Violent Collision Led Largest Spiral Galaxy Ever Seen to Beget Tiny New Galaxy - Wired

A violent collision millions of years in the past may have stretched out one arm of the spiral galaxy NGC 6872, making it the largest known spiral galaxy and perhaps generating a new galaxy in its wake.

From tip to tip the spiral galaxy NGC 6872 stretches 522,000 light-years, more than five times the length of our own Milky Way. It is located 212 million light-years from Earth in the southern constellation Pavo.

NGC 6872?s enormous span is partly the result of gravitational interactions with a nearby galactic neighbor, the much smaller IC 4970 (the bright spot above NCG 6872). Galaxies are known to fly past one another as they merge, wreaking havoc as they tug at each other's stars. Based on computer models, astronomers think that IC 4970 made its most recent close approach to NGC 6872 about 130 million years ago, sweeping through its northeast arm and disturbing copious amounts of gas, dust, and stars. IC 4970 may have also stolen a bit of cold gas from the larger galaxy, which went to powering the supermassive black hole in its center.

This research also suggests this pass created the seeds for a tiny new galaxy, circled at the far left of the image. The baby galaxy shines most brightly in ultraviolet light, meaning it contains a rich supply of hot young stars less than 200 million years old. This finding is based on archival data from the Galaxy Evolution Explorer (GALEX) mission, a space telescope built and flown by NASA that is now run by researchers at Caltech. GALEX sees in the ultraviolet, which is why it was able to spot the details of this interesting system. 

Image: NASA's Goddard Space Flight Center/ESO/JPL-Caltech/DSS