Dynamical origins of wide binary collisions: from the Solar system to galaxy mergers

Abstract

Direct collisions of two gravitating objects are common in astrophysics. Around ~30% of Kuiper-belt objects are binaries, and ~20% are probably contact-binaries consisting of two lobes, where Arrokoth is the most studied one so far. On galactic and extra-galactic scales, the merger rate of binary black-holes and MS stars could be enhanced by triple and multiple evolution. The celebrated Lidov-Kozai mechanism excites the binary's eccentricity in multiple systems, and is scale-free, hence can be applied to many astrophysical scales. We show that both Arrokoth's evolution, as well as stellar multiple evolution is governed by semi-secular evolution followed by chaotic episodes. We discuss the evolutionary pathways and the origins of chaos. On Solar-system scales, we're able to explain both Arrokoth's peculiar obliquity and slow rotation. On galactic scales we show that the compact object merger rate is enhanced in galactic nuclei, while the galactic tidal field could also be a highly collisional environment and leads to a plethora of transients. Finally, we apply our results to mergers of triple supermassive BHs observable in the LISA band.