The dark matter density profile of the lensing cluster MS2137-23: 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, rho_d 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.