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Using numerical simulation, we examine the triggering of starbursts in mergers of comparable mass disk galaxies. The simulations involve a range of merging geometries and internal structures for the merging galaxies. We find that while geometry plays a small role in fueling starbursts, the presence or absence of a dense central bulge in the disk galaxies has a strong impact on the nature of merger induced starbursts. Galaxies lacking a central bulge are prone to bar formation early in a merger, driving nuclear inflows and moderately strong starbursts while the galaxies are still widely separated. In contrast, the presence of a bulge component stabilizes the disks against these early inflows, delaying the starbursts until the final merging phase. When such galaxies finally do merge, the ensuing starbursts represent an increase in the star formation rate of nearly two orders of magnitude. The strongest of these ``ultraluminous" starbursts involve coplanar encounters, while starbursts in inclined disks are somewhat less intense and occur slightly later in the merger evolution. Unlike bulgeless galaxy mergers, the morphology and star forming properties of merging galaxies with central bulges provide a good match to ultraluminous infrared galaxies. Our results suggest that the internal structure of the merging galaxies, more so than their merging geometry, may be the key to producing ultraluminous infrared galaxies.
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