Gasdynamics and starbursts in major mergers

Using numerical simulation, we study the development of gaseous inflows and triggering of starburst activity in mergers of disk galaxies of comparable mass. Our models cover a range of orbits and internal structures for the merging galaxies. In all encounters studied, the galaxies experience strong gaseous inflows and, using a density-dependent Schmidt law to model star formation, moderate to intense starburst activity. We find that galaxy structure plays a dominant role in regulating activity. The gaseous inflows are strongest when galaxies with dense central bulges are in the final stages of merging, while inflows in bulgeless galaxies are weaker and occur earlier in the interaction. Orbital geometry plays only a relatively modest role in the onset of collisionally induced activity. Through an analysis of the torques acting on the gas, we show that these inflows are generally driven by gravitational torques from the host galaxy (rather than the companion) and that dense bulges act to stabilize galaxies against bar modes and inflow until the galaxies merge, at which point rapidly varying gravitational torques drive strong dissipation and inflow of gas in the merging pair. The strongest inflows (and associated starburst activity) develop in coplanar encounters, while the activity in inclined mergers is somewhat less intense and occurs slightly later during the merger. To the extent that a Schmidt law is a reasonable description of star formation in these systems, the starbursts that develop in mergers of galaxies with central bulges represent an increase in the star formation rate of two orders of magnitude over that in isolated galaxies. We find that the gaseous and stellar morphology and star-forming properties of these systems provide a good match to those of observed ultraluminous infrared galaxies. Our results imply that the internal structure of the merging galaxies, rather than orbital geometry, may be the key factor in producing ultraluminous infrared galaxies.