A smashing view from Hubble

Long ago, a galaxy far away smashed into another galaxy - creating a beautiful, terrible knot of cosmic chaos. The view of that galactic collision, captured by the Hubble Space Telescope, serves as a preview of what might well happen when the Andromeda Galaxy slams into our Milky Way galaxy billions of years from now.

The picture from Hubble, released today, shows the mighty crash of two galaxies similar to the Milky Way, but 250 million light-years away in the constellation Cancer. The weird-looking, two-tailed result is known as NGC 2623 or Arp 243.

Scientists say NGC 2623 appears to be in the late stages of a galactic merger. The supermassive black holes at the center of the two original galaxies have combined to form a super-energetic nucleus. The energy released by the clash has sparked the formation of large star clusters much brighter than the brightest clusters we see in our own celestial neighborhood.

The stars shine particularly brightly in the long tidal tails that were thrown off during the collision. Each of those tails is roughly 65,000 to 80,000 light-years long, which comes close to rivaling the width of the Milky Way's main disk.

Aaron Evans, an astronomer at the University of Virginia at Charlottesville who led the observational team, said NGC 2623's burst of starbirth has been going on for a long time. "If we assume the oldest star clusters that are extremely bright were formed at the beginning of the interaction, the age would be around 100 million years," he told me.

The star clusters, which show up as sparks of blue in the Hubble image, were likely shocked into existence as a result of the galactic collision. "The stars themselves don't actually touch, but what does happen is that the gas in the galaxies is affected gravitationally by the interaction," Evans explained.

Looking at luminous galaxies

Evans and his colleagues came across the smash-up in the course of conducting a survey of luminous infrared galaxies, which are called LIRGs for short. The survey, which draws upon imagery from the Hubble, Chandra and Spitzer space telescopes as well as the Galaxy Evolution Explorer, is known by yet another acronym (GOALS, which stands for Great Observatories All-sky LIRG Survey). Data from the European Space Agency's XMM-Newton satellite contributed to the NGC 2623 study.

"The main reason why we chose this galaxy is that it had this spectacular region of star formation off the nucleus," Evans said. The region was so spectacular that the team wrote a research paper about it, which was published more than a year ago in The Astrophysical Journal. An overview of the GOALS observations appeared in the June issue of the Publications of the Astronomical Society of the Pacific.

Evans explained that the goal of GOALS is to understand the processes that cause galaxies to light up. More than 200 galaxies have been sampled so far. "The sample as a whole is essentially designed so we can get snapshots of these varous galaxies as they're merging," Evans said.

One of the things that distinguishes NGC 2623 from most of the other galaxies is that the emissions from its central black hole appears to be relatively weak, considering the level of star formation that's being observed.

Galactic crash scenes rank among the most popular pictures produced by Hubble: This year's "people's choice" for Hubble observations was a tangled-up double-galaxy called Arp 274. Yet another assortment of interacting galaxies, known as Stephan's Quintet, was among the first pictures unveiled after the space telescope's upgrade. (The NGC 2623 image data was collected by Hubble's Advanced Camera for Surveys before it went on the blink in 2007.)

Beautiful ... from a distance

Such collisions may look divine from a distance, but you wouldn't want to be in the middle of one. Astronomers say that's what's likely to happen right here in the Milky Way 3 billion to 4 billion years from now. The nearby Andromeda Galaxy is currently on a collision course, approaching us at a speed of more than 300,000 mph, or 500,000 kilometers per hour. (Note to self: Arrange to be somewhere else in 3 billion years.)

If anyone is still hanging around the Milky Way at that time, they wouldn't necessarily be hit by Andromedan flotsam or jetsam. But they would see a sky fairly crackling with newborn stars and exploding with blasts of ultraviolet radiation. It wouldn't be a pretty sight. 

"Yeah, that would be very bad," Evans agreed.

"In terms of star formation in our galaxy triggered by the Milky Way - Andromeda merger, the real danger to us is having a massive star go supernova in our vicinity," he said in a follow-up e-mail.

So 250 million light-years is just about the right distance from which to appreciate the cosmic mayhem. Here are links to still more galactic crash scenes:

• Galaxies go into shock
• Hubble's 'fountain of youth'
• Galaxies in gridlock
• Season's greeting from space
• Close encounter with a galaxy cluster
• ESO shows you the Cosmic Bird
• X-rated galaxies
• Stars gone wild

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