The shape of dark matter subhaloes in the Aquarius simulations

We analyse the Aquarius simulations to characterize the shape of dark matter haloes with peak circular velocity in the range 8 < V_max < 200 km s^-1, and perform a convergence study using the various Aquarius resolution levels. For the converged objects, we determine the principal axis (a ≥ b ≥ c) of the normalized inertia tensor as a function of radius. We find that the triaxiality of field haloes is an increasing function of halo mass, so that the smallest haloes in our sample are ∼40-50 per cent rounder than Milky Way-like objects at the radius where the circular velocity peaks, r_max. We find that the distribution of subhalo axis ratios is consistent with that of field haloes of comparable V_max. Inner and outer contours within each object are well aligned, with the major axis preferentially pointing in the radial direction for subhaloes closest to the centre of their host halo. We also analyse the dynamical structure of subhaloes likely to host luminous satellites comparable to the classical dwarf spheroidals in the Local Group. These haloes have axis ratios that increase with radius, and which are mildly triaxial with <b/a> ∼ 0.75 and <c/a> ∼ 0.60 at r ∼ 1 kpc. Their velocity ellipsoid become strongly tangentially biased in the outskirts as a consequence of tidal stripping.