Multi-cloud crushing – the collective survival of cold clouds in galactic outflows

ABSTRACT The ram-pressure acceleration of cold gas by hot outflows plays a crucial role in the dynamics of multiphase galactic winds. Recent numerical studies incorporating radiative cooling have identified a size threshold for idealized cold clouds to survive within high-velocity outflows. This study extends the investigation to a more complex morphology of cold gas as observed in the interstellar medium. We conduct 3D hydrodynamic simulations of ensembles of individual spherical $\sim 10^4$ K clouds to systematically explore under which conditions the cold clouds can survive. We find that cloud ensembles can survive collectively – even when individual clouds, if isolated, would be rapidly destroyed. Our results indicate that, besides the morphology, factors such as tight packing, small intercloud distance, and higher fragmentation facilitate survival. We propose a novel multi-cloud survival criterion that accounts for collective properties of the cloud system, including total gas mass and the geometric configuration based on an effective volume filling fraction of the cold gas $F_V$. This fraction is computed by constructing a composite volume from individual enclosing conical boxes aligned with the wind, incorporating spatial overlap, and cloud-tail spreading. The box dimensions scale with the critical survival radius $r_{\rm crit}$ from the single-cloud criterion. We find a threshold $F_{V,{\rm crit}}\approx 0.24$ among our simulations that robustly separates surviving from destroyed systems across diverse geometric configurations. Our findings emphasize the critical importance of initial cloud distribution and fragmentation in governing the long-term evolution and survival of cold gas structures, providing insight into observed multiphase outflows and circumgalactic medium dynamics.