High-speed crash makes hot, 'sterile' galaxies

By Larry O'Hanlon

updated 12:37 p.m. ET Oct. 9, 2008

A new wider view of two very well-known galaxies has revealed a big surprise: They are connected by faint, starless filaments of hydrogen gas which trace back to a very high-speed intergalactic collision.

The smash-up between galaxies M86 and NGC4438 not been suspected before, and may explain why M86, which is visible to the naked eye, is unable to give birth to new stars.

"Stars and gases behave very differently in collisions," explained astronomer Jeffrey Kenney of Yale University and lead author of a paper on the discovery in the Nov. 2008 issue of Astrophysical Journal Letters.

During galactic smash-ups stars rarely collide, since there is so much space between them. But gases do slam into gases. The faster the collision, the higher the temperature the gases reach.

In the case of M86, its gases are millions of degrees and radiate in X-rays. Until now, however, there was no easy explanation for all this blistering hot gas. The new evidence of M86's collision may solve that mystery.

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What's more, the super-hot gas also probably explains why M86 is unable to produce new stars. To make stars you need colossal clouds of frigid gas that will collapse to begin star-producing nuclear reactions.

Super-hot gases are far too agitated to clump together and collapse to form such new heavenly bodies. As a result, the wickedly hot galactic atmosphere of M86 leaves it bereft of baby stars and dominated by older stars that formed before something — probably galactic gas collisions — turned up the heat.

"This has been an ongoing mystery for years," said Kenney of the absence of young stars in elliptical galaxies, which also happen to be the largest galaxies in the universe.

The faint streamers of hydrogen gas between the two galaxies were previously detected at the edges of images of both galaxies M86 and NGC4438, but it wasn't until new technologies enabled a wider, deeper view of the space between them that the connection was discovered, said astronomer Bill Keel of the University of Alabama.

"These galaxies have a history," concludes Keel. The discovery underscores the growing realization that no galaxy is an island, he said. "It's no longer an isolated, stable system." It's part of a larger process of collisions, mergers and near misses.

Keel said he is hoping Kenney and his colleagues will search for more telltale gas filaments between other galaxies in the Virgo cluster, where both M86 and NGC 4438 reside. M86 is the brightest galaxy in the Virgo cluster, a neighbor galaxy cluster about 50 million light-years away from our own Local Group cluster.

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