The origin and kinematics of cold gas in galactic winds: insight from numerical simulations

We study the origin of Na I-absorbing gas in ultraluminous infrared galaxies motivated by the recent observations by Martin of extremely superthermal linewidths in this cool gas. We model the effects of repeated supernova explosions driving supershells in the central regions of molecular disks with Md = 1010 M sun, using cylindrically symmetric gas dynamical simulations run with ZEUS-3D. The shocked swept-up shells quickly cool and fragment by Rayleigh-Taylor (R-T) instability as they accelerate out of the dense, stratified disks. The numerical resolution of the cooling and compression at the shock fronts determines the peak shell density, and so the speed of R-T fragmentation. We identify cooled shells and shell fragments as Na I-absorbing gas and study its kinematics along various sightlines across the grid. We find that simulations with a numerical resolution of <=0.2 pc produce multiple R-T fragmented shells in a given line of sight that appear to explain the observed kinematics. We suggest that the observed wide Na I absorption lines, langvrang = 320 ± 120 km s-1, are produced by these multiple fragmented shells traveling at different velocities. We also suggest that some shell fragments can be accelerated above the observed average terminal velocity of 750 km s-1 by the same energy-driven wind with an instantaneous starburst of ~109 M sun. The mass carried by these fragments is only a small fraction of the total shell mass, while the bulk of mass is traveling with velocities consistent with the observed average shell velocity 330 ± 100 km s-1. Our results show that an energy-driven bubble causing R-T instabilities can explain the kinematics of cool gas seen in the Na I observations without invoking additional physics relying primarily on momentum conservation, such as entrainment of gas by Kelvin-Helmholtz instabilities, ram pressure driving of cold clouds by a hot wind, or radiation pressure acting on dust