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Mergers between gas--rich disks and less--massive dwarf galaxies are studied using numerical simulation. As the orbit of a dwarf decays, owing to the effects of dynamical friction, the primary disk develops large--amplitude spirals and transient bars in response to its tidal forcing. While these features arise in both the stars and the gas in the disk, the non--axisymmetric structures in the gas differ slightly from those in the stars. In particular, as a consequence of the formation of strong shocks in the gas and the effects of radiative cooling, the gas response tends to lead the stellar response, enabling the stars to strongly torque the gas. These torques deprive the gas of its angular momentum, forcing a significant fraction of it into the inner regions of the disk. The radial inflows of gas induced by these mergers result in the accumulation of large quantities of interstellar gas in the nuclear regions of the host disks. In some cases, nearly half of all the gas initially distributed throughout the disk winds up in a thin disk of several hundred parsecs in extent. Given the high densities in the nuclear gas, it is plausible to identify the concentrations of dense gas in the remnants with those observed to accompany intense starbursts in some active galaxies.
Models which include a plausible description of star formation in the merging system show that the radial gas inflow is not adversely affected by feedback. As the orbit of the dwarf galaxy decays, the global star formation rates in the disk galaxy remain virtually unchanged until the merging dwarf has nearly reached the central regions. At this point the star formation rates rise by an order of magnitude, with the great majority of the star formation being associated with the nuclear gas concentration. The lifetime of this central starburst is $\sim 2.5\times 10^8$ years, and the starburst consumes half the total disk gas. This starburst population is very compact ($r < 0.5$ kpc), suggesting that these merger-induced starbursts are not likely to contribute significantly to the building of extended bulges in spiral and S0 galaxies.
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