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More dark galaxies reveal themselves: hundreds of nearly starless suspects found in nearby cluster.

Hundreds of shady characters are lurking in a nearby neighborhood of galaxies. The Coma cluster houses nearly 20 times as many dark galaxies as previously known, researchers report. These shadowy figures--some as large as the Milky Way but with just one-thousandth the number of stars--could be dead ends in galactic evolution.

The cluster houses at least 854 of these barely perceptible galaxies, and there could be well over 1,000. These "ultra-diffuse galaxies" appear to have had much of their star-forming gas stolen, Jin Koda, an astronomer at Stony Brook University in New York, and colleagues report. The findings appear online June 24 in Astrophysical Journal Letters.

Last year, another team found 47 ultra-diffuse galaxies in the Coma cluster (SN: 12/13/14, p. 9), a bevy of thousands of galaxies that sits roughly 330 million light-years away in the constellation Coma Berenices. Koda and colleagues wondered if a bigger telescope could find even more dark galaxies, so they dug through images of the cluster taken by the 8-meter-wide Subaru Telescope in Hawaii.

"It's a beautiful result," says Yale astronomer Pieter van Dokkum, who led the discovery of the first 47 dark galaxies.

These galaxies are relics from an earlier time. They haven't formed any stars in the last 4 billion to 10 billion years, Koda says. The galaxies aren't scattered around the cluster haphazardly, as would be expected if they were new arrivals falling into Coma randomly. They are instead arranged symmetrically around the heart of the cluster, indicating that they have been lurking within Coma for a long time.

Their longevity is surprising. Star-starved galaxies are gravitationally tugged to and fro by their brighter, more massive brethren. With so few stars, the dark galaxies should have been torn apart long ago. "For these fluffy-looking galaxies to survive, they need something like dark matter protecting them," Koda says.

All galaxies are held together by dark matter, elusive particles that neither emit nor absorb light, revealing themselves only by their gravitational influence. These murky galaxies, however, take it to an extreme. To survive the rough-and-tumble streets of Coma, the dark galaxies must be over 99 percent dark matter--a far cry from the roughly 85 percent that's typical of galaxies.

"These things were not expected to be there," van Dokkum says. They could be failed galaxies, he says. Something might have stripped them of their gas, leaving behind a smattering of stars and a massive storehouse of dark matter.

One way to sweep out the gas, Koda says, is with a wave of supernova explosions. If enough stars exploded fast enough, maybe they could have launched all of the spare gas out of the galaxy.

Caption: Eight dark galaxies (circled) are among more than 800 in the Coma cluster (shown). Those circled in green are newly discovered.


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Title Annotation:ATOM & COSMOS
Author:Crockett, Christopher
Publication:Science News
Geographic Code:1USA
Date:Jul 25, 2015
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