a growing massive black hole. “The fact that my simulations lined up with what the Kannappan group found made me excited to explore the implications for how galaxies evolve,” Richardson said.
Census of Growing Black Holes
Polimera took on the challenge of constructing a new census of growing black holes, with attention to both traditional and mixed-message types. She obtained published measurements of visible light spectral features to test for black holes in thousands of galaxies found in two surveys led by Kannappan, RESOLVE and ECO. These surveys include ultraviolet and radio data ideal for studying star formation, and they have an unusual design: whereas most astronomical surveys select samples that favor big and bright galaxies, RESOLVE and ECO are complete inventories of huge volumes of the present-day universe in which dwarf galaxies are abundant.
More Prevalent in Dwarf Galaxies?
“It was important to me that we didn’t bias our black hole search toward dwarf galaxies,” Polimera said. “But in looking at the whole census, I found that the new type of growing black holes almost always showed up in dwarfs. I was taken aback by the numbers when I first saw them.”
Dwarf galaxies with high levels of star formation experience efficient cosmic gas accretion, which may also yield efficient gas accretion onto their central massive black holes. More than 80 percent of all 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 my mind was: Have we missed a way that extreme star formation alone could explain these galaxies?” She led an exhaustive 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 holes were real.
The Last Word
“Our paper published today focuses on detection of growing black holes using optical spectroscopy,” Kannappan wrote in her email to The Daily Galaxy.”In follow-up work led by Mugdha, we are examining growing black holes detected with mid-infrared photometry. So far it seems that optical and mid-infrared black hole detections rarely coincide for dwarf galaxies, so using both techniques can give us a more complete census of the growing black hole population in dwarfs. The numbers are even larger than what we find using optical spectroscopy alone.
We look to a time when most galaxies had the characteristics of today’s dwarfs.”
“To understand these numbers in the big picture of galaxy evolution, my first question would be, what if anything is special about the dwarf galaxies in which we happen to detect growing black holes? The answer bears on how commonly a dwarf galaxy may contain a dark black hole that is not growing, like a firefly that is not glowing at the moment. It also bears on what causes a given black hole to light up. Second, what will we see when we apply our new detection method to distant galaxies? When we look at distant galaxies, we see them as they were long ago, when their light left them. So we look back in time, to a time when most galaxies had the characteristics of today’s dwarfs: primordial composition and strong star formation. It will be exciting to use our new method to trace the evolving population of growing, massive black holes over cosmic time.”
Source: Mugdha S. Polimera et al, RESOLVE and ECO: Finding Low-metallicity z ∼ 0 Dwarf AGN Candidates Using Optimized Emission-line Diagnostics, The Astrophysical Journal (2022). DOI: 10.3847/1538-4357/ac6595
Image credit: The newly discovered massive black holes reside in dwarf galaxies, where their radiation competes with the light of abundant young stars. Credit: NASA & ESA/Hubble, artistic conception of black hole with jet by M. Polimera.
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