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

Surprisingly compact substructures in galaxies and galaxy clusters, as well as field halos, observed by gravitational lensing could be challenging to explain solely on the basis of collisionless dark matter (DM). To investigate such objects, recent studies have focussed on the gravothermal collapse that occurs in halos consisting of self-interacting dark matter (SIDM). However, simple models of elastic scattering could face issues when attempting to explain the nature of these compact objects during very late 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 that conventional SIDM models are subject to. We investigate the evolution of a DM halo for a model consisting of two species with unequal masses, which features only non-gravitational interactions among the different species, but not within their own structures. By employing N-body simulations, we studied the effect of unequal-mass scattering on the evolution of an isolated DM halo. In particular, we simulated the late stage of its evolution with a high central density. We find that our two-species models can produce density cores with sizes that are dependent on the mass ratio between the two species. Moreover, mass segregation caused by the unequal particle masses leads to an enhanced finite final central density, or at least a slowly growing state, which depends on the mass ratio and the mass fraction of the two DM species. We find that SIDM models consisting of two DM species have the capacity to simultaneously explain DM halos with density cores and systems that are denser in their centre than would otherwise be expected from collisionless DM, while avoiding the gravothermal catastrophe scenario. This approach offers a compelling alternative to single-species models, along with a rich phenomenology.