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Major mergers between gas-rich disk galaxies are studied using numerical simulation. We find that while gaseous dissipation is a generic result in these events, the detailed evolution of the gaseous component is dependent on the internal structure of the galaxies. In galaxies which lack a dense central bulge, the disks develop strong bars shortly after their initial encounter in response to their tidal forcing. The gas in the bar tends to lead the stellar component, allowing the stars to strongly torque the disk gas. These torques deprive the gas of angular momentum and drive a significant fraction of it into dense cores in the centers of each galaxy before the galaxies actually merge. During the final merging, these dense cores coalesce at the center of the remnant.
In contrast, compact central bulges in the progenitor galaxies act to stabilize the disks against strong bar modes early in the interaction. Instead, transient spiral arms and rings form in the disks, which are less efficient at torquing the disk gas and driving nuclear inflows. As a result, the dense gaseous cores are prevented from forming until the galaxies ultimately merge. At this point, the heightened gravitational torques drive the gas in both disks into a single concentrated mass at the remnant center. Although the total gas mass in this compact core is similar to that found in the remnant of a bulgeless galaxy merger, the differences in the dissipative evolution of the gaseous components in the differing scenarios will have important ramifications for the triggering of starbursts and nuclear activity in galaxy mergers.
