NASA, ESA, M. Cisternas
A new study using images from the Hubble Space Telescope has found that galaxies with powerful black holes at their cores - called Active Galactic Nuclei or AGN (on the left) need not rely on galaxy collisions to grow. Non-AGN galaxies are shown on the right. The photos come from the COSMOS survey using the Hubble Space Telescope.
updated 1/5/2011 11:26:58 AM ET 2011-01-05T16:26:58

A clash of titans would seem to be the perfect cause for a giant explosion, and for years scientists thought it was massive collisions between galaxies that triggered the black hole violence seen at the heart of many active galaxies.

A new study suggests, however, these galaxy mergers may not be at fault, and that less obvious sources are the culprit.

Our Milky Way galaxy and most others have supermassive black holes at their hearts. Some of these monster black holes are relatively calm, but others known as "active galactic nuclei" or AGN can spew out more radiation than our entire galaxy does, and from a patch of space no larger than our solar system. Astronomers suspect this energy is released when matter heats up as it gets sucked into the black holes.

Until now, the main suspects for what made these supermassive black holes into active galactic nuclei were galactic mergers. The crashes were thought to be what drove matter into the black holes, ramping up their activity. [ Gallery: Black Holes of the Universe ]

But after looking at telltale signs of galactic mergers among 140 active galaxies, as well as more than 1,200 comparable inactive galaxies, over the last 8 billion years, a team of astronomers found no significant link between the galaxy crashes and black hole outbursts.

"The implication is that the universe is not evolving in such a violent way as previously thought, at least for the last 8 billion years," research team leader Mauricio Cisternas, an astronomer at the Max Planck Institute for Astronomy in Heidelberg, Germany, told The universe is thought to be about 13.7 billion years old.

Cisternas and other scientists analyzed galaxies from the COSMOS survey, which investigates an area of sky comprehensively mapped by Hubble and other telescopes at different wavelengths. In that area, which is roughly 10 times the area covered by the moon, they identified active galaxies by using X-ray observations from the European Space Agency's XMM-Newton space telescope, and studied them in greater detail with optical images taken by the Hubble Space Telescope.

A galactic police lineup
For each of the active galaxies the researchers inspected, they chose nine comparable nonactive galaxies of roughly the same cosmic age and thus stage of evolution. (One can estimate how old a galaxy is by figuring out its distance from us in light years; knowing how many years it took the galaxy's light to get here can tell you its age.) They examined all these galaxies for evidence of mergers.

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"You can usually tell when galaxies have been involved in a merger," said researcher Knud Jahnke, a astronomer at Max Planck. "Instead of the neat, geometric spiral or smooth elliptical shapes you usually see in Hubble images, colliding galaxies typically look distorted and warped."

At the researchers' request, 10 galaxy experts from eight institutions independently evaluated whether each of the galaxies was distorted, without being told which ones contained an active galactic nucleus. (The researchers had removed the bright spot in the images of active galaxies that had such nuclei.)

None of the experts' findings established a significant link between a galaxy's activity and its involvement in a major merger. The researchers concluded that the cause of at least three-quarters and possibly all of active galactic nucleus activity over the last 8 billion years must have a different explanation.

"We do not rule out that mergers actually might cause AGN activity in some cases," Jahnke told "But they do not dominate the buildup of black hole mass over the last 8 billion years."

When galaxies collide
Not every major galactic collision has to result in gas falling into the black holes at the centers of galaxies, Cisternas said.

For example, he explained, gas can get stalled at some point as it gets drawn toward the cores, and is kept far away from the black hole. "Another possibility is that due to the violence of the merger event, a relevant fraction of the gas is simply stripped away," he said.

So what might have caused the violence witnessed in active galaxies? Potential culprits include collisions of molecular clouds, instabilities within galaxies, or gravitational disruptions by other galaxies flying by. Any of them could have fed matter into the black holes to spike up activity.

The researchers plan on looking at mergers from even further in the past to see whether they are linked with active galaxies.

"At these times there were more frequent mergers, by a factor of 10 or so, and the amount of gas in galaxies was much higher," Jahnke explained. "Hence physics could be different and other mechanisms could be at work."

The scientists will detail their findings Jan. 10 in the Astrophysical Journal.

