Light versus dark in strong-lens galaxies: Dark matter haloes that are
  rounder than their stars
  We measure the projected density profile, shape and alignment of the stellar
and dark matter mass distribution in 11 strong-lens galaxies. We find that the
projected dark matter density profile - under the assumption of a Chabrier
stellar initial mass function - shows significant variation from galaxy to
galaxy. Those with an outermost image beyond $\sim 10$ kpc are very well fit by
a projected NFW profile; those with images within 10 kpc appear to be more
concentrated than NFW, as expected if their dark haloes contract due to
baryonic cooling. We find that over several half-light radii, the dark matter
haloes of these lenses are rounder than their stellar mass distributions. While
the haloes are never more elliptical than $e_{dm} = 0.2$, their stars can
extend to $e_* > 0.2$. Galaxies with high dark matter ellipticity and weak
external shear show strong alignment between light and dark; those with strong
shear ($\gamma \gtrsim 0.1$) can be highly misaligned. This is reassuring since
isolated misaligned galaxies are expected to be unstable. Our results provide a
new constraint on galaxy formation models. For a given cosmology, these must
explain the origin of both very round dark matter haloes and misaligned
strong-lens systems.
