Fading in the Flow: Suppression of cold gas growth in expanding galactic outflows

TL;DR

This study investigates how the expansion of galactic winds influences the survival, morphology, and cold gas growth in outflow clouds, revealing that expansion suppresses cold gas formation and alters observable emission features.

Contribution

The paper introduces a novel model accounting for wind expansion effects on clouds, highlighting the importance of local pressure equilibrium and differential expansion in cold gas dynamics.

Findings

Clouds remain isobaric with the wind, leading to density decline and dissolution downstream.

Expansion suppresses cold gas mass growth by diffusing the boundary layer.

The model predicts a head-to-tail emission gradient consistent with observations.

Abstract

Multiphase outflows, revealed by multi-wavelength observations, are crucial in redistributing gas and metals within and around galaxies. These outflows are often modelled theoretically using wind tunnel simulations of a cold ($\sim 10^4$ K) cloud interacting with a uniform hot ($\sim 10^6$ K) wind. However, real outflows expand downstream, a feature overlooked in most idealised simulations. We study how an expanding wind affects the survival, morphology, and dynamics of a cloud. We conduct idealised hydrodynamic simulations with optically thin radiative cooling of a cloud in an expanding wind, modelled using the adiabatic Chevalier & Clegg (1985) solution. We find that clouds remain locally isobaric with the wind, leading to a steep decline in their density contrast and eventual dissolution downstream. Compared to a plane-parallel wind, this suppresses cold gas mass growth because as clouds travel downstream, the surrounding mixed boundary layer becomes diffuse and less radiative. Our analytical scaling arguments show that cloud expansion and local pressure equilibrium are the key regulators of cold mass growth. Unlike traditional simulations, our model accounts for the differential expansion experienced by the long cometary tails of clouds in wind tunnels. This creates a strong head-to-tail emission gradient in the filamentary cold gas, which is more consistent with observations. We also demonstrate that the dynamics of individual clouds can substantially alter the radial properties of their host multiphase outflows.