Mystery Space Object May Be Ejected Black Hole

John Roach

for National Geographic News

Published May 7, 2010

A mystery object in a galaxy far, far away could be a supermassive black hole that got booted from its home galaxy's center, according to a new study.

Then again, the strange body could be a rare type of supernova or an oddball "midsize" black hole—more massive than black holes born when single stars explode but "lighter" than the supermassive ones at the centers of galaxies.

"All three of those [options] are exotic and have something peculiar to them," said study co-author Peter Jonker, an astronomer with the Netherlands Institute for Space Research in Utrecht.

Off-center Black Holes Wanted

Jonker and his colleagues found the mystery object while on the hunt for off-center supermassive black holes that are thought to form when two galaxies merge. (Related: "Colossal Four-Galaxy Collision Discovered.")

Most, if not all, galaxies are thought to have supermassive black holes at their cores. Recent computer simulations suggest that when two galaxies merge, so do their central black holes.

But the newly formed black hole combo "actually receives a kick" from gravitational forces generated by the galactic merger, Jonker said. The kick, according to the models, "launches this newly formed black hole out of the center of the galaxy." (See "Hundreds of 'Rogue' Black Holes May Roam Milky Way.")

Sorting through archived data from the Chandra X-ray Observatory, the team found an interesting candidate in a galaxy half a billion light years away from Earth. The extremely bright x-ray object is about ten thousand light-years from its galactic center.

Based on the Chandra data, however, the astronomers couldn't rule out the possibility that the newfound object actually lies behind the galaxy in question.

So the team compared their x-ray information with archived optical data from the Hubble Space Telescope. They found that the mystery object emits a bright blue light in visible wavelengths. (See NASA astronomer's picks for the top Hubble pictures of the past 20 years.)

"If you look through [a] galaxy toward something in the background, you go through a layer of material"—interstellar dust—"that preferentially takes out the blue light," so you wouldn't see it, Jonker explained.

The object's blue hue helps confirm that it belongs to the galaxy in question, the team reports in a paper accepted for publication in the Monthly Notices of the Royal Astronomical Society.

"Having ruled out the background source, then the question is, OK, what then can it be?" Jonker said.