First evidence of a collision between two unrelated open clusters in the Milky Way

Abstract

We report the first evidence of an ongoing collision between two star clusters in our Galaxy, namely IC 4665 and Collinder 350. These are open clusters located at a distance of ~330 pc from the Sun and ~100 pc above the Galactic plane, and they both have prograde motions with only a small difference in their velocities (Collinder 350 moves $\sim 5\, \rm {km\, s^{-1}}$ faster than IC 4665); as inferred from ESA/Gaia based catalogue. Interestingly, the two clusters are physically separated by only ~36 pc in space; a distance that is smaller than the sum of their respective radii. Furthermore, the clusters exhibit signatures of elongated stellar density distributions, and we also detect an onset of an inter-cluster stellar bridge. Moreover, the orbit analysis suggests that the younger cluster IC 4665 (age = 53 Myr) must have formed at a distance >500 pc away from Collinder 350 (age = 617 Myr). These findings together imply that the two clusters do not represent merging of two objects in a binary system; rather, what we are witnessing is an actual collision between two independently formed star clusters. This collision phenomenon provides a unique opportunity to explore new aspects of formation and evolution theory of star clusters.