Program: GN-2012B-Q-42

Numerical simulations predict dark matter halos to have steep inner density profiles. While for massive elliptical galaxies this prediction is consistent with observations, measurements in galaxy clusters revealed significantly shallower inner dark matter slopes than predicted, setting a problem for the standard cold dark matter paradigm. The shape of the dark matter distribution depends both on the properties of dark matter itself and on the baryonic physics that takes place during the formation and evolution of luminous structures. New and better observations are needed, particularly at mass scales intermediate between galaxies and clusters, to distinguish between the wealth of possible models. The gravitational lens system SDSSJ1148+1930 (the “cosmic horseshoe”) offers a unique opportunity to measure the inner slope of a group-scale dark matter halo. We recently discovered a radial arc in the system. The radial arc provides strong constraints on the shape of the mass distribution of the lens galaxy, once its redshift is known. Therefore, we request time to measure the redshift of the object associated with the radial arc. This would provide the first robust measurement of the inner slope of the dark matter halo of a group-scale system, which can then be used as a benchmark for simulations.