Radial Gas Flows in Colliding Galaxies: Connecting Simulations and Observations

Abstract

We investigate the detailed response of gas to the formation oftransient and long-lived dynamical structures induced in the earlystages of a disk-disk collision and identify observational signatures ofradial gas inflow through a detailed examination of the collisionsimulation of an equal-mass bulge-dominated galaxy. Our analysis anddiscussion mainly focuses on the evolution of the diffuse and dense gasin the early stages of the collision, when the two disks are interactingbut have not yet merged. Stars respond to the tidal interaction byforming both transient arms and long-lived m=2 bars, but the gasresponse is more transient, flowing directly toward the central regionswithin about 10^{8} yr after the initial collision. The rate ofinflow declines when more than half of the total gas supply reaches theinner few kiloparsecs, where the gas forms a dense nuclear ring insidethe stellar bar. The average gas inflow rate to the central 1.8 kpc is~7 M_{solar} yr^{-1} with a peak rate of 17M_{solar} yr^{-1}. Gas with high volume density is foundin the inner parts of the postcollision disks at size scales close tothe spatial resolution of the simulations, and this may be a directresult 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 highredshifts is inferred. The evolution of the structural parameters suchas asymmetry and concentration of both stars and gas are studied indetail. Further, a new structure parameter (the compactness parameter K)that traces the evolution of the size scale of the gas relative to thestellar disk is introduced, and this may be a useful tracer to determinethe merger chronology of colliding systems. Noncircular gas kinematicsdriven by the perturbation of the nonaxisymmetric structure can producedistinct emission features in the ``forbidden velocity quadrants'' ofthe position-velocity diagram (PVD). The dynamical mass calculated usingthe rotation curve derived from fitting the emission envelope of the PVDcan determine the true mass to within 20%-40%. The evolution of themolecular fraction(M_{H 2} /M_{H 2}+HI) is a potentialtracer to quantitatively assign the age of the interaction, but theapplication to real systems may require additional observationaldiagnostics to properly assess the exact chronology of the mergerevolution.