Giant cold satellites from low-concentration haloes

ABSTRACT The dwarf satellite galaxies of the Milky Way Crater II and Antlia II have uncommonly low dynamical mass densities, due to their large size and low velocity dispersion. Previous work have found it difficult to identify formation scenarios within the Λ cold dark matter (ΛCDM) framework and have invoked cored dark matter haloes, processed by tides. I show that the tidal evolution of ΛCDM NFW haloes is richer than previously recognized. In haloes that fall short of the mass–concentration relation, tidal heating causes the innermost regions to expand significantly, resulting in the formation of giant, kinematically cold satellites like Crater II and Antlia II. While these satellites are reaching apocentre, extra-tidal material can cause an even more inflated appearance. If originally underconcentrated, Crater II and Antlia II may well have experienced very little mass loss, as in fact hinted by their observed metallicity. On a cosmological scale, satellites with low dynamical mass densities are not a rare occurrence, but were more frequent in the past. If indeed a satellite of NGC 1052, the same mechanism may similarly have led to the formation of NGC 1052-DF2.