Self-interacting dark matter interpretation of Crater II

The satellite galaxy Crater II of the Milky Way is extremely cold andexceptionally diffuse. These unusual properties are challenging to understandin the standard model of cold dark matter. We investigate the formation ofCrater II in self-interacting dark matter (SIDM), where dark matter particlescan scatter and thermalize. We conduct a series of controlled N-bodysimulations to model the tidal evolution of Crater II, varying theself-interacting cross section, orbit parameters, and initial stellardistribution. Dark matter self-interactions lead to halo core formation and thedistribution of stars expands accordingly. A cored SIDM halo also boosts tidalmass loss, allowing for a high orbit. Our simulations show that SIDM halos witha $1~{\rm kpc}$ core can simultaneously explain the low stellar velocitydispersion and the large half-light radius of Crater II, remaining robust tothe initial distribution of stars. For the orbit motivated by the measurementsfrom Gaia Early Data Release 3, the favored self-interacting cross section isapproximately $60~{\rm cm^2/g}$ on the mass scale of Crater II.