NGC 4038 and NGC 4039 in Corvus (the Raven) are otherwise known as the Antennae Galaxies. They are thought to be in a (temporary) starburst phase, with a rapid conversion of gas into stars (perhaps 30x higher than what is occurring in our Milky Way). As is the case with other starburst galaxies, a tidal tail is often visible, a reflection of a close encounter with another galaxy.
Technical details as follows:
RCOS 16″ @ f/11.3; Apogee U9; Paramount ME; LRGB with 7 hours Lum and 3 hours/channel RGB; processed in PixInsight and Photoshop; SSRO @ CTIO, Chile; data acquired in March/April 2010.