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Largest spiral galaxy measured and pictured

First Posted: Jan 11, 2013 10:51 AM EST

NGC 6872, the largest spiral galaxy in the known universe, was now measured by astronomers. They employed existing materials from several telescopes, including the Very Large Telescope of the European Southern Observatory (ESO),  the Two Micron All Sky Survey, NASA's Spitzer Space Telescope, and NASA's Galaxy Evolution Explorer (GALEX) mission, the specialized tool for this purpose. The combined archival data yielded an insightful image of this gargantuan galaxy, showing that it measures half-a-million light years, 522,000 to be exact, between the two far ends of the spirl arms. In comparison, our own Milky Way galaxy, which is still a large galaxy itself, measures up to about 100,000 light-years in diameter.

giant galaxy crash
(Photo : NASA's Goddard Space Flight Center/ESO/JPL-Caltech/DSS)
This composite of the giant barred spiral galaxy NGC 6872 combines visible light images from the European Southern Observatory's Very Large Telescope with far-ultraviolet (1,528 angstroms) data from NASA's GALEX and 3.6-micron infrared data acquired by NASA's Spitzer Space Telescope. A previously unsuspected tidal dwarf galaxy candidate (circled) appears only in the ultraviolet, indicating the presence of many hot young stars. IC 4970, the small disk galaxy interacting with NGC 6872, is located above the spiral's central region.

The team of astronomers from the United States, Chile and Brazil also found an explanation for the unusually stretched out form of the spiral galaxy. Employing GALEX and its ability to accurately record the ultraviolet sprectrum, which is emitted by newly formed and very bright stars, they could not only measure the whole extension and mass of NGC 6872, but also discover and categorize a smaller companion galaxy that interacts in a crucial way with the enormous neighbor.
The disc galaxy IC 4790, with one-fifth the mass of the large galaxy, seems to consist mainly of young stars.

"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 team member Duilia de Mello, a professor of astronomy at Catholic University.

The researchers believe that those two galaxies may be interacting in unexpected ways to create what could be a third, smaller galaxy in the area. Which is the opposite of the expected outcome, which is that larger galaxies incorporate smaller galaxies. Such a new galaxy could have started forming after a collision between NGC 6872 and IC 4790, one which astronomers think may have also jumpstarted star formation in NGC 6872 after it had once slowed, a collision that could have roughly happened as illustrated in this provided picture:

giant galaxy crash
(Photo : NASA)
Computer simulations of the collision between NGC 6872 and IC 4970 reproduce the basic features of the galaxies as we see them today. They indicate that IC 4970's closest encounter occurred 130 million years ago and that the smaller galaxy followed a path (dashed curve) close to the plane of the spiral's disk and in the same direction it rotates.

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