Kinematics and Dynamics of Multiphase Outflows in Simulations of the Star-Forming Galactic ISM

TL;DR

This study develops a method to analyze multiphase galactic outflows in simulations, revealing how hot, warm, and intermediate phases exchange mass, momentum, and energy, and challenging the ballistic model at high velocities.

Contribution

It introduces a new quantitative approach to study phase interactions in galactic outflows using simulation data, highlighting the importance of hot-warm flux exchanges and cloud size effects.

Findings

Ballistic model is valid at intermediate velocities.

Warm phase gains mass mainly from cooling of intermediate phase.

Hot-to-warm momentum transfer is significant.

Abstract

Galactic outflows produced by stellar feedback are known to be multiphase in nature. Both observations and simulations indicate that the material within several kpc of galactic disk mid-planes consists of warm clouds embedded within a hot wind. A theoretical understanding of the outflow phenomenon, including both winds and fountain flows, requires study of the interactions among thermal phases. We develop a method to quantify these interactions via measurements of mass, momentum, and energy flux exchanges using temporally and spatially averaged quantities and conservation laws. We apply this method to a star-forming ISM MHD simulation based on the TIGRESS framework, for Solar neighbourhood conditions. To evaluate the extent of interactions among the phases, we first examine the validity of the ``ballistic model,'' which predicts trajectories of the warm phase ($5050\,\rm{K}<T<2\times10^4\,\rm{K}$) treated as non-interacting clouds. This model is successful at intermediate vertical velocities ($ 50$ km s$^{-1}$ $\lesssim |v_z| \lesssim 100 $ km s$^{-1}$), but at higher velocities we observe an excess in simulated warm outflow compared to the ballistic model. This discrepancy cannot be fully accounted for by cooling of high-velocity intermediate-temperature ($2\times10^4\,\rm{K}<T<5\times10^5\,\rm{K}$) gas. By examining the fluxes of mass, momentum and energy, we conclude that warm phase gains mass via cooling of the intermediate phase, while momentum transfer occurs from the hot ($T>5\times10^5\,\rm{K}$) to the warm phase. The large energy flux from the hot outflow that is transferred to the warm and intermediate phases is quickly radiated away. A simple interaction model implies an effective warm cloud size in the fountain flow of a few 100~pc, showing that warm-hot flux exchange mainly involves a few large clouds rather than many small ones.