Radial Gas Flows in Colliding Galaxies: Connecting Simulations and Observations

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. Our analysis and discussion mainly focuses on the evolution of the diffuse and dense gas in the early stages of the collision, when the two disks are interacting but have not yet merged. 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 108 yr after the initial collision. The rate of inflow declines when more than half of the total gas supply reaches the inner few kiloparsecs, 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 M☉ yr-1 with a peak rate of 17 M☉ yr-1. Gas with high volume density is found in the inner parts of the postcollision disks at size scales close to the spatial resolution of the simulations, and this may be a direct result of shocks traced by the discontinuity in the gas velocity field. 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 such as asymmetry and concentration of both stars and gas are studied in detail. Further, a new structure parameter (the compactness parameter K) that traces the evolution of the size scale of the gas relative to the stellar disk is introduced, and this may be a useful tracer to determine the merger chronology of colliding systems. Noncircular gas kinematics driven by the perturbation of the nonaxisymmetric 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%-40%. The evolution of the molecular fraction (M/M) is a potential tracer to quantitatively assign the age of the interaction, but the application to real systems may require additional observational diagnostics to properly assess the exact chronology of the merger evolution.