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Galaxy interactions can be the most efficient way to produce strong torques, and transfer the angular momentum away. During the interaction period, strong bars are triggered in the galaxy disks, and through the same mechanisms as described before, gas is driven inwards. During the merger, a complete change of geometry also brings most of the gas to the center. The main consequence is the trigger of spectacular starbursts, in particular in major mergers, i.e. merging about equal masses disk galaxies. The star forming activity is concentrated in nuclei, with some exceptions (as the antennae or Arp 299 for instance). The same gas inflow can also fuel an active nucleus, either directly, or indirectly through the coeval dense nuclear clusters formed in the starburst (cf section 2.2).

In these huge starbursts discovered by IRAS, CO concentrations suggest that the the cause of the starburst is the concentration of huge gas masses in the center; the molecular gas represents a significant fraction of the dynamical mass (Scoville et al 1991). This gas must be brought to the center in a time- scale short enough with respect to the feedback time-scale of star-formation, i.e. a few 107yrs (cf Larson 1987). This requires very strong gravity torques.