Light versus dark in strong-lens galaxies: Dark matter haloes that are rounder than their stars

Understand

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.