To collapse or not to collapse: Halo evolution with self-interacting dark matter mass segregation (2506.06272v2)

Abstract: Surprisingly compact substructures in galaxies and galaxy clusters, but also field halos, have been observed by gravitational lensing. They could be difficult to explain with collisionless dark matter (DM). To explain those objects, recent studies focused on the gravothermal collapse that halos consisting of self-interacting dark matter (SIDM) can undergo. However, simple models of elastic scattering could face problems explaining those compact objects during very later stages of the collapse and the post-collapse phase, where a black hole may have formed from DM. We aim to explain compact halos while avoiding the gravothermal catastrophe to which typical SIDM models are subject. Therefore, we investigate the evolution of a DM halo for an SIDM model consisting of two species with unequal masses, which features only interactions between the different species but not within themselves. Employing $N$-body simulations, we study the effect of unequal-mass SIDM models on the evolution of an isolated DM halo. In particular, the late stages of its evolution with high central densities are simulated. We find that our two-species SIDM models can produce density cores with their size depending on the mass ratio of the two species. Moreover, mass segregation caused by the unequal particle masses leads to a finite final density state or at least a slowly growing density, which depends on the mass ratio and the mass fraction of the two DM species. SIDM models consisting of two DM species can simultaneously explain DM halos with density cores, as well as systems that are denser in their centre than expected from collisionless DM, while avoiding the gravothermal catastrophe. They are a compelling alternative to single-species models, offering a rich phenomenology.