The Launching of Cold Clouds by Galaxy Outflows. Ii. The Role of Thermal Conduction

Abstract

We explore the impact of electron thermal conduction on the evolution of radiatively cooled cold clouds embedded in flows of hot and fast material as it occurs in outflowing galaxies. Performing a parameter study of threedimensional adaptive mesh refinement hydrodynamical simulations, we show that electron thermal conduction causes cold clouds to evaporate, but it can also extend their lifetimes by compressing them into dense filaments. We distinguish between low column-density clouds, which are disrupted on very short times, and high-column density clouds with much longer disruption times that are set by a balance between impinging thermal energy and evaporation. We provide fits to the cloud lifetimes and velocities that can be used in galaxy-scale simulations of outflows in which the evolution of individual clouds cannot be modeled with the required resolution. Moreover, we show that the clouds are only accelerated to a small fraction of the ambient velocity because compression by evaporation causes the clouds to present a small cross-section to the ambient flow. This means that either magnetic fields must suppress thermal conduction, or that the cold clouds observed in galaxy outflows are not formed of cold material carried out from the galaxy.

Hydrodynamical Coupling of Mass and Momentum in Multiphase Galactic Winds

“…As with previous studies of galactic winds, we model this second component as dense clouds initially at rest with respect to the hot wind (e.g., Scannapieco & Brüggen 2015;Brüggen & Scannapieco 2016). Our aim is to extend previous studies by examining the detailed momentum and energy coupling between different wind phases, so we consider both idealized spherical clouds and more realistic turbulent clouds with a distribution of interior densities set by turbulent processes.…”

“…Rather than efficiently mixing with the hot post-shock wind, radiatively cooling clouds tend to get strung out into filaments containing individual "cloudlets" of dense gas that can survive much longer. Other authors have investigated the effects of conduction (e.g., Marcolini et al 2005;Orlando et al 2005;Brüggen & Scannapieco 2016) and magnetic fields (e.g., Mac Low et al 1994;Fragile et al 2005;Shin et al 2008;McCourt et al 2015; Banda-Barragán et al 2016) on the cloud-shock interaction, with varying results for the stabilization of the cloud.…”

The Production of Cold Gas Within Galaxy Outflows

“…Veilleux et al 2005). However, this model runs into serious difficulties in explaining the observations, both because: (i) shocks and conduction from the exterior medium tend to compress the cloud perpendicular to the direction of the flow, greatly reducing the momentum flux it receives; and (ii) instabilities and evaporation lead to rapid cloud disruption (Klein et al 1994;Mac Low & Zahnle 1994;Orlando et al 2006Orlando et al , 2008Scannapieco & Brüggen 2015;Brüggen & Scannapieco 2016). Together these imply that the lifetimes of the clumps are likely to be much shorter that the time required to accelerate them to the observed speeds.…”

Magnetized High Velocity Clouds in the Galactic Halo: A New Distance Constraint

“…Thus, for the sake of clarity, we have run our simulations adiabatically, deferring the more challenging task to include cooling for future work. In addition to cooling, other physical effects that may be relevant when modelling cloud-wind interactions include: thermal conduction (Armillotta et al 2016;Brüggen & Scannapieco 2016); turbulence (e.g Pittard & Parkin 2016); and fractal density structure (e.g. Bland-Hawthorn et al 2007;Schneider & Robertson 2017).…”