The Launching of Cold Clouds by Galaxy Outflows I: Hydrodynamic Interactions with Radiative Cooling

TL;DR

This study uses adaptive-mesh simulations to analyze how cold clouds in galaxy outflows evolve, focusing on mass loss and acceleration, revealing that cloud lifetime and acceleration depend on Mach number but not on cloud size.

Contribution

It provides a detailed hydrodynamic analysis of cold cloud evolution in galaxy outflows, incorporating radiative cooling and long-term tracking, with new scalings for cloud lifetime and acceleration.

Findings

Cloud lifetime and acceleration are independent of cloud radius when scaled by the cloud crushing time.

Supersonic flows create a Mach cone that influences cloud stability and stretching.

Simulation resolution is sufficient to model cloud evolution without magnetic fields or thermal conduction.

Abstract

To better understand the nature of the multiphase material found in outflowing galaxies, we study the evolution of cold clouds embedded in flows of hot and fast material. Using a suite of adaptive-mesh refinement simulations that include radiative cooling, we investigate both cloud mass loss and cloud acceleration under the full range of conditions observed in galaxy outflows. The simulations are designed to track the cloud center of mass, enabling us to study the cloud evolution at long disruption times. For supersonic flows, a Mach cone forms around the cloud, which damps the Kelvin-Helmholtz instability but also establishes a streamwise pressure gradient that stretches the cloud apart. If time is expressed in units of the cloud crushing time, both the cloud lifetime and the cloud acceleration rate are independent of cloud radius, and we find simple scalings for these quantities as a function of the Mach number of the external medium. A resolution study suggests that our simulations have sufficient resolution to accurately describe the evolution of cold clouds in the absence of thermal conduction and magnetic fields, physical processes whose roles will be studied in forthcoming papers.