Astronomers are accustomed to seeing supermassive black holes at the heart of large galaxies like our own Milky Way and neighboring galaxies like Andromeda. However, a new analysis suggests there’s a giant black hole hiding in a most unusual place: inside one of the smallest galaxies known to exist. Fornax UCD3 is a tiny galaxy, but it has a monster black hole in the middle.
Fornax UCD3 is what’s known as an ultracompact dwarf galaxy. These objects have a total mass in the tens of millions of solar masses. That might sound like a lot, but keep in mind the sun is a rather small-ish star. There are plenty of stars that weigh in at hundreds of solar masses all on their own. The radius of an ultracompact dwarf galaxy rarely goes beyond 300 light years, making them the densest stellar regions in the known universe.
According to Anton Afanasiev from the Sternberg Astronomical Institute at Moscow State University, the black hole at the center of UCD3 has a mass of roughly 3.5 million suns. The entire galaxy is just a few dozen million solar masses. The team came to this conclusion using data from SINFONI, an infrared spectrograph installed at one of the Very Large Telescope (VLT) facilities in Chile. The giveaway was an unusual pattern in angular velocity within UCD3. When stars get near a massive object like a black hole, they begin accelerating in different directions. The average speed doesn’t change, but “velocity dispersion” does.
Scientists compared the velocity dispersion in UCD3 with established models of black holes. The most likely mass for the object is 3.5 million solar masses. The team also ran simulations with no black hole, but that possibility was ruled out with more than 99 percent confidence.
This is the fourth black hole discovered in an ultracompact dwarf galaxy, but it’s the largest with about 4 percent of the galaxy’s total mass. In larger galaxies, the central black hole is less than a percent of the total mass. This discovery lends credence to an increasingly popular explanation for the existence of ultra-compact dwarf galaxies. Astronomers think that average galaxies made close passes with much more massive ones early in their evolution. As a result, most of their stars were pulled away, leaving the core with a large black hole.
To add further weight to this hypothesis, astronomers need to detect more ultra-compact dwarf galaxies with supermassive black holes. The Moscow State University team plans to do just that in the coming years.
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