We analyze the Aquarius simulations to characterize the shape of dark matter halos with peak circular velocity in the range 8 < V _ { max } < 200 km / s , and perform a convergence study using the various Aquarius resolution levels .
For the converged objects , we determine the principal axis ( a \geq b \geq c ) of the normalized inertia tensor as a function of radius .
We find that the triaxiality of field halos is an increasing function of halo mass , so that the smallest halos in our sample are \sim 40 - 50 \% 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 halos 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 subhalos closest to the center of their host halo .
We also analyze the dynamical structure of subhalos likely to host luminous satellites comparable to the classical dwarf spheroidals in the Local Group .
These halos have axis ratios that increase with radius , and which are mildly triaxial with \langle b / a \rangle \sim 0.75 and \langle c / a \rangle \sim 0.60 at r \sim 1 kpc .
Their velocity ellipsoid become strongly tangentially biased in the outskirts as a consequence of tidal stripping .