The Structure of Multiphase Galactic Winds

Abstract

We present a novel analytic framework to model the steady-state structure of multiphase galactic winds comprised of a hot, volume-filling component and a cold, clumpy component. We first derive general expressions for the structure of the hot phase for arbitrary mass, momentum, and energy sources terms. Next, informed by recent simulations, we parameterize the cloud-wind mass transfer rates, which are set by the competition between turbulent mixing and radiative cooling. This enables us to cast the cloud-wind interaction as a source term for the hot phase and thereby simultaneously solve for the evolution of both phases fully accounting for their bidirectional influence. With this model, we explore the nature of galactic winds over a broad range of conditions. We find that: (i) with realistic parameter choices, we naturally produce a hot, low-density wind that transports energy while entraining a significant flux of cold clouds, (ii) mixing dominates the cold cloud acceleration and decelerates the hot wind, (iii) during mixing thermalization of relative kinetic energy provides significant heating, (iv) 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, and (v) 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.

Self-ionizing galactic winds

“…The interaction also slows down the dynamical cooling of the wind due to the creation of bow shocks at the head of the clouds (Cooper et al 2008;Vijayan et al 2018) and thereby converting a fraction of the overall kinetic energy to thermal energy. Description of the wind including such interactions has recently been studied to some extent (Fielding & Bryan 2021;Nguyen & Thompson 2021). Given that the dynamical cooling time increases due to wind-cloud interaction, we speculate from sec 1 that the NEI effects on the wind would be suppressed.…”

“…Simulations of winds have shown much more complicated dynamics involving acceleration and mixing of cold clouds, creation of entropy due to bow shocks, and non-spherical wind geometries (Cooper et al 2008;Melioli et al 2013;Vijayan et al 2018;Schneider et al 2020). Theoretical models have been able to include some of these features to some extent Nguyen & Thompson 2021;Fielding & Bryan 2021).…”

“…This excess radiation can be a factor of few to a few orders of magnitude higher compared to the HM12 background and, therefore, is extremely crucial for modeling warm/cold gas in/around galactic winds. In this paper, we do not include any multiphase hydrodynamical interactions between the fast wind and cold clouds or departure from spherical symmetry as in Fielding & Bryan (2021); Nguyen & Thompson (2021).…”

“…We also note that we find a negligible amount of O ii (ion fraction 10 −5 ) in our simulations due to a higher photo-ionization rate at low temperatures. We speculate that any observable O i or O ii in the wind is probably not a part of the freely expanding wind and must originate from other sources, such as the entrainment of cold clouds from the ISM and interaction between these clouds and the wind (Prochaska et al 2011;Scarlata & Panagia 2015;Fielding & Bryan 2021)…”