bridge of gas and stars between these galaxies indicates they have
passed close to each other experiencing violent tides from mutual
gravity. Known together as Arp 240 but individually as NGC 5257 and NGC
5258, computer modelling and the ages of star clusters indicate that
the two galaxies completed a first passage near each other only about
250 million years ago. Gravitational tides not only pulled away
matter—they compress gas and so caused star formation in both galaxies.
Galactic mergers are thought to be common, with Arp 240 representing a snapshot of a brief stage in this inevitable process.
Arp 240 pair are about 300 million light-years distant and can be seen
with a small telescope toward the constellation of Virgo. Repeated
close passages should ultimately result in a merger and with the
emergence of a single combined galaxy.Source