Determination of the Dark Matter Profile of Abell 2199 from Integrated Starlight

Abstract

We have obtained deep, long-slit spectroscopy along the major axis of NGC 6166, the cD galaxy in the cluster Abell 2199, in order to measure the kinematics of intracluster starlight at large radii. The velocity dispersion rises beyond a few kpc, and steadily rises to 660 km/s at a radius of 60 kpc, nearly reaching the velocity dispersion of the cluster (775+-50 km/s). These data suggest that the stars in the halo of the cD trace the potential of the cluster and that the kinematics of these intracluster stars can be used to constrain the mass profile of the cluster. In addition, we find evidence for systematic rotation in the intracluster stars beyond 20 kpc. The surface brightness and velocity dispersion profiles can be fit using a single component mass model only by making unphysical assumptions about the level of anisotropy for both the stars in the cD galaxy and for the kinematics of the galaxies in the cluster. Two-component mass models for the cD and its halo are explored using the kinematics of known cluster members as an additional constraint beyond the extent of the stellar kinematics. Assuming isotropy, the observed kinematics can be reproduced only if the halo has a soft core, i.e., Alpha < 1 (Alpha=1 is excluded due to low implied stellar mass-to-light ratios). This result is inconsistent with the predictions of current N-body simulations with CDM. Assuming that our best-fit mass models for the Abell 2199 were placed at cosmological redshifts between 0.2 <= z <= 0.5, the derived tangential critical radii range from 5'' to 40'' consistent with the strong lensing observed in intermediate redshift clusters. We also present Monte Carlo simulations that test the reliability of velocity dispersions when the S/N is low.