The Dark Matter Density Profile of the Lensing Cluster MS 2137−23: A Test of the Cold Dark Matter Paradigm

We present new spectroscopic observations of the gravitational arcs and the brightest cluster galaxy (BCG) in the cluster MS2137-23 (z=0. 313) obtained with the Echelle Spectrograph and Imager on the Keck II telescope. We find that the tangential and radial arcs arise from sources at almost identical redshifts (z=1. 501, 1. 502). We combine the measured stellar velocity dispersion profile of the BCG with a lensing analysis to constrain the distribution of dark and stellar matter in the central 100 kpc of the cluster. Our data indicate a remarkably flat inner slope for the dark matter profile, rhod propto r^-beta, with beta<0. 9 at 99% CL. Steep inner slopes obtained in cold dark matter cosmological simulations -- such as Navarro Frenk & White (beta=1) or Moore (1. 5) universal dark matter profiles -- are ruled out at better than 99% CL. As baryon collapse is likely to have steepened the dark matter profile from its original form, our data provides a powerful test of the cold dark matter paradigm at the cluster mass scale.