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

We report the first evidence of an on-going 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 ~5 km/s 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.