Fully compressive tides in galaxy mergers
  The disruptive effect of galactic tides is a textbook example of
gravitational dynamics. However, depending on the shape of the potential, tides
can also become fully compressive. When that is the case, they might trigger or
strengthen the formation of galactic substructures (star clusters, tidal dwarf
galaxies), instead of destroying them. We perform N-body simulations of
interacting galaxies to quantify this effect. We demonstrate that tidal
compression occurs repeatedly during a galaxy merger, independently of the
specific choice of parameterization. With a model tailored to the Antennae
galaxies, we show that the distribution of compressive tides matches the
locations and timescales of observed substructures. After extending our study
to a broad range of parameters, we conclude that neither the importance of the
compressive tides (~15% of the stellar mass) nor their duration (~ 10 Myr) are
strongly affected by changes in the progenitors' configurations and orbits.
Moreover, we show that individual clumps of matter can enter compressive
regions several times in the course of a simulation. We speculate that this may
spawn multiple star formation episodes in some star clusters, through e.g.,
enhanced gas retention.
