Heavenly Taffy: Galaxies in collision

Some bodies just like sticking together, whatever the means. Astronomers have discovered a pair of colliding galaxies connected by a bridge of high-speed electrons and elongated magnetic fields. A map of the bridge’s radio emissions depicts bands that stretch like taffy between the galaxies.

GALACTIC TAFFY. Map shows two colliding galaxies (white) joined by a bridge of radio-wave-emitting electrons (pink). Condon

The galaxy pair, designated UGC 813/6, is only the second known to exhibit the taffy structure, notes James J. Condon of the National Radio Astronomy Observatory in Charlottesville, Va. By examining this structure, astronomers can reconstruct the original distribution of interstellar gas and magnetic fields within each of the galaxies, Condon says.

Condon’s team spotted the first pair of taffy galaxies in 1990. The second pair confirms that the structure, though rare, isn’t a fluke. Condon, with George Helou and Thomas H. Jarrett of the California Institute of Technology in Pasadena, describe the findings in the April Astronomical Journal.

The team’s observations, taken with the Very Large Array radio telescope in Socorro, N.M., suggest that the magnetic fields and high-speed electrons that fill the bridge were ripped from the galaxies when the two rammed into each other. To generate the bridge, which spans some 50,000 light-years, galaxies must contain dense gas clouds and strike head-on, Condon says.

The new findings support the notion that each end of a magnetic field line within a galaxy is anchored to a dense gas cloud. As the galaxies collide and pass through each other, their dense clouds create a magnetic bridge, stretching out their magnetic field lines much as a taffy machine pulls taffy, Condon says. Energetic charged particles spiral around the field lines, illuminating the bridge as they radiate radio waves.

The head-on crashes that lead to bridges are in stark contrast to the majority of collisions, in which two galaxies come close enough for their mutual gravity to yank out streams of gas and stars yet remain a few hundred thousand light-years apart.

“The observations provide a powerful new diagnostic tool for studying galaxies that have just undergone the remarkable feat of having passed bodily through another similar galaxy,” comments Philip N. Appleton of Caltech. Because the bridge’s radio emission fades after an astronomically brief 100 million years, “the radiation from particles trapped in these distended magnetic fields acts like cosmic signposts, telling astronomers that something remarkable has just happened,” he adds.

Because the most energetic electrons in the bridge radiate their energy the fastest, their intensity acts as a clock, indicating how long ago the galactic collision took place. Condon and his colleagues estimate that the impact that created the newly observed taffy pair occurred some 50 million years before the observed radio emission was generated.

The galaxies are now speeding away from each other at roughly 400 kilometers per second, but gravity will ultimately pull them back for a final merger.

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