Go with the Flow: Understanding Inflow Mechanisms in Galaxy Collisions
  Dynamical interactions between colliding spiral galaxies strongly affect the
state and distribution of their interstellar gas. Observations indicate that
interactions funnel gas toward the nuclei, fueling bursts of star formation and
nuclear activity. To date, most numerical simulations of galaxy mergers have
assumed that the gaseous and stellar disks initially have the same distribution
and size. However, observations of isolated disk galaxies show that this is
seldom the case; in fact, most spirals have as much or more gas beyond their
optical radii as they do within. Can gas in such extended disks be efficiently
transported to the nuclei during interactions?
  To address this question, we examine the effect of various parameters on the
transport of gas to the nuclei of interacting galaxies. In addition to the
relative radii of the gaseous and stellar disks, these parameters include the
pericentric separation, disk orientation, fractional gas mass, presence of a
bulge, treatment of gas thermodynamics, and the spatial resolution of the
numerical simulation. We found that gas accumulates in most of our simulated
nuclei, but the efficiency of inflow is largely dependent upon the encounter
geometry. Dissipation alone is not enough to produce inflows; an efficient
mechanism for extracting angular momentum from the gas is necessary. Several
different mechanisms are seen in these experiments. Aside from mode-driven
inflows (such as, but not limited to, bars) and ram-pressure sweeping, both of
which have been previously described and well studied, we supply the first
quantitative study of an often-seen process: the formation of massive gas
clumps in Jeans-unstable tidal shocks, and their subsequent delivery to the
nuclei via dynamical friction.
