This paper studies how collisions between dark matter particles and the merger history of small galaxy halos work together to shape their centers. The authors focus on Self-Interacting Dark Matter, or SIDM, a class of models where dark matter particles occasionally scatter off one another. Those scatterings move heat through a halo and can lead to a process called gravothermal collapse, a runaway increase of density in the halo center when heat flows the wrong way.

The team ran six high-resolution “zoom-in” simulations of dark-matter-only halos with masses around 10 billion times the mass of the Sun. Zoom-in means they follow each halo in detail inside a larger cosmological environment. They used the GIZMO code with a Monte Carlo treatment of particle scattering. For most runs they used a large constant cross section of σ/m = 70 cm2 per gram, which is a measure of how likely particles are to scatter at the low relative speeds typical of dwarf halos. They also ran one case with σ/m = 30 cm2 g−1 to test sensitivity.

Their main finding is that merger history matters. Three of the six halos, the ones with the most peaceful merger histories, show clear signs of gravothermal core collapse in these simulations. The other three halos, which experienced more sustained mergers, do not collapse. The simulations show that mergers inject orbital kinetic energy into the halo. That extra energy alters the internal heat transport and can delay or prevent the runaway central collapse.

The study also finds a novel effect: in two halos that do not collapse, merger-driven heat transport pushes their central densities below what a standard gravothermal fluid model predicts. In other words, mergers can both prevent collapse and make centers even less dense than the simplest SIDM models would expect. This suggests merger history could help produce dark-matter-deficient galaxies and increase the diversity of galaxy rotation curves beyond what halo concentration alone would predict.