Enormous black hole resides at core of tiny galaxy

A supermassive black hole lives at the center of a supertiny galaxy, astronomers report in the Sept. 18 Nature. Fifteen percent of the galaxy’s mass is locked up in this one black hole.

“That’s an outrageous number,” says John Kormendy, an astrophysicist at the University of Texas at Austin who was not involved with the study. “It cries out for an understanding that we don’t have.”

Supermassive black holes reside at the center of nearly every large galaxy, including the Milky Way. But these black holes typically make up only a few tenths of a percent of their host galaxy’s weight.

Study leader Anil Seth, an astrophysicist at the University of Utah in Salt Lake City, argues that this black hole’s host galaxy, named M60-UCD1, used to be much larger. It is most likely a remnant of a galaxy that has been slowly shredded by numerous brushes with its neighbor, a giant galaxy named M60.

M60-UCD1 is about 54 million light-years away in the constellation Virgo. It’s a type of galaxy known as an ultra compact dwarf, a dense ball of over 100 million stars, stretching about 150 light-years across. Ultra compact dwarfs, or UCDs, are so tiny that they can easily be mistaken for stars near Earth.

“Of all the kinds of galaxies we know about,” Kormendy says, “UCDs are the least settled puzzle.” Understanding where they come from, he adds, could help astronomers better understand how all galaxies evolve.

UCDs have also been overlooked as homes for supermassive black holes because determining the speeds of stars within such puny galaxies is difficult. But ignoring UCDs in a count of supermassive black holes, Seth says, would be like taking a census of the United States and not counting everyone west of the Mississippi.

Seth and colleagues determined that M60-UCD1 had a central, supermassive black hole by measuring the speeds of stars in different parts of the galaxy with the Gemini North telescope in Hawaii. The telescope has deformable mirrors that cancel the turbulence of Earth’s atmosphere, providing extra sharp images. The team noticed that stars closer to the galaxy’s center whipped around much faster than could be explained by the gravity of the stars themselves. The high speeds revealed a black hole at the galaxy’s core weighing roughly 21 million times as much as the sun.

If M60-UCD1 is typical of other dwarfs, he adds, there might be twice as many supermassive black holes in the local universe than previously thought. Having an accurate picture of where supermassive black holes live can help astronomers figure out how they formed.

The possibility that UCDs host supermassive black holes is fascinating, Kormendy says, though he would like to see evidence from more than one galaxy. He is puzzled by how M60-UCD1 could have survived 10 close encounters with the neighboring galaxy, as Seth’s team’s simulation describes. Galaxies usually coalesce after just a few passes, he says.

Another possibility, says Kormendy, is that M60, the nearby massive galaxy, ejected the black hole as it gobbled down other galaxies. If M60 flung a black hole into space, that black hole could have dragged a dense cluster of stars along with it. Alternatively, M60-UCD1 might be a remnant from the early universe, when supermassive black holes grew much faster than the galaxies in which they lived.

Seth says future observations will help unravel some of M60-UCD1’s mysteries. To confirm the black hole hypothesis, the team plans to observe the galaxy with radio telescopes, which might reveal radiation from jets of charged particles spewed out by a black hole swallowing gobs of interstellar gas and dust. The researchers are also expanding their work to other UCDs to learn whether M60-UCD1 is a special case.