Radial Gas Flows in Colliding Galaxies: Connecting Simulations and Observations

(abridged) We investigate the detailed response of gas to the formation of transient and long-lived dynamical structures induced in the early stages of a disk-disk collision, and identify observational signatures of radial gas inflow through a detailed examination of the collision simulation of an equal mass bulge dominated galaxy. Stars respond to the tidal interaction by forming both transient arms and long lived m=2 bars, but the gas response is more transient, flowing directly toward the central regions within about 10^8 years after the initial collision. The rate of inflow declines when more than half of the total gas supply reaches the inner few kpc, where the gas forms a dense nuclear ring inside the stellar bar. The average gas inflow rate to the central 1.8 kpc is \~7 Msun/yr with a peak rate of 17 Msun/yr. The evolution of gas in a bulgeless progenitor galaxy is also discussed, and a possible link to the ``chain galaxy'' population observed at high redshifts is inferred. The evolution of the structural parameters (the asymmetry and concentration) of both stars and gas are studied in detail. Further, a new structural parameter (the compactness parameter K) that traces the evolution of the size scale of the gas relative to the stellar disk is introduced. Non-circular gas kinematics driven by the perturbation of the non-axisymmetric structure can produce distinct emission features in the "forbidden velocity quadrants'' of the position-velocity diagram (PVD). The dynamical mass calculated using the rotation curve derived from fitting the emission envelope of the PVD can determine the true mass to within 20% to 40%. The evolution of the molecular fraction $M_H2/M_(H2 + HI) and the compactness (K) are potential tracers to quantitatively assign the age of the interaction.