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

Abstract

We present new spectroscopic observations of the gravitational arcs and the brightest cluster galaxy (BCG) in the cluster MS 2137-23 (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, ρ_d~r^-β, with β<0.9 at a 99% CL. Steep inner slopes obtained in cold dark matter cosmological simulations-such as Navarro, Frenk, & White (β=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 provide a powerful test of the cold dark matter paradigm at the cluster mass scale. Based on observations collected at the Keck Observatory, which is operated jointly by the California Institute of Technology and the University of California, and with the NASA/ESA Hubble Space Telescope, obtained at Space Telescope Science Institute, which is operated by Association of Universities for Research in Astronomy, under NASA contract NAS5-26555.