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Unstrange Phenomena

25 May 2010

Why Black Holes Burst With Light When Galaxies Merge

*** Scientists got good news about the odd behavior of supermassive black holes from NASA's cosmic gossip, the Swift satellite.




Photo by NASA/Swift/NOAO/Michael Koss and Richard Mushotzky The optical counterparts of many active galactic nuclei (circled) detected by the Swift BAT Hard X-ray Survey. Galaxies are in the process of merging. These nuclei were known prior to the Swift survey. Swift has found dozens of new ones in more distant galaxies. - space.com


From Denny: Astronomers finally solve a mystery that has bedeviled them for decades. They have wracked their brains trying to figure out why black holes suddenly "light up" when galaxies collide and merge. It does seem like a contradiction that something that sucks up energy and is "black" could suddenly be lit up like a Christmas tree.

NASA Swift satellite

Astronomers got the good news recently from NASA's Swift satellite that relayed back the information like a good cosmic gossip. The word is that supermassive black holes are lighting up like crazy with hard X-rays when their parent galaxies merge.

The numbers the astronomers throw out to us are massive and practically beyond comprehension. Currently, it's just 1% of supermassive black holes that display this peacock preening behavior of putting on a good show in the universe. What's the amount of energy they give off? It's astounding at 10 billion times the sun's energy. Yeah, I'd say that would qualify them to be labeled as Active Galactic Nuclei or AGN. What an understatement! :)

Our Milky Way Galaxy's center black hole

Supermassive black holes can contain a lot of energy, on the scale of one million to one billion the times of the sun's mass. Oh, gee, our humble little Milky Way galaxy only holds in its black hole about three to four solar masses. It must have a small fuel tank. In fact, by normal standards, our Milky Way black hole is downright quiet. It experiences outbursts and then periods of quiet. Maybe it's sulking like any teenager.

"It's kind of lucky that we're in the 90 percent that aren't very active, so there are not a lot of hard X-rays being produced and coming our way and disturbing Earth's atmosphere," says Meg Urry, an astrophysicist at Yale University in New Haven, Connecticut.

Supermassive black holes

These high-energy displays of supermassive black holes have had scientists creating theories for years. Until now, there was no way to prove or disprove those theories. Now, some of those past theories were confirmed, especially the ones that promoted the idea that the violence of galactic mergers could fuel the growth of central black holes.

"We find that about 25 percent of black holes found by Swift are in the process of merging," comments Michael Koss, University of Maryland in College Park, at a NASA teleconference. "Many of these galaxies are very close to us, so we see the severe distortion of the galaxy shapes," he explained. "In addition, we see that the galaxies are very close to each other and therefore will merge and interact very strongly."

Within the next billion years or so scientists figure about 60% of those active galaxies will merge completely, creating giant black holes.

Scientists know that black holes grow by making like a vacuum cleaner and cleaning up the nearby gaseous material or by merging with other black holes. These galactic mergers are a way for the suppermassive black holes to actually feed as they also grow.

NASA's Swift satellite and NuStar

NASA's Swift satellite's survey range was calibrated for about 650 million light-years away. It watches for gamma-ray bursts. Did you know that gamma-rays are the most powerful form of energy in the universe since the Big Bang?

Get this, in 2012, there will launch the NuStar mission that will make a great complement to Swift. The NuStar uses two sets of mirror arrays. Those arrays will focus hard X-rays. The NuStar will be able to study even more distant galaxies, going deeper into space and back in time, from seven or even eight billion years ago. OK, now I'm impressed.


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