Scientists figure out why black holes 'light up' when galaxies collide

When galaxies merge, some black holes will suddenly emit 10 billion times the energy of the sun. Using data from NASA's Swift satellite, scientists now know why this happens.

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    The optical counterparts of many active galactic nuclei (circled) detected by the Swift BAT Hard X-ray Survey clearly show galaxies in the process of merging. These nuclei were known prior to the Swift survey, but Swift has found dozens of new ones in more distant galaxies.
    NASA/Swift/NOAO/Michael Koss and Richard Mushotzky (Univ. of Maryland)
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Supermassive black holes appear to light up with hard X-rays when their parent galaxies decide to merge, according to a survey by NASA's peeping Swift satellite. The new findings solve a mystery that has kept astronomers in the dark for decades.

Just 1 percent of supermassive black holes currently put on such exhibitionist behavior by giving off as much as 10 billion times the sun's energy, as so-called active galactic nuclei (AGN). The supermassive black holes themselves hold anywhere from a million to a billion times the sun's mass, while the Milky Way galaxy's black hole is about 3 or 4 million solar masses.

Researchers had scratched their heads over why such high-energy displays occurred, until now. The findings announced today confirm past theories that suggested that violence from galactic mergers can fuel the growth of central black holes.

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"We find that about 25 percent of black holes found by Swift are in the process of merging," said team member Michael Koss of the University of Maryland in College Park, during a NASA teleconference.

About 60 percent of such active galaxies will merge completely within the next billion years to create giant black holes.

Hard X-ray action

Swift used its Burst Alert Telescope (BAT) to detect any telltale signs of hard X-rays, which rank between gamma-rays and X-rays on the electromagnetic spectrum of light.

Such hard X-rays can pass through interstellar gas or dust which otherwise blocks ultraviolet, optical and soft-X-ray light. Infrared radiation can also pass through the material, but may represent emissions from a galaxy's star nurseries rather than the central black holes.

The hard X-ray survey allowed astronomers to feel confident that they had spotted the majority of AGN within Swift's survey range of about 650 million light-years away. (A light-year is the distance that light can travel in one year — about 6 trillion miles (10 trillion km).)

Koss and his colleagues then followed up by spending 20 nights peering at the AGN with a 2-meter telescope at Kitt Peak National Observatory near Tucson, Ariz.

"Many of these galaxies are very close to us, so we see the severe distortion of the galaxy shapes," Koss explained. "In addition, we see that the galaxies are very close to each other and therefore will merge and interact very strongly."

Feeding on mergers

The galactic mergers represent a way in which supermassive black holes can feed as they grow, according to Joel Bregman, an astrophysicist at the University of Michigan in Ann Arbor, who was not involved with the study. Astronomers had previously known that black holes could grow by either merging with other black holes, or by sucking down nearby gaseous material.

"What this work does is it shows us the [black holes] feeding on gas channeled into the center [of the galaxies] at a fairly early stage," Bregman said. In other words, supermassive black holes can kick into high gear early on before their host galaxies have even begun to merge.

Past findings have also show that both merging galaxies and AGN are both much more common in the earlier universe, which had many more small galaxies than now. That indirectly supports the idea of galactic mergers triggering the growth of black holes.

But unlike the merger galaxies that spew plenty of hard X-rays, our Milky Way galaxy has a fairly quiet black hole at its center. The black hole likely goes through periods of outbursts and relative quiet, Bregman noted.

About 10 percent of similar-size galaxies are much more active than the Milky Way, said Meg Urry, an astrophysicist at Yale University in New Haven, Conn., who was not a member of the Swift team.

"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," Urry said.

Peeking into the past

Swift's eyes could get a complementary boost from NASA's upcoming NuStar mission, which is slated for launch in 2012. NuStar would use two sets of mirror arrays that focus hard X-rays to study even more distant galaxies from 7 or 8 billion years ago.

"It will find more distant black hole growth and AGN, so the question is whether those are triggered the same way as the [Swift] sample," Urry said in response to a question. "We think they are, but you'd like to observe them and see if that's the case."

For now, the Swift satellite continues its main mission of watching for gamma-ray bursts, which represent the most powerful form of energy in the universe since the Big Bang.

"We just detected our 508th gamma-ray burst about 30 minutes ago," said Neil Gehrels, principal investigator for Swift at the NASA Goddard Space Flight Center in Greenbelt, Md., during the NASA teleconference. NASA celebrated Swift's 500th discovery of gamma ray bursts back in April.

The Swift study will appear in the June 20 issue of The Astrophysical Journal Letters.

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