  Physics lectures always refer to the tides as a disruptive effect. However,
tides can also be compressive. When the potential of two galaxies overlap, as
happens during a merger, fully compressive tides can develop and have a strong
impact on the dynamics of substructures such as star clusters or tidal dwarf
galaxies. Using N-body simulations of a large set of mergers, we noticed the
importance of these tidal modes at cluster scale. With a model of the Antennae
galaxies, we conclude that the positions and timescales of these tidal modes
match the actual distribution of young clusters. A detailed study of the
statistics of the compressive tides shows a stunning correlation between this
purely gravitational effect and the observed properties of the star clusters.
In this contribution, we introduce the concept of compressive tide and show its
relevance in the special case of the Antennae galaxies. We extend our
conclusions to a broad range of parameters and discuss their implications on
several critical points such as the infant mortality, multiple star formation
epochs in clusters or the age distribution.
