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

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Abstract

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 ~10 kpc are very well fit by a projected Navarro-Frenk-White (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 edm = 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 (γ ≲ 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.

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Bruderer, C., Read, J. I., Coles, J. P., Leier, D., Falco, E. E., Ferreras, I., & Saha, P. (2016). Light versus dark in strong-lens galaxies: Dark matter haloes that are rounder than their stars. Monthly Notices of the Royal Astronomical Society, 456(1), 870–884. https://doi.org/10.1093/mnras/stv2582

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