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Multiphase Galactic Winds

A novel analytic model for a galactic wind peppered with embedded cold clouds

Published onJan 21, 2022
Multiphase Galactic Winds

This schematic figure outlines an analytic framework for galactic winds recently developed by Fielding & Bryan (2022).[1] The framework enables solutions accounting for co-evolution of the cold and hot phases of a galactic wind as they interact with each other while flowing out of a galaxy.

Its essential new component is a robust model for the turbulent radiative mixing layers that mediate interactions between the phases. Motion of the hot phase relative to a cold cloud induces the development of a turbulent mixing layer on the cloud's surface. The turbulence leads to mixing and produces intermediate temperature gas susceptible to extremely rapid radiative cooling. Condensation of the hot phase and growth of the cold cloud occurs if radiative cooling in the mixing layer proceeds more rapidly than the turbulence can shred the cloud. Alternatively, if the turbulence shreds the cloud more rapidly than it can cool then the cloud is destroyed and its mass, momentum, and energy are added to the hot phase.

The paper explores how this two-way interaction between the hot and cold phases affects the structure of a galactic wind, finding that:

  1. realistic parameter choices naturally lead to a hot, low-density wind that transports energy while entraining a significant flux of cold clouds,

  2. mixing dominates cold cloud acceleration and decelerates the hot wind,

  3. thermalization of kinetic energy during mixing provides significant heating,

  4. systems with low hot-phase mass loading factors and/or star formation rates can sustain higher initial cold phase mass loading factors, but the clouds are quickly shredded

  5. systems with large hot-phase mass loading factors and/or star formation rates cannot sustain large initial cold-phase mass loading factors, but the clouds tend to grow with radius.

Our results highlight the necessity of accounting for the multiphase structure of galactic winds, both physically and observationally, and have important implications for feedback in galactic systems.

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