From: Andy Soos, ENN
Published November 23, 2010 03:28 PM

Galactic Outburst

Astronomers using NASA's Spitzer Space Telescope have found a stunning burst of star formation that beams out as much infrared light as an entire galaxy. The collision of two spiral galaxies has triggered this explosion, which is cloaked by dust that renders its stars nearly invisible in other wavelengths of light. Although bright as this is, it pales in comparison to a quasar. The brightest known quasar is 3C 273 in the constellation of Virgo. This quasar's luminosity is about 2 trillion times that of our sun, or about 100 times that of the total light of average giant galaxies like our Milky Way. The starburst newly revealed by Spitzer stands as the most luminous ever seen taking place away from the centers, or nuclei, of merging parent galaxies. It blazes ten times brighter than the nearby Universe's previous most famous starburst that gleams in another galactic smashup known as the Antennae Galaxy.


The new findings show that galaxy mergers can pack a real star-making wallop far from the respective galactic centers, where star-forming dust and gases typically pool.

"This discovery proves that merging galaxies can generate powerful starbursts outside of the centers of the parent galaxies," says Hanae Inami, first author of a paper detailing the results in the July issue of The Astronomical Journal.  She adds: "The infrared light emission of the starburst dominates its host galaxy and rivals that of the most luminous galaxies we see that are relatively close to our home, the Milky Way."

Galaxy mergers are now known to be more common than was previously thought. They were even more common in the early universe than they are today. The early universe was smaller, so galaxies were closer together and therefore more prone to smash-ups. Even apparently isolated galaxies can show signs of past mergers in their internal structure.Our own Milky Way contains the debris of the many smaller galaxies it has brushed against and devoured in the past. And it has not stopped munching away at its neighbors: It is currently absorbing the Sagittarius dwarf elliptical galaxy.

Inami, Armus and their colleagues spotted the buried starburst with Spitzer in the interacting galaxies known as II Zw 096. This galactic train wreck - located around 500 million light years away in the constellation Delphinus (the Dolphin) - will continue to unfold for a few hundred million years. Gravitational forces have already dissolved the once-pinwheel shape of one of II Zw 096's pair of merging galaxies.

The ultra-bright starburst region spans 700 light-years or so - just a tiny portion of II Zw 096, which streams across some 50,000 to 60,000 light-years - yet it blasts out 80 percent of the infrared light from this galactic tumult. Based on Spitzer data, researchers estimate the starburst is cranking out stars at the breakneck pace of around 100 solar masses, or masses of our Sun, per year.

"Most of the far-infrared emission in II Zw 096, and hence most of the power, is coming from a region that is not associated with the centers of the merging galaxies," Inami explains.  In galaxy mergers, individual stars rarely slam into one another because of the vast distances separating them; even in the comparatively crowded central hubs of spiral galaxies, trillions of kilometers still often yawn between the stars. 

Typically the first sign of a collision is a bridge of matter connecting two galaxies as gravity's first gentle tugs tease out dust and gas. As the outer reaches of the galaxies begin to interact, long streamers of gas and dust, called tidal tails, sweep back to wrap around the galactic cores.But giant, diffuse clouds of gas and dust in galaxies do crash together - passing through each other somewhat like ocean waves - and in turn spur the gravitational collapse of dense pockets of matter into new stars. These young, hot stars shine intensely in the energetic ultraviolet part of the spectrum.

In the case of II Zw 096, however, a thick shroud of gas and dust still surrounds this stellar brood. The blanket of material absorbs the stars' light and re-radiates it in the lower-energy, infrared wavelengths that gleam clear through the dust to Spitzer's camera.Merging galaxies such as II Zw 096 also offer a sneak peek at the fate of our Milky Way in some 4.5 billion years when it is expected to plow into its nearest large galactic neighbor, the Andromeda Galaxy. Off-nuclear starbursts such as that in II Zw 096 and the Antennae Galaxy could occur in the vicinity of our Solar System, perhaps, which is located about two-thirds of the way out from the Milky Way's glowing, bulging center.

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