An international team of astronomers and astrophysicists, led by University of Utah, has detected supermassive black holes in two ultra-compact dwarf galaxies: VUCD3 and M59cO.
Ultra-compact dwarf galaxies (UCDs) were discovered in the 1990s through spectroscopic surveys of the Fornax cluster.
With masses ranging from a few million to a hundred million solar masses and sizes less than 300 light years across, UCDs are among the densest stellar systems in the Universe.
In 2014, University of Utah astronomer Dr. Anil Seth and co-authors discovered that an ultra-compact dwarf galaxy called M60-UCD1 contained a supermassive black hole, then the smallest known galaxy to harbor such an enormous black hole.
According to the researchers, M60-UCD1 is also the most massive UCD: if you lived inside this galaxy, the night sky would dazzle with at least one million stars visible to the naked eye.
Now, the same team has found two more UCDs with supermassive black holes: VUCD3 and M59cO.
“It’s pretty amazing when you really think about it. These UCDs are around 0.1% 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,” said team member Christopher Ahn, doctoral candidate in the Department of Physics Astronomy at the University of Utah and first author of a paper in the Astrophysical Journal (arXiv.org preprint) describing the discovery.
VUCD3 and M59cO are 53 million and 60 million light-years away, respectively. These galaxies orbit much more massive galaxies in the Virgo galaxy cluster: Messier 87 and Messier 59, respectively.
VUCD3’s black hole has a mass equivalent to 4.4 million suns, making up about 13% of the galaxy’s total mass, and M59cO’s black hole has a mass of 5.8 million suns, making up about 18% of its total mass.
By comparison, the supermassive black hole at the center of the Milky Way has a mass of 4 million suns, but makes up less than 0.01% of the galaxy’s total mass.
“Together, the three examples (M60-UCD1, VUCD3 and M59cO) suggest that black holes lurk at the center of most of UCDs, potentially doubling the number of supermassive black holes known in the Universe,” the astronomers said.
“We still don’t fully understand how galaxies form and evolve over time. These objects can tell us how galaxies merge and collide,” Ahn added. “Maybe a fraction of the centers of all galaxies are actually these compact galaxies stripped of their outer parts.”
One explanation for the supermassive black hole inside UCDs is that the galaxies were once made up of billions of stars.
The astronomers believe that the dwarfs were ‘swallowed up’ and ripped apart by the gravity of much larger galaxies.
“We know that galaxies merge and combine all the time — that’s how galaxies evolve. Our Milky Way is eating up galaxies as we speak,” Dr. Seth said.
“Our general picture of how galaxies form is that little galaxies merge to form big galaxies. But we have a really incomplete picture of that. UCDs provide us a longer timeline to be able to look at what’s happened in the past.”
Christopher P. Ahn et al. 2017. Detection of Supermassive Black Holes in Two Virgo Ultracompact Dwarf Galaxies. ApJ 839, 72; doi: 10.3847/1538-4357/aa6972