The inner structure of ΛCDM haloes -: II.: Halo mass profiles and low surface brightness galaxy rotation curves

We use a set of high-resolution cosmological N-body simulations to investigate the inner mass profile of galaxy-sized cold dark matter (CDM) haloes. These simulations extend the numerical convergence study presented in Paper I of this series, and demonstrate that the mass profile of CDM galaxy haloes can be robustly estimated beyond a minimum converged radius of order r(conv) similar to 1 h(-1) kpc in our highest-resolution runs. The density profiles of simulated haloes become progressively shallower from the virial radius inwards, and show no sign of approaching a well-defined power law near the centre. At r(conv), the density profile is steeper than expected from the formula proposed by Navarro, Frenk & White, which has a rho proportional to r(-1) cusp, but significantly shallower than the steeply divergent rho proportional to r(-1.5) cusp proposed by Moore et al. We perform a direct comparison of the spherically averaged dark matter circular velocity profiles with Ha rotation curves of a sample of low surface brightness (LSB) galaxies. We find that most galaxies in the sample (about 70 per cent) have rotation curves that are consistent with the structure of CDM haloes. Of the remainder, 20 per cent have rotation curves which cannot be fit by any smooth fitting function with few free parameters, and 10 per cent are inconsistent with CDM haloes. However, the latter consist mostly of rotation curves that do not extend to large enough radii to accurately determine their shapes and maximum velocities. We conclude that the inner structure of CDM haloes is not manifestly inconsistent with the rotation curves of LSB galaxies.