A new image of the collision of two galaxies thought to be similar to our Milky Way shows early signs of the formation of one of nature's most massive black holes.
The picture, taken with NASA's Chandra X-ray Observatory, shows two galaxies about 250 million light-years away in the midst of a head-on crash that has been going on for 10 million years, relatively short in astronomical terms, according to the new data. Chandra scientists said the picture is the best X-ray view ever of a galaxy merger.
Researchers say the collision likely triggered the formation of huge numbers of new stars, sent shock waves rumbling through intergalactic space, and could eventually lead to the formation of a supermassive black hole in the center of the combined galaxy, called Arp 200.
"The Chandra observations show that things really get messed up when two galaxies run into each other at full speed," said David Clements of the Imperial College, London, one of the team members involved in the study. "The event affects everything from the formation of massive black holes to the dispersal of heavy elements into the universe."
Astronomers think that many galaxies, including our own, may experience similar collisions at some time during their lives. Arp 220 is considered to be a prototype for understanding what conditions were like in the early universe, when massive galaxies and supermassive black holes were presumably formed by numerous galaxy collisions. It is the closest example of an "ultra-luminous" galaxy, one that gives off a hundred times as much radiation as our galaxy.
The Chandra image shows a bright central region at the waist of a glowing, hour-glass-shaped cloud of multimillion-degree gas. Rushing out of the galaxy at hundreds of thousands of miles per hour, the super-heated as forms a so-called superwind, thought to be due to explosive activity generated by the formation of hundreds of millions of new stars.
Farther out, spanning a distance of 75,000 light years, are giant lobes of hot gas that could be galactic remnants flung into intergalactic space by the early impact of the collision.
Chandra observations allowed astronomers to pinpoint an X-ray source at the exact location of the nucleus of one of the pre-merger galaxies. Another fainter X-ray source nearby may coincide with the nucleus of the other galaxy remnant.
The two X-ray sources are suspected of being black holes, which are thought to spew X-rays from just outside their spheres as matter spirals inward and is superheated. The two are more luminous than stellar black holes, which are typically as massive as a few Suns. But they are relatively weak compared to other supermassive black holes.
So the extraordinary luminosity of Arp 220 may be due to the rapid rate of star formation, astronomers said.
However, in a few hundred million years, this balance of power may change. The two black holes could merge to produce a central supermassive black hole, researchers said. This new arrangement could cause much more gas to fall into the central black hole, creating a power source equal to or greater than that due to star formation.
"The unusual concentration of X-ray sources in the very center of Arp 220 suggests that we could be observing the early stages of the creation of a supermassive black hole and the eventual rise to power of an active galactic nucleus," said Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, another member of the team.
Clements and McDowell were joined on this research by an international group of researchers from the United States, United Kingdom and Spain.
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