Small Galaxy Packs Massive Black Hole

A Hubble Space Telescope image shows a dwarf galaxy, VCC128, at the center, and the enlargement at right shows a double nucleus that suggests the presence of a black hole. CREDIT: NASA/Hubble Space Telescope. Below, artist's depiction of the massive black hole (black dot) at the center of a stellar envelope and the ring of stars. Credit: Victor Debattista

Supermassive black holes anchor most if not all large galaxies. But astronomers have been unsure whether very small galaxies tend to pack serious black holes at their centers.

A new discovery suggests they can.

Astronomers have found evidence of a massive black hole at the heart of a dwarf elliptical galaxy. The galaxy is just 1 percent the mass of our own Milky Way, and its black hole is estimated to be somewhere between 1 million to 50 million times the mass of our Sun. That's a lightweight compared to some supermassive black holes-which can be billions of times as massive as a star-but considerably more hefty than stellar black holes, which weigh only as much as a few suns.

The object, VCC128, is about 54 million light-years away in the Virgo Cluster of galaxies.

"It's a very small galaxy, on the outskirts of the cluster," said Victor Debattista, a postdoctoral researcher in astronomy at the University of Washington. "It is effectively the smallest galaxy in which there is a supermassive black hole."

The finding was presented in Seattle today at a meeting of the American Astronomical Society.

Black holes cannot be seen, because they trap matter and light. Debattista and colleagues detected the object in archived Hubble Space Telescope images. What they actually saw was a two blobs of light at the galaxy's center [image]. This double nucleus, as they call it, is actually two bright spots on a ring of stars that surround a very massive central object.

The discovery raises the question of whether other dwarf galaxies with bright centers might have massive black holes.

"The fact that we found a black hole is impressive because it's been thought that a galaxy this small should not be able to host a black hole," Debattista said. "It had been speculated that dwarf galaxies like this could not make black holes."

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