Astronomers Discover a Trove of Gigantic Black Holes

Astronomers have discovered a previously unknown treasure trove of massive black holes in dwarf galaxies. The newly discovered black holes provide a glimpse into the life of the supermassive black hole at the center of our own Milky Way galaxy.

A team led by researchers at the University of North Carolina at Chapel Hill discovered a previously unknown treasure trove of massive black holes in dwarf galaxies. The newly discovered black holes provide a glimpse into the life of the supermassive black hole at the center of our own Milky Way galaxy.

The Milky Way is thought to have formed as a giant spiral galaxy from the mergers of many smaller dwarf galaxies. The Magellanic Clouds, which can be seen in the southern sky, are dwarf galaxies that will merge into the Milky Way. Each dwarf that falls in may bring a central massive black hole tens or hundreds of thousands of times the mass of our sun, potentially destined to be devoured by the Milky Way’s central supermassive black hole.

However, it is unknown how frequently dwarf galaxies contain a massive black hole, leaving a critical 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.

It was important to me that we didn’t bias our black hole search toward dwarf galaxies. 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.

Mugdha Polimera

“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.

Sending mixed messages

Black holes are typically detected when they are actively growing by ingesting gas and stardust swirling around them, which makes them glow intensely. UNC-Chapel Hill Professor Sheila Kannappan, Polimera’s Ph.D. advisor and coauthor of the study, 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.”

The problem is that, while growing black holes emit distinctive high-energy radiation, so can young newborn stars. Traditionally, astronomers used diagnostic tests to distinguish growing black holes from new star formation, which rely on detailed features of each galaxy’s visible light when spread out into a spectrum like a rainbow.

Undergraduate students working with Kannappan attempted to apply these traditional tests to galaxy survey data, which led to the discovery. The team realized that some of the galaxies were sending contradictory signals: two tests would show growing black holes, but a third would only show star formation.

“Previous work had just rejected ambiguous cases like these from statistical analysis,” Kannappan explained. “But I had a hunch they might be undiscovered black holes in dwarf galaxies.” She suspected that the third, sometimes contradictory, test was more sensitive to typical dwarf properties than the other two: their simple elemental composition (mostly primordial hydrogen and helium from the Big Bang) and their high rate of star formation.

Astronomers find hidden trove of massive black holes

With theoretical simulations, study coauthor Chris Richardson, an associate professor at Elon University, confirmed that the mixed-message test results exactly matched what theory would predict for a primordial-composition, highly star-forming dwarf galaxy with a growing massive black hole. “The fact that my simulations agreed with what the Kannappan group discovered piqued my interest in investigating the implications for how galaxies evolve,” Richardson said.

A census of growing black holes

Polimera took on the task of creating a new census of expanding black holes, paying special 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 discovered in two Kannappan-led surveys, RESOLVE and ECO. These surveys include ultraviolet and radio data that are ideal for studying star formation, and they are designed in an unusual way: 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.

“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.”

The new type accounted for more than 80% of all growing black holes she discovered in dwarf galaxies. The outcome appeared to be too good to be true. “We were all nervous,” Polimera admitted. “My first thought was, have we missed a way that extreme star formation alone could explain these galaxies?” She oversaw a thorough search for alternative explanations involving star formation, modeling uncertainties, or exotic astrophysics. Finally, the team had to admit that the newly discovered black holes were real.

Kannappan said, “We’re still pinching ourselves. We’re ecstatic to pursue a slew of follow-up ideas. The black holes we discovered are the fundamental building blocks of supermassive black holes such as the one in our own Milky Way galaxy. There’s so much we want to learn about them.”