SALT LAKE CITY — Three years ago, a team of astronomers led by the University of Utah discovered a supermassive black hole in a tiny galaxy — an unprecedented find.
At that time, it was the smallest galaxy known to harbor such a giant black hole, which are usually found at the heart of much larger galaxies, like the Milky Way. Recently, however, the same group of University of Utah astronomers and colleagues found two more supermassive black holes in equally small galaxies called ultra-compact dwarf galaxies.
These three discoveries suggest, not only that dwarf galaxies are most likely bits and pieces leftover from massive galaxies that have collided, but that there may be twice as many supermassive black holes in the universe than previously thought.
“We still don’t fully understand how galaxies form and evolve over time. These objects can tell us how galaxies merge and collide,” said Chris Ahn, postdoctoral candidate in the Department of Physics & Astronomy and lead author of the international study that was published Monday in The Astrophysical Journal.
“Because we’ve found supermassive black holes in all three of these ultra-compact dwarf galaxies, we’ve had the data available to us so we could actually study the internal dynamics of what’s going on in these galaxies … and that’s why we think it could potentially double the number of supermassive black holes.”
For astronomers, there are generally two main ways to learn more about the universe outside ours and about how galaxies evolve. According to Ahn, when astronomers look into the night sky, they have a snapshot in time and the best way to delve into that snapshot is by either looking at objects that are very far away or expanding the sample size of studied galaxies.
The farther away the galaxy, the more difficult it is to obtain quality data, so the ability to look at a diverse set of galaxies that offer trustworthy data can offer a significant contribution to how galaxies form and evolve over time.
“We know galaxies merge all the time,” Ahn said. “Our own Milky Way is on a collision course with our closest, big galaxy Andromeda … We have an idea of how galaxies form and evolve over time, but it’s not a complete picture.”
This discovery makes it clear, however, that many smaller galaxies are tiny leftovers of massive galaxies that were stripped of their outer layers after colliding into other, larger galaxies.
The ultra-compact dwarf galaxies discovered, dubbed VUCD3 and M59cO, orbit massive galaxies in the Virgo galaxy cluster. VUCD3’s black hole has a mass equivalent to 4.4 million suns, making up about 13 percent of the galaxy’s total mass, and M59cO’s black hole has a mass of 5.8 million suns, making up about 18 percent of its total mass.
“It’s pretty amazing when you really think about it. These ultra-compact dwarfs are around 0.1 percent the size of the Milky Way, yet they host supermassive black holes that are bigger than the black hole at the center of our own galaxy,” Ahn said in a news release.