Perpetrator of Galactic Hit-and-Run Found

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Scientists have discovered why the irregularly shaped and hydrogen-shrouded starburst galaxy known as NCG 4449, located 12.4 million light years away, is so weird. It bears scars from a past close encounter with a newly discovered companion dwarf galaxy.

The evidence exonerates another previously known companion galaxy, DDO 125, which is located about 130,000 light years away from NCG 4449. Instead, new observations of NGC 4449 taken between May 29 and June 1, 2011 (made in the course of commissioning a new 0.7-meter telescope at Saturn Lodge Observatory, Calif.) turned up what scientists call “a profoundly tidally distorted” dwarf galaxy NGC 4449B.

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The new companion lies about 29,000 light years from the center of NGC 4449. Computer models show the contorted shapes of the two galaxies fit.

“We speculate that NGC 4449B is on its first encounter with NGC 4449 and experienced a close passage near the nucleus of NGC 4449,” lead researcher Michael Rich, with the University of California, Los Angeles, writes in this week’s Nature.

“This conclusion is supported by the morphology of NGC 4449B, the plume pointing at the nucleus, and the approximate agreement with the structure and timescales of the simulation,” Rich wrote.

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Scientists believe the encounter played a role in igniting the present burst of star formation in NGC 4449.

Image: Computer rendering of the newly found dwarf galaxy (right), shown in comparison with our Milky Way, that is believed to be a companion of the irregular starburst galaxy NCG 4449. To give a sense of scale, the dwarf galaxy would reach from the center of our galaxy to the location of our sun (labeled). Credit: Robert Hurt, Jet Propulsion Laboratory/Claire Welsh, Nature

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