Angular Momentum Transport in Extended Galactic Disks
  We demonstrate a significant difference in the angular momentum transport
properties of galactic disks between regions in which the interstellar medium
is single phase or two phase. Our study is motivated by observations of HI in
extended galactic disks which indicate velocity dispersions of nonthermal
origin, suggesting that turbulence in the gas may be contributing significantly
to the observed dispersion. To address this, we have implemented a shearing-box
framework within the FLASH code. The new code was used to perform local
simulations of galactic disks that incorporate differential rotation,
self-gravity, vertical stratification, hydrodynamics and cooling. These
simulations explore plausible mechanisms for driving turbulent motions via the
thermal and self-gravitational instabilities coupling to differential rotation.
Where a two-phase medium develops, gravitational angular momentum transporting
stresses are much greater, creating a possible mechanism for transferring
energy from galactic rotation to turbulence. In simulations where the disk
conditions do not trigger the formation of a two-phase medium, it is found that
perturbations to the flow damp without leading to a sustained mechanism for
driving turbulence. The differing angular momentum transport properties of the
single- and two-phase regimes of the disk suggest that a significant,
dynamically motivated division can be drawn between the two, even when this
division occurs far outside the star formation cutoff in a galactic disk.
