Fully compressive tides in galaxy mergers

TL;DR

This study demonstrates that fully compressive galactic tides occur repeatedly during galaxy mergers, potentially triggering star formation and substructure formation, with their effects being relatively independent of initial conditions.

Contribution

The paper provides the first quantification of fully compressive tides in galaxy mergers through N-body simulations, linking them to observed substructures and star formation episodes.

Findings

Compressive tides occur repeatedly during galaxy mergers.

Approximately 15% of stellar mass experiences compressive tides.

Compressive regions can be revisited multiple times, influencing star formation.

Abstract

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.