  The strongest starbursts are observed towards galaxy nuclei, or circumnuclear
regions. However in interacting galaxies, star formation is also triggered in
overlap regions far from nuclei, in spiral arms and sometimes in tidal tails.
What is the relative importance of these starbursts? What kind of starformation
is dominating, as a function of redshift? These different starbursts occur in
different dynamical conditions (global and local): gravitational instabilities,
density waves, radial flows, shear, cloud collisions, density accumulations,
and they have been investigated with the help of numerical simulations.
Gravitational instabilities are necessary to initiate star formation, but they
are not sufficient; galactic disks are self-regulated through these
instabilities to have their Toomre Q parameter of the order of 1, and thus this
criterium is in practice unable to predict the onset of intense star formation.
Super star clusters are a characteristic SF mode in starbursts, and might be
due to the rapid formation of large gas complexes. Star formation can propagate
radially inwards, due to gravity torques and gas inflow, but also outwards, due
to superwinds, and energy outflows: both expanding or collapsing waves are
observed in circumnuclear regions. Mergers are more efficient in forming stars
at high redshift, because of larger gas content, and shorter dynamical times.
The relation between nuclear starbursts and nuclear activity is based on the
same fueling mechanisms, but also on reciprocal triggering and regulations.
