High Velocity-dispersion Cold Gas in ULIRG Outflows. I: Direct
  Simulations
  Observations have revealed cold gas with large velocity dispersions (~300
km/s) within the hot outflows of ultra-luminous infrared galaxies (ULIRGs).
This gas may trace its origin to the Rayleigh-Taylor (RT) fragmentation of a
super-bubble or may arise on smaller scales. We model a ULIRG outflow at two
scales to recreate this gas in three-dimensional hydrodynamic simulations using
FLASH. Although resolution is limited, these models successfully produce cold
gas in outflows with large velocity dispersions. Our small-scale models produce
this cold gas through RT fragmentation of the super-bubble wall, but the
large-scale models produce the cold gas after hot bubbles fragment the disc's
gas into cold clouds which are then accelerated by thermal pressure, and
supplemented by cooling within the outflow. We produce simple mock spectra to
compare these simulations to observed absorption spectra and find line-widths
of ~250 km/s, agreeing with the lower end of observations.
