On the theory of galactic winds

Galactic winds are ubiquitous in most rapidly star-forming galaxies in both the local and high-redshift universe. They shape the galaxy luminosity function, flattening its faint-end slope compared to the halo mass function, and affect the chemical evolution of galaxies, determining the mass-metallicity relation, regulating star formation over cosmic time, and polluting the intergalactic medium (IGM) with metals. Although important, the physics of galactic winds is still unclear. Many theoretical mechanisms have been proposed. Winds may be driven by the heating of the interstellar medium (ISM) by overlapping supernovae (SNe), cosmic rays, the radiation pressure by continuum absorption and scattering of starlight on dust grains, or the momentum input from SNe. However, the comparison between theory and observation is incomplete. The growing observations of emission and absorption of cold molecular, cool atomic, and ionized gas in galactic outflows in a large number of galaxies have not been well explained by any models over a vast range of galaxy parameters. A full understanding of these issues requires both better theoretical explorations and comparisons with new and existing observations. Here, I develop theoretical models of both radiation pressure- and supernova-driven galactic winds, and compared these models with observations.;First of all, galactic winds driven from uniformly bright self-gravitating disks radiating near the Eddington limit, which is relevant to rapidly star-forming galaxies and gravitating AGN disks. I show that uniformly bright self-gravitating disks radiating at the Eddington limit are fundamentally unstable to driving large-scale winds. I apply this theory to galactic winds from ultra-luminous infrared galaxies (ULIRGs) that approach the Eddington limit for dust.;Secondly, galactic superwinds may be driven by very hot outflows generated by overlapping supernovae within the host galaxy. We use the Chevalier & Clegg (CC85) wind model and the observed correlation between X-ray luminosities of galaxies and their SFRs to constrain the mass loss rates across a wide range of star formation rates (SFRs), from dwarf starbursts to ultra-luminous infrared galaxies. In addition, we highlight the fact that heavily mass-loaded winds cannot be described by the adiabatic CC85 model because they become strongly radiative.;Furthermore, efficient thermalization of overlapping supernovae within star-forming galaxies may produce a supernova-heated fluid that drives galactic winds. For fiducial assumptions about the timescale for Kelvin-Helmholz (KH) instabilities from high-resolution simulations (which neglect magnetic fields) we show that cool clouds with temperature from Tc ~ 102-104 K seen in emission and absorption in galactic winds cannot be accelerated to observed velocities by the ram pressure of a hot wind. Taking into account both the radial structure of the hot flow and gravity, we show that this conclusion holds over a wide range of galaxy, cloud, and hot wind properties. This finding calls into question the prevailing picture whereby the cool atomic gas seen in galactic winds is entrained and accelerated by the hot flow. Given these difficulties with ram pressure acceleration, we discuss alternative models for the origin of high velocity cool gas outflows. Another possibility is that magnetic fields in cool clouds are sufficiently important that they prolong the cloud's life. For Tc = 103 K and 104 K clouds, we show that if conductive evaporation can be neglected, the KH timescale must be ~ 10 and 3 times longer, respectively, than the values from hydrodynamical simulations in order for cool cloud velocities to reach those seen in observations. (Abstract shortened by UMI.).