The abundance of core-collapsed subhaloes in SIDM: insights from structure formation in ΛCDM

Abstract

ABSTRACT Dark matter haloes enter a phase of gravothermal core-collapse in the presence of self-interactions. This phase is expected to be subdominant given the long time-scales involved. However, it has been shown that collapse is accelerated in subhaloes in strong tidal environments. Cosmological simulations in ΛCDM give complete distributions of subhalo orbits and their profiles. We use these to estimate the fraction of the subhaloes in different host halo environments that are in the core-collapse phase. Our hosts range from the Large Magellanic cloud (LMC)-like hosts to clusters. We use fluid simulations of self-interacting dark matter (SIDM) to evolve subhaloes in their hosts, including the effect of tidal truncation at the time of pericentre crossing. We find, for parameters that allow the interaction cross-section to be high at dwarf scales, at least $10\%$ of all subhaloes are expected to have intrinsically collapsed within Hubble time up to the group mass host scales. This fraction increases significantly, becoming at least 20% when tidal interactions are considered. To identify these objects in observations, we find that we need to measure their densities at very small radial scales, where the subhaloes show a bimodal distribution of densities or we need to measure the slopes of their inner density profiles near the scale radius, which are much steeper than NFW slopes. Current measurements of central slopes of classical dwarfs do not show preference for being completely collapsed, however they are consistent with being cored or in a phase of transition between expansion and collapse.