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

TL;DR

This study extends cloud-crushing physics by simulating ensembles of radiatively cooling cold clouds in a hot galactic wind, rather than single clouds. By constructing an effective volume filling fraction $F_V$ from conical boxes aligned with the wind and linking it to the single-cloud survival criterion via the critical radius $r_{ m crit}$, the authors derive a universal threshold $F_{V,{ m crit}}\approx 0.24$ that separates surviving from destroyed multi-cloud systems across morphologies. They demonstrate that wind-aligned configurations can survive and even grow via tail merging and mass feeding, while orthogonal arrangements are prone to destruction unless strong downstream replenishment occurs; fragmentation into many small clouds generally enhances survival. The results offer a practical framework for predicting cold gas persistence in multiphase galactic winds and CGM dynamics, with implications for feedback models and the interpretation of observations of fast, multiphase 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 three-dimensional hydrodynamic simulations of ensembles of individual spherical 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 inter-cloud 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 universal threshold $F_{V,{\rm crit}}\approx 0.24$ 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 CGM dynamics.

Figures (26)

• Figure 1: A setup of the cloud-crushing problem and an overview of common parameters. The zoom-in schematically introduces the physics of a turbulent mixing layer, the ultimate driver of cloud growth. Colored red, green, and blue are the hot, mixed, and cold gas respectively, where the mixed gas of intermediate temperature quickly cools and thus feeds the cold gas reservoir.
• Figure 2: The evolution of a cloud for survival and destruction. The timestamps are identical for both cases. The left-hand side shows 8 cells per $r_{\mathrm{cloud}}$ resolution, and destruction of a cloud with $t_{\mathrm{cool,mix}}/t_{\mathrm{cc}} \approx 7.55$. The right-hand side shows a 32 cells per $r_{\mathrm{cloud}}$ resolution survival of a cloud with $t_{\mathrm{cool,mix}}/t_{\mathrm{cc}} \approx 7.55\times 10^{-2}$. Visualized is a mass-weighted projection plot of the density in the simulation box. Indicated above the snapshots one finds several measurable quantities, indicating the gas mass and relative velocity in the box. Entrainment allows the cloud to surpass the initial mass after several $t_{\mathrm{cc}}$.
• Figure 3: A visualization of different properties introduced in the problem. Most plots are mass-weighted means, except the pressure and specific scalar (tracer) plots. Most notably, one observes several shock-fronts throughout the cloud and its surrounding The data is taken from a high-resolution survival run.
• Figure 4: Study of the transitional regime between survival and destruction at different ratios of $t_{\mathrm{cool,mix}}/t_{\mathrm{cc}}$. Shown for different values of $t_{\mathrm{cool,mix}}/t_{\mathrm{cc}}$ is the cold gas mass fraction up to 40 cloud crushing times. The simulations were carried out with a single cloud to study behavior close to the survival criterion, whose expected outcome is denoted by two different linestyles.
• Figure 5: Mass-weighted mean of the density in a non-periodic wind-aligned cloud setup. There is no spatial separation between the clouds and we visualize only gas mass that is denser than one percent of the initial cloud's density. On the left side, we see a setup of 15 clouds that are being destroyed while the right-hand side shows early stages of cloud survival and entrainment for a total of 50 clouds. Both initial morphologies form a tail that can enable cloud growth only if enough clouds are placed initially.