An Origin for Multi-Phase Gas in Galactic Winds and Halos

TL;DR

This paper proposes that radiative cooling of hot galactic outflows can produce the observed high velocity cool gas in galactic winds and halos, explaining their multi-phase nature and distribution.

Contribution

It introduces a model where hot outflows cool radiatively, forming multi-phase gas and explaining the presence of cool gas far from galactic centers, with implications for galaxy evolution.

Findings

Cool gas forms at radii up to tens of kpc from hot outflows.

The cool gas can produce observable X-ray and UV/optical emission.

Cooled winds can deposit cool gas into galactic halos.

Abstract

The physical origin of high velocity cool gas seen in galactic winds remains unknown. Following Wang (1995), we argue that radiative cooling in initially hot thermally-driven outflows can produce fast neutral atomic and photoionized cool gas. The inevitability of adiabatic cooling from the flow's initial 10^7-10^8K temperature and the shape of the cooling function for T<10^7K imply that outflows with hot gas mass-loss rate relative to star formation rate of beta=Mdot_hot/Mdot_star > 0.5 cool radiatively on scales ranging from the size of the energy injection region to tens of kpc. We highlight the beta and star formation rate surface density dependence of the column density, emission measure, radiative efficiency, and velocity. At r_cool, the gas produces X-ray and then UV/optical line emission with a total power bounded by 10^{-2} L_star if the flow is powered by steady-state star formation with luminosity L_star. The wind is thermally unstable at r_cool, potentially leading to a multi-phase medium. Cooled winds decelerate significantly in the extended gravitational potential of galaxies. The cool gas precipitated from hot outflows may explain its prevalence in galactic halos. We forward a picture of winds whereby cool clouds are initially accelerated by the ram pressure of the hot flow, but are rapidly shredded by hydrodynamical instabilities, thereby increasing beta, seeding radiative and thermal instability, and cool gas rebirth. If the cooled wind shocks as it sweeps up the circumgalactic medium, its cooling time is short, thus depositing cool gas far out into the halo. Finally, conduction can dominate energy transport in low-beta hot winds, leading to flatter temperature profiles than otherwise expected, potentially consistent with X-ray observations of some starbursts.