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Starlight shadows protogalaxy finding.

Starlight Shadows Protogalaxy Finding

When is a hydrogen cloud a budding galaxy and when is it just gas? That's the astronomical question prompted by researchers with opposing views -- and separate radio and optical observations -- of a newly discovered celestial object that some believe represents a nearby, late-blooming galaxy.

The news broke last week when radio astronomers announced finding an isolated, apparently starless cloud of hydrogen gas, 10 times the Milky Way's diameter, about to give birth to a galaxy. That interpretation of the discovery excites astronomers because it challenges the accepted notion that all galaxies formed shortly after the Big Bang, some 10 to 15 billion years ago. In addition, the cloud's relative proximity, about 65 million light-years from Earth near the Virgo cluster, puts it "astronomically in our backyard," says co-discoverer Martha P. Haynes of Cornell University in Ithaca, N.Y. This makes possible detailed observations using both optical and radio telescopes.

But previous optical images of the cloud region may give an entirely different interpretation of the object, say astronomers Mike Irwin and Richard McMahon of the University of Cambridge in England. Tipped off last summer that the sky region under study by Haynes and her co-worker might prove interesting to examine optically, Irwin and McMahon scanned photographic plates made in the 1970s and found a pocket of stars in the supposedly starless region. Moreover, the starlight coincided with one of the two clumps of dense hydrogen in the cloud's radio image detected by Haynes and Riccardo Giovanelli at the Arecibo Observatory in Puerto Rico. For Irwin, the finding clinches his belief that the cloud is not a budding galaxy or protogalaxy but just an unusually large gas envelope surrounding a slowly evolving, already-formed galaxy known as an irregular dwarf.

"They [Giovanelli and Haynes] jumped to a conclusion because they did not see an optical counterpart," says Irwin. "Our observation argues for a dwarf galaxy."

Giovanelli says a small pocket of starlight does not eliminate the cloud as a protogalaxy. Astronomers must make further optical measurements, he adds, to determine if the stars actually reside in the cloud and move with it. Optical observations cannot begin until late next month, when the cloud region returns to the twilight sky, McMahon notes.

"Nobody yet knows what this thing is," says Arthur M. Wolfe, an astronomer at the University of California, San Diego. "What's great about this object is it may be appearing now, nearby, as it did in the early universe," notes Wolfe. He adds that the cloud's large size argues against the theory that galaxies form when small shreds of matter collide. "It may not be a bunch of little guys, but big guys, that form galaxies."
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Title Annotation:discovery of elliptical cloud near Virgo
Author:Cowen, Ron
Publication:Science News
Date:Sep 9, 1989
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