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 mas