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Photos: Month in Space: January 2014

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  1. Southern stargazing

    Stars, galaxies and nebulas dot the skies over the European Southern Observatory's La Silla Paranal Observatory in Chile, in a picture released on Jan. 7. This image also shows three of the four movable units that feed light into the Very Large Telescope Interferometer, the world's most advanced optical instrument. Combining to form one larger telescope, they are greater than the sum of their parts: They reveal details that would otherwise be visible only through a telescope as large as the distance between them. (Y. Beletsky / ESO) Back to slideshow navigation
  2. A balloon's view

    Cameras captured the Grandville High School RoboDawgs' balloon floating through Earth's upper atmosphere during its ascent on Dec. 28, 2013. The Grandville RoboDawgs’ first winter balloon launch reached an estimated altitude of 130,000 feet, or about 25 miles, according to coaches Mike Evele and Doug Hepfer. It skyrocketed past the team’s previous 100,000-feet record set in June. The RoboDawgs started with just one robotics team in 1998, but they've grown to support more than 30 teams at public schools in Grandville, Mich. (Kyle Moroney / AP) Back to slideshow navigation
  3. Spacemen at work

    Russian cosmonauts Oleg Kotov, right, and Sergey Ryazanskiy perform maintenance on the International Space Station on Jan. 27. During the six-hour, eight-minute spacewalk, Kotov and Ryazanskiy completed the installation of a pair of high-fidelity cameras that experienced connectivity issues during a Dec. 27 spacewalk. The cosmonauts also retrieved scientific gear outside the station's Russian segment. (NASA) Back to slideshow navigation
  4. Special delivery

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  5. Accidental art

    A piece of art? A time-lapse photo? A flickering light show? At first glance, this image looks nothing like the images we're used to seeing from the Hubble Space Telescope. But it's a genuine Hubble frame that was released on Jan. 27. Hubble's team suspects that the telescope's Fine Guidance System locked onto a bad guide star, potentially a double star or binary. This caused an error in the tracking system, resulting in a remarkable picture of brightly colored stellar streaks. The prominent red streaks are from stars in the globular cluster NGC 288. (NASA / ESA) Back to slideshow navigation
  6. Supersonic test flight

    A camera looking back over Virgin Galactic's SpaceShipTwo's fuselage shows the rocket burn with a Mojave Desert vista in the background during a test flight of the rocket plane on Jan. 10. Cameras were mounted on the exterior of SpaceShipTwo as well as its carrier airplane, WhiteKnightTwo, to monitor the rocket engine's performance. The test was aimed at setting the stage for honest-to-goodness flights into outer space later this year, and eventual commercial space tours.

    More about SpaceShipTwo on PhotoBlog (Virgin Galactic) Back to slideshow navigation
  7. Red lagoon

    The VLT Survey Telescope at the European Southern Observatory's Paranal Observatory in Chile captured this richly detailed new image of the Lagoon Nebula, released on Jan. 22. This giant cloud of gas and dust is creating intensely bright young stars, and is home to young stellar clusters. This image is a tiny part of just one of 11 public surveys of the sky now in progress using ESO telescopes. (ESO/VPHAS team) Back to slideshow navigation
  8. Fire on the mountain

    This image provided by NASA shows a satellite view of smoke from the Colby Fire, taken by the Multi-angle Imaging SpectroRadiometer aboard NASA's Terra spacecraft as it passed over Southern California on Jan. 16. The fire burned more than 1,863 acres and forced the evacuation of 3,700 people. (NASA via AP) Back to slideshow navigation
  9. Where stars are born

    An image captured by NASA's Spitzer Space Telescope shows the Orion Nebula, an immense stellar nursery some 1,500 light-years away. This false-color infrared view, released on Jan. 15, spans about 40 light-years across the region. The brightest portion of the nebula is centered on Orion's young, massive, hot stars, known as the Trapezium Cluster. But Spitzer also can detect stars still in the process of formation, seen here in red hues. (NASA / JPL-Caltech) Back to slideshow navigation
  10. Cygnus takes flight

    Orbital Sciences Corp.'s Antares rocket rises from NASA's Wallops Flight Facility on Wallops Island, Va, on Jan. 9. The rocket sent Orbital's Cygnus cargo capsule on its first official resupply mission to the International Space Station. (Chris Perry / NASA) Back to slideshow navigation
  11. A long, long time ago...

    This long-exposure picture from the Hubble Space Telescope, released Jan. 8, is the deepest image ever made of any cluster of galaxies. The cluster known as Abell 2744 appears in the foreground. It contains several hundred galaxies as they looked 3.5 billion years ago. Abell 2744 acts as a gravitational lens to warp space, brightening and magnifying images of nearly 3,000 distant background galaxies. The more distant galaxies appear as they did more than 12 billion years ago, not long after the Big Bang. (NASA / NASA via AFP - Getty Images) Back to slideshow navigation
  12. Frosty halo

    Sun dogs are bright spots that appear in the sky around the sun when light is refracted through ice crystals in the atmosphere. These sun dogs appeared on Jan. 5 amid brutally cold temperatures along Highway 83, north of Bismarck, N.D. The temperature was about 22 degrees below zero Fahrenheit, with a 50-below-zero wind chill.

    Slideshow: The Year in Space (Brian Peterson / The Bismarck Tribune via AP) Back to slideshow navigation
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