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