Multiphase fragmentation: Molecular shattering

Abstract

Abstract Recent observations suggest galaxies may ubiquitously host a molecular component to their multiphase circumgalactic medium (CGM). However, the structure and kinematics of the molecular CGM remains understudied theoretically and largely unconstrained observationally. Recent work suggests molecular gas clouds with efficient cooling survive acceleration in hot winds similar to atomic clouds. Yet the fragmentation of molecular clouds into a large number of fragments (‘shattering’) when subjected to external shocks or undergoing rapid cooling remains unstudied. We perform radiative, inviscid hydrodynamics simulations of clouds perturbed out of pressure equilibrium to explore the process of shattering to molecular temperatures. We find molecular clouds larger than a critical size can shatter into a mist of tiny droplets, with the critical size deviating significantly from the atomic case. We find that cold clouds shatter only if the sound crossing time exceeds the local maximum of the cooling time ∼8000 K. Moreover, we find evidence for a universal mechanism to ‘shatter’ cold clouds into a ‘mist’ of tiny droplets as a result of rotational fragmentation – a process we dub ‘splintering.’ Our results have implications for resolving the molecular phase of the CGM in observations and cosmological simulations.