Tiny galaxy hosts huge black hole

From:; http://news.bbc.co.uk/2/hi/science/nature/6244323.stm

Jan 9, 2007

VCC128 appears in the images to have a double nucleus

Astronomers have found evidence of a supermassive black hole at the heart of a tiny galaxy.

VCC128 is an elliptical dwarf galaxy, about 1% the size of our own Milky Way, located in the Virgo Cluster, which is about 59 million light-years away.

The finding is a puzzle, say scientists: a galaxy this small should have ejected its large black hole.

Details were presented at the 209th meeting of the American Astronomical Society (AAS) in Seattle, Washington.

"It's the biggest type of black hole in the tiniest, most insignificant galaxy," Victor Debattista, from the University of Washington's astronomy department and lead researcher on the study, told BBC News.

Gravitational kicks

Black holes lie at the centre of many galaxies and have gravitational fields so powerful that nothing - not even light - can escape.

Supermassive black holes are so large that their mass can equal anywhere between 100,000 and 10 billion times that of our own Sun.

VCC128 is the smallest galaxy yet found in which there is a supermassive black hole.

According to current theory, galaxies grow through mergers with other galaxies.

When two galaxies merge, their central black holes form a binary system and they revolve around each other, eventually coalescing into a single object.

But there is a paradox here. As black holes merge, a powerful gravitational kick occurs. For larger galaxies, with stronger gravitational fields, this is unlikely to dislodge the new hole from the galactic centre.

In small galaxies, however, the kick has the potential eject the black hole from the galaxy completely.


"The fact we were able to find a very massive black hole here - conceivably more massive than the one at the centre of our own Milky Way - tells us that, in some cases at least, black holes can form and be retained by small galaxies," said Dr Debattista.

"It also shows us that these kicks cannot be universal. Sometimes these black holes aren't growing just through mergers, but by accreting gas from their surroundings."

Dr Debattista had wondered whether VCC128 was once a much larger galaxy that was somehow stripped of its stars.

But, he said, the light profile of the object is indicative of a galaxy that was born this way.

The astronomers made the discovery while sifting through old data from the Hubble Space Telescope. They carried out follow-up observations with the 3.5m (11.5ft) telescope at the Apache Point Observatory in New Mexico.

In the images, VCC128 has two bright points at its centre - a double nucleus. The scientists interpret the two points to be concentrations of stars collected at opposite edges of a ring around the black hole.