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Supernova burps rid galaxies of hot gas.

Supernova burps rid galaxies of hot gas

The concerted action of thousands of exploding supernovas concentrated in small regions of a young, gas-rich galaxy could provide the power necessary for the ejection of gigantic blobs of matter. Shot out a high speeds, these expanding clumps would carry metal-laden gas to a galaxy's fringes and beyond. Such a scenario may help explain how evolving spiral galaxies redistribute chemical elements and how young spiral galaxies, which seem to have larger disks than their older counterparts, can shrink while maintaining a certain density.

This picture of galactic evolution comes out of a simple computer model developed by Jane C. Charlton, now at the University of Arizona in Tucson, and Edwin E. Salpeter of Cornell University in Ithaca, N.Y., who described their findings in the Nov. 1 ASTROPHYSICAL JOURNAL.

"It's certainly a highly speculative model," Charlton says. "Whether these things happen at all isn't clear." Nonetheless, the model makes several interesting predictions that may enable observers to check it.

In their model, Charlton and Salpeter assume that young galaxies tend to form massive stars that last only a few million years before exploding. Much of the released energy goes into heating up interstellar gas, creating large, hot blobs with individual masses about 10 million times that of the sun.

Although these blobs spread out laterally nearly as much as they travel outward, they can be treated mathematically in the same way as cannonballs fired from the Earth's surface. Some of these galactic cannonballs would move fast enough to escape the galaxy's gravitational field, while slower ones would eventually fall back into the galactic disk.

The escaping blobs would carry away angular momentum, meaning the galaxy would spin down while shrinking its extended disk. Computer simulations show that ejecting 10 to 20 percent of a disk's mass in the form of blobs over 10 billion years would be sufficient to cut the galaxy's disk to half of its initial size.

Blobs with insufficient velocity to escape the galaxy would eventually reenter the disk, enriching it with metallic elements created in the supernova explosions. Computer simulation results predict that such a galaxy's outskirts would have an unexpectedly high concentration of metallic elements.

"As you go out, the metal abundance first goes down and then goes up," Salpeter says. "That's certainly opposite to what most people would guess." Astronomers now have an incentive to look for this type of variation in metal abundance.
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Author:Peterson, I.
Publication:Science News
Date:Nov 11, 1989
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