On the kinematic and thermodynamic state of clouds in complex wind-multi-cloud environments using a Friends-of-Friends analysis

TL;DR

This paper introduces a Friends-of-Friends method to analyze cloud interactions in wind environments, revealing detailed cloud evolution, dominant entrainment processes, and implications for galactic outflows and observations.

Contribution

The study develops a novel Friends-of-Friends analysis technique to characterize individual cloud morphology and dynamics in complex wind-cloud interactions, advancing understanding of multi-phase outflows.

Findings

Condensation becomes dominant in momentum transfer after shock passage.

Internal cloud motions generate effective dynamic pressure exceeding thermal pressure.

Cooling and kinetic energy injection sustain multi-phase galactic winds.

Abstract

We investigate the interaction between a shock-driven hot wind and a cold multi-cloud layer, for conditions commonly found in interstellar and circumgalactic gas. We present a method for identifying distinct clouds using a Friends-of-Friends algorithm. This approach unveils novel detailed information about individual clouds and their collective behaviour. By tracing the evolution of individual clouds, our method provides comprehensive descriptions of cloud morphology, including measures of the elongation and fractal dimension. Combining the kinematics and morphology of clouds, we refine previous models for drag and entrainment processes. Our by-cloud analysis allows to discern the dominant entrainment processes at different times. We find that after the initial shock passage, momentum transfer due to condensation becomes increasingly important, compared to ram pressure, which dominates at early times. We also find that internal motions within clouds act as an effective dynamic pressure that exceeds the thermal pressure by an order of magnitude. Our analysis shows how the highly efficient cooling of the warm mixed gas at temperatures $\sim 10^{5}$ K is effectively balanced by the kinetic energy injected by the hot wind into the warm and cold phases via shocks and shear motions. Compression-driven condensation and turbulence dissipation maintain a multi-phase outflow and can help explain the presence of dense gas in galaxy-scale winds. Finally, we show that applying our Friends-of-Friends analysis to $\rm{H}_\rm{I}$-emitting gas and correcting for beam size and telescope sensitivity can explain two populations of $\rm{H}_\rm{I}$ clouds within the Milky-Way nuclear wind as structures pertaining to the same outflow.