UNH Space Science Center research scientist Dacheng Lin leads a group of astrophysicists who have recently discovered a massive black hole wandering at the edge of a galaxy about 4.5 billion light years away.
“The discovery was somewhat of a coincidence,” Lin said. “The original project was to understand the nature of a few thousand X-ray sources in a large catalog [half a million] of sources detected by the X-ray observatory XMM-Newton. The wandering black hole that we discovered came to my attention due to its special X-ray spectra different from those seen in the two main classes of X-ray sources.”
There are two primary classes of X-ray sources. One class is normal stars like our sun. The other class is called active galactic nuclei, which are supermassive (more than one million solar mass) black holes at the center of galaxies, emitting X-rays when surrounding gas spirals into them.
Based on its bright X-ray emission, this wandering blac- k hole is massive, about 100,000 times the sun’s mass. The peculiar X-ray flare emitting from the far-off galaxy caught Lin’s attention.
“I looked through data in our astronomical data archive, and by examining the high-resolution images taken by the Hubble Space Telescope, I found that this X-ray source is not akin with the center of a galaxy, but is more close to a faint optical source lingering at the outskirts of a galaxy,” Lin said. “Therefore we identified it as a wandering black hole at the edge of a galaxy.”
What makes this discovery important is the odd behavior of the black hole in question. Usually, the destruction of a star that wanders too close to a black hole, a process called tidal disruption, is found to happen when a black hole is at the center of a galaxy. But this is the first observation of a wandering black hole disrupting a star, resulting in the bright X-ray emissions that caught Lin’s eye.
One main theory for the formation of wandering massive black holes is the merging of galaxies. In the early universe, galaxies were closer together than they are now and would frequently collide into each other. When a smaller galaxy collided with a large one, the outer part of the small galaxy would be easily disrupted and the black hole in its center would wander around the large galaxy that it collided with. Therefore, it can be expected to have massive black holes wandering at the outskirts in large galaxies. But these wandering black holes are normally very quiet and very hard to detect, which explains why only a small amount of them have been found.
“With our discovery of this wandering black hole, along with the records of other high luminosity black holes, strongly supports the merging theory of the evolution and formation of galaxies,” Lin said, implying that there are many more massive black holes in the universe that have yet to be discovered.