Scientists have found massive black holes lurking in plain sight in dwarf galaxies, providing a glimpse into the life narrative of the supermassive black hole at the center of our own Milky Way galaxy.
The Milky Way is thought to have formed from the merging of several smaller dwarf galaxies into a giant 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 could carry with it a center enormous black hole with 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 massive black hole, leaving a critical gap in our knowledge of how black holes and galaxies form together. According to new research published in the Astrophysical Journal, massive black holes are many times more abundant in dwarf galaxies than previously thought.
“This result blew my mind because these black holes were previously hidden in plain sight,” said Mugdha Polimera, the main author of the study.
When black holes are actively expanding, they swallow gas and stardust from the surrounding area, which causes them to glow brightly.
“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 coauthor Sheila Kannappan.
The problem is that, just as expanding black holes emit distinct high-energy radiation, so do young newborn stars. Astronomers have traditionally used diagnostic procedures to distinguish expanding black holes from fresh star formation, which rely on the precise properties of each galaxy’s visible light when spread out into a spectrum like a rainbow.
The journey to discovery began when Kannappan’s undergraduate students attempted to apply traditional tests to galaxy survey data. The researchers discovered that some galaxies were giving mixed signals: two tests indicated expanding black holes, while a third indicated solely 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 felt that the third, sometimes conflicting, test was more sensitive to characteristic dwarf traits such as their simple elemental composition (primarily primordial hydrogen and helium from the Big Bang) and their high rate of generating new stars than the other two.
The results of the mixed-message test exactly matched what theory would predict for a primordial-composition, strongly star-forming dwarf galaxy containing a growing large black hole, according to study co-author 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 added.
Polimera took on the task of compiling a new census of expanding black holes, taking into account both traditional 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 an unusual design that includes UV and radio data that is perfect for studying star formation. RESOLVE and ECO are thorough inventory of enormous regions of the present-day cosmos in which dwarf galaxies are prevalent, whereas most astronomical surveys select samples that favor big and luminous galaxies.
“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 type accounted for more than 80% of all developing black holes she discovered in dwarf galaxies.
The outcome appeared to be too excellent. “We all got nervous,” Polimera admitted. “The first question that came to my mind was: Have we missed a way that extreme star formation alone could explain these galaxies?” She oversaw a thorough investigation into alternate possibilities involving star formation, modeling flaws, and exotic astrophysics. Finally, the team had to admit that the freshly discovered black holes were real.
Kannappan commented, “We’re still pinching ourselves.” “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.”
Image Credit: UNC
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