Multiphase fragmentation: molecular shattering

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 remain 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 hydrodynamic 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 at similar to 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.

Automated summary

Recent observations suggest that galaxies may have a molecular component in their circumgalactic medium. However, the structure and dynamics of the molecular circumgalactic medium are still poorly understood. This study uses simulations to explore the process of fragmentation and shattering of molecular clouds, finding that larger clouds can shatter into tiny droplets, and cold clouds can undergo a process of splintering. These findings have implications for understanding the molecular phase of the circumgalactic medium in observations and cosmological simulations.