Radial Gas Flows in Colliding Galaxies: Connecting Simulations and Observations

(abridged) We investigate the detailed response of gas to the formation oftransient and long-lived dynamical structures induced in the early stages of adisk-disk collision, and identify observational signatures of radial gas inflowthrough a detailed examination of the collision simulation of an equal massbulge dominated galaxy. Stars respond to the tidal interaction by forming bothtransient 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 theinitial collision. The rate of inflow declines when more than half of the totalgas supply reaches the inner few kpc, where the gas forms a dense nuclear ringinside 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 bulgelessprogenitor galaxy is also discussed, and a possible link to the ``chaingalaxy'' population observed at high redshifts is inferred. The evolution ofthe structural parameters (the asymmetry and concentration) of both stars andgas are studied in detail. Further, a new structural parameter (the compactnessparameter K) that traces the evolution of the size scale of the gas relative tothe stellar disk is introduced. Non-circular gas kinematics driven by theperturbation of the non-axisymmetric structure can produce distinct emissionfeatures in the "forbidden velocity quadrants'' of the position-velocitydiagram (PVD). The dynamical mass calculated using the rotation curve derivedfrom fitting the emission envelope of the PVD can determine the true mass towithin 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 ageof the interaction.