Astronomers Discover Hidden Trove of Massive Black Holes – “We All Got Nervous”

The genesis of our galaxy's supermassive black hole has been revealed thanks to new black holes discovered in dwarf galaxies.

A team of researchers lead by astronomers from the University of North Carolina at Chapel Hill's Department of Physics and Astronomy discovered a previously unknown hoard of huge black holes in dwarf galaxies. The newly found black holes provide insight into the life of the supermassive black hole at the heart of our galaxy, the Milky Way. 

Hiding in plain sight

The Milky Way is thought to have formed from the merging of several smaller dwarf galaxies into a gigantic spiral galaxy. The Magellanic Clouds, for example, are dwarf galaxies in the southern sky that eventually merge into the Milky Way. Each dwarf that enters the Milky Way might carry with it a center enormous black hole with a mass tens or hundreds of thousands of times that of our sun, destined to be consumed by the Milky Way's central supermassive black hole.

However, it is unknown how often dwarf galaxies include a big black hole, creating a critical gap in our knowledge of how black holes and galaxies form together. A new study published in the Astrophysical Journal on May 24, 2022, fills in some of the gaps by finding that big black holes are far more abundant in dwarf galaxies than previously assumed.

“This result really blew my mind because these black holes were previously hiding in plain sight,” Mugdha Polimera, the study's primary author and a Ph.D. student in the Department of Physics and Astronomy, stated.

Sending mixed messages

When black holes are actively expanding by eating gas and stardust whirling around them, which causes them to shine brightly, they can be spotted. 

Professor Sheila Kannappan of the Department of Physics and Astronomy, Polimera's Ph.D. adviser and collaborator of the work, compared black holes to fireflies. “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.” 

Growing black holes emit characteristic high-energy radiation, but so do tiny newborn stars. Previously, astronomers used diagnostic tests to distinguish developing black holes from fresh star formation, which relied on precise properties of each galaxy's visible light when stretched out into a rainbow spectrum.

Undergraduate students working with Kannappan attempted to apply these standard criteria to data from galaxy surveys, which led to their finding. The researchers discovered that certain galaxies were delivering contradictory messages: two tests indicated expanding black holes, while a third indicated simply 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 explained. She felt that the third, sometimes conflicting, test was more sensitive to characteristic dwarf traits such as their simple elemental makeup (primarily primordial hydrogen and helium from the Big Bang) and their rapid pace of generating new stars than the other two.

The findings of the mixed-message test perfectly matched what theory would predict for a primordial-composition, strongly star-forming dwarf galaxy harboring a developing large black hole, according to study coauthor Chris Richardson, an associate professor at Elon University. “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 remarked.

A census of growing black holes

Polimera took on the task of compiling a new census of expanding black holes, taking into account both conventional and mixed-message varieties. She used previously published measurements of visible light spectral properties to look for black holes in hundreds of galaxies discovered by Kannappan's RESOLVE and ECO surveys. These surveys feature a unique design that includes UV and radio data that is perfect for investigating star formation. RESOLVE and ECO are complete inventories of huge volumes of the present-day universe, in which dwarf galaxies are abundant. 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.

“It was important to me that we didn’t bias our black hole search toward dwarf galaxies,” Polimera added. “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.” 

The new form accounted for more than 80% of all developing black holes she discovered in dwarf galaxies.

The outcome appeared to be far too excellent. "We were all nervous," Polimera said. "The first thing that came to me was: Have we overlooked a way to explain these galaxies by high star creation alone?" She oversaw a thorough investigation into alternate possibilities involving star formation, modeling flaws, and exotic astrophysics. Finally, the crew had no choice but to accept that the freshly discovered black holes were genuine.

“We’re still pinching ourselves,” Kannappan stated. “We’re excited to pursue a zillion follow-up ideas. The black holes we’ve found are the basic building blocks of supermassive black holes like the one in our own Milky Way. There’s so much we want to learn about them.”
Astronomers Discover Hidden Trove of Massive Black Holes – “We All Got Nervous” Astronomers Discover Hidden Trove of Massive Black Holes – “We All Got Nervous” Reviewed by Lilit on May 30, 2022 Rating: 5
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