It is well established now that dynamical perturbations are often triggering star formation in galaxy disks. The strongest starbursts are always occuring in interacting or merging galaxies. However, the detailed processes leading to star formation at large-scale are not yet well understood.

What is the relative importance of local and global triggering? Is the main star formation occuring in the nuclei, from gas driven inwards by gravity torques? or in spiral arms, all over the disk, or in overlapping regions of interacting galaxies?

Many mechanisms have been proposed to trigger star formation, such as 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.

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. I will review observational evidence and theoretical expectations, and discuss why mergers are more efficient in forming stars at high redshift.
 
 

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