Largest-Known Spiral Galaxy: NGC 6872

Astronomers using NASA’s Galaxy Evolution Explorer have spotted what they say is the biggest spiral galaxy ever seen.

This image shows the giant barred spiral galaxy NGC 6872 (NASA’s Goddard Space Flight Center / ESO / JPL-Caltech / DSS)

The object – giant barred spiral galaxy NGC 6872 – is located in the southern constellation Pavo about 212 million light-years away. It spans more than 522,000 light-years – five times the size of Milky Way Galaxy.

“Without GALEX’s ability to detect the ultraviolet light of the youngest, hottest stars, we would never have recognized the full extent of this intriguing system,” said Dr Rafael Eufrasio, a research assistant at NASA’s Goddard Space Flight Center in Greenbelt, who presented the discovery at the American Astronomical Society meeting in Long Beach, California.

NGC 6872′s size stems from its interaction with a much smaller disk galaxy named IC 4970, which has only about one-fifth the mass of NGC 6872.

Astronomers think large galaxies grew through mergers and acquisitions – assembling over billions of years by absorbing numerous smaller systems. Intriguingly, the gravitational interaction of NGC 6872 and IC 4970 may have done the opposite, spawning what may develop into a new small galaxy.

“The northeastern arm of NGC 6872 is the most disturbed and is rippling with star formation, but at its far end, visible only in the ultraviolet, is an object that appears to be a tidal dwarf galaxy similar to those seen in other interacting systems,” said co-author Prof Duilia de Mello of Catholic University of America in Washington.

The tidal dwarf candidate is brighter in ultraviolet than other regions of the galaxy, a sign it bears a rich supply of hot young stars less than 200 million years old.

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Bibliographic information: Eufrasio RT et al. When the UV Unveils the Largest Spiral. 221st AAS Meeting. Long Beach, CA. January 7, 2013