Understanding the ‘feeble giant’ Crater II with tidally stretched wave dark matter

Abstract

ABSTRACT The unusually large ‘dwarf’ galaxy Crater II, with its small velocity dispersion, ≃3 km s−1, defies expectations that low-mass galaxies should be small and dense. We combine the latest stellar and velocity dispersion profiles finding Crater II has a prominent dark core of radius $\simeq 0.71^{+0.09}_{-0.08}$ kpc, surrounded by a low density halo, with a transition visible between the core and the halo. We show that this profile matches the distinctive core-halo profile predicted by ‘Wave Dark Matter’ as a Bose-Einstein condensate, ψDM, where the ground state soliton core is surrounded by a tenuous halo of interfering waves, with a marked density transition predicted between the core and halo. Similar core-halo structure is seen in most dwarf spheroidal galaxies (dSphs), but with smaller cores, ≃0.25 kpc and higher velocity dispersions, ≃9 km s−1, and we argue here that Crater II may have been a typical dSph that has lost most of its halo mass to tidal stripping, so its velocity dispersion is lower by a factor of 3 and the soliton is wider by a factor of 3, following the inverse scaling required by the Uncertainty Principle. This tidal solution for Crater II in the context of ψDM is supported by its small pericenter of ≃20 kpc established by Gaia, implying significant tidal stripping of Crater II by the Milky Way is expected.