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 failed to identify formation scenarios within the $Λ$CDM framework and have invoked cored dark matter haloes, processed by tides. I show that the tidal evolution of $Λ$CDM NFW haloes is richer than previously recognised: tidal heating causes the innermost regions of haloes that fall short of the mass-concentration relation to expand significantly, resulting in the formation of giant, kinematically cold satellites like Crater II and Antlia II. Furthermore, while the satellite is reaching apocenter, extra-tidal material can cause an even more inflated appearance. When present, as likely for the larger Antlia II, nominally unbound material can be recognised thanks to its somewhat hotter kinematics and line-of-sight velocity gradient. Contrary to other formation scenarios, Crater II and Antlia II may well have experienced very little mass loss, as in fact hinted by their observed metallicity. If indeed a satellite of NGC1052, tidal evolution of a low-concentration halo may similarly have led to the formation of NGC1052-DF2.