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The unusually low velocity dispersion and large size of Crater II pose achallenge to our understanding of dwarf galaxies in the Lambda Cold Dark Matter(LCDM) cosmogony. The low velocity dispersion suggests either a dark halo massmuch lower than the minimum expected from hydrogen cooling limit arguments, orone that is in the late stages of extreme tidal stripping. The tidalinterpretation has been favoured in recent work and is supported by the smallpericentric distances consistent with available kinematic estimates. We useN-body simulations to examine this interpretation in detail, assuming aNavarro-Frenk-White (NFW) profile for Crater II's progenitor halo. Our mainfinding is that, although the low velocity dispersion can indeed result fromthe effect of tides, the large size of Crater II is inconsistent with thishypothesis. This is because galaxies stripped to match the observed velocitydispersion are also reduced to sizes much smaller than the observed half-lightradius of Crater II. Unless its size has been substantially overestimated,reconciling this system with LCDM requires that either (i) it is not bound andnear equilibrium (unlikely, given its crossing time is shorter than the timeelapsed since pericentre), or that (ii) its progenitor halo deviates from theassumed NFW profile. The latter alternative may signal that baryons can affectthe inner halo cusp even in extremely faint dwarfs or, more intriguingly, maysignal effects associated with the intimate nature of the dark matter, such asfinite self-interactions, or other such deviations from the canonical LCDMparadigm.

Galactic tides and the Crater II dwarf spheroidal: a challenge to LCDM?