April 24, 2008 -- What might be described as the universe's mightiest sumo matches, or perhaps rowdiest square dances, have been caught on camera and brought together for the first time into an extraordinary album of galactic collisions.
Each image, captured by the Hubble Space Telescope (HST), is a snapshot of an ongoing process of galaxies slamming or twirling into each other.
"The gravitational dance is taking place over hundreds of million of years," explains Hubble researcher Lars Lindberg Christensen of the European Space Agency (ESA) in Garching, Germany. "A lot of the same things are happening, but still a lot of different things happen as well."
For instance, some images show galaxies just beginning to collide while others show the process well underway. There are also differences in the speeds of the collisions, it appears. Some galaxies look sort of like a water balloon being punctured by a bullet, Christensen told Discovery News.
Despite the violent appearances, however, the stars within the galaxies are not themselves colliding, since they are too far apart. All the action is really in the gases and dust, which do collide and create terrific shockwaves.
These shocks are thought to then trigger vast waves of star creation. In fact it's the heat of all those new stars -- in the form of infrared light collected by the HST -- that gave away the collisions in many of these images.
"Collisions basically result in rampant star formation in galaxies as gas is compressed by the interaction," said astronomer Aaron Evans of the University of Virgina, Stony Brook University and the National Radio Astronomy Observatories. "It turns out that most of the star formation in these galaxies is obscured, and thus the light from these stars is absorbed by dust and emitted as heat."
This heat is also the best handle astronomers have on the overall importance of merger-induced star formation comes from measuring the total energy budget of the Universe, said Evans. Astronomers figure that about half of the light generated since the Big Bang is infrared light -- mostly generated in these sorts of galactic mergers.
Other common features in these collisions are quasars. These are believed to be super massive black holes at the centers of the colliding galaxies that give off powerful bursts of X-rays when they are feeding on matter. Some researchers suspect that collisions are the best time to see super massive black holes feeding because of all that gas and dust that's flying helter-skelter during the cosmic dance.
Even our Milky Way is not immune. It has already gobbled up some smaller galaxies and is on a crash course with Andromeda. That dance ought to begin in about 2 billion years, say astronomers.
In the end, all of the colliding spiral galaxies will form what are called elliptical galaxies -- which are more like colossal spheres of stars. These are thought to be the old-age homes for stars, since most of the gas and dust are used up in elliptical galaxies and few new stars form in them.
"Every massive galaxy may not go through a 'major' merger such as those seen in most of the HST images, but every galaxy goes through at least a 'minor' merger," Evans told Discovery News. "By minor merger, I mean a merger of a massive galaxy like our own and a much less massive satellite galaxy like the Magellanic Clouds. These types of accretion events happen all of the time."
The 59 images were collected and processed over much of the last year and released today in celebration of the 18th anniversary of the launch of the HST. The collection is also the largest ever released to the public at one time from the HST.
Most of the collisions are part of a larger study of galaxies that are exceptionally bright in infrared light called the Great Observatories All-sky LIRG Survey. The survey brings together images from Hubble, NASA's infrared Spitzer Space Observatory, NASA's Chandra X-Ray Observatory and NASA's Galaxy Explorer.
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