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Astronomers and colleagues have found two ultra-compact dwarf galaxies, VUCD3 and M59cO, with supermassive black holes. The findings suggest that the dwarfs are likely tiny leftovers of larger galaxies that were stripped of their outer layers after colliding into other, larger galaxies M87 and M59, respectively. Credit: NASA/Space Telescope

A pair of supermassive black holes have been found in a duo of ultra-compact dwarf galaxies.

A team of astronomers from the University of Utah discovered two more black holes in the smaller galaxies, three years after discovering a similar supermassive black hole in an ultra-compact dwarf galaxy.

After the first discovery the team suggested that the dwarfs were likely tiny leftovers of larger galaxies that were stripped of their outer layers after colliding into other, larger galaxies.

However, with three of these astronomical abnormalities now confirmed, the researchers suggest that black holes lurk at the center of most ultra-compact dwarf galaxies, potentially doubling the number of supermassive black holes known in the universe.

The black holes make up a high percentage of the compact galaxies’ total mass, supporting the theory that the dwarfs are remnants of massive galaxies that were ripped apart by larger galaxies.

“We still don't fully understand how galaxies form and evolve over time,” Chris Ahn, a doctoral candidate in the Department of Physics & Astronomy and lead author of the study, said in a statement. “These objects can tell us how galaxies merge and collide.

“Maybe a fraction of the centers of all galaxies are actually these compact galaxies stripped of their outer parts,” he added.

The research team measured the two ultra-compact dwarf galaxies—VUCD3 and M59cO—that lie far beyond the spiral arms of the Milky Way, orbiting massive galaxies in the Virgo galaxy cluster, and detected a supermassive black hole in both galaxies.

VUCD3’s black hole has a mass equivalent to 4.4 million suns, making up about 13 percent of the galaxy’s total mass, while M59cO’s black hole has a mass of 5.8 million suns, making up about 18 percent of its total mass.

While these black holes make up a significant portion of their galaxies total mass, the giant black hole at the center of the Milky Way has a mass of four million suns but makes up less than .01 percent of the galaxy’s total mass.

“It's pretty amazing when you really think about it,” Ahn said. “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.”

The astronomers measured the movement of the stars to calculate the ultra-compact dwarf galaxies mass by using the Gemini North telescope on the Mauna Kea volcano in Hawaii. They corrected for the distortions caused by Earth’s atmosphere.

The research team shot a laser into the sky to mimic a star and moved a mirror around it hundreds of times a second to undo the distortion. The next step was to apply the technique to the ultra-compact dwarf galaxies, which are so small that the corrections are necessary to measure the motions inside the object, which brings the once blurry galaxy into focus.

The images were then analyzed from the Hubble Space Telescope to measure the distribution of the stars in each galaxy and created a computer simulation that best fit their observations.

They discovered that the motion of the stars at the center of the galaxies moved much faster than those on the outside, an identifier of a black hole.

The study was The Astrophysical Journal.

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