Effective Self-interaction of Dark Matter from Gravitational Scattering

Abstract

I show that gravitational scattering of dark matter objects of mass ∼10 4 M ⊙ and speeds of ∼10 km s −1 , provides the cross section per unit mass required in self-interacting dark matter models that alleviate the small-scale structure challenges to the collisionless cold dark matter model. For primordial objects of mass 10 4 M 4 M ⊙ , moving at the velocity dispersion characteristic of dwarf galaxies, 10 v 1 km s −1 , the cross section per unit mass for gravitational scattering is ∼ 10 ( M 4 / v 1 4 ) cm 2 g − 1 . The steep decline in interaction with increasing velocity explains why self-interaction is not evident in data on massive galaxies and clusters of galaxies.