Galaxy Clusters with Rounded Central Density Profiles
Abstract
Observational determinations of the mass distribution within galaxy clusters show that some clusters do not fit the density profile expected from hierarchical clustering of CDM material, which yields a sharp, cuspy center. Tyson et al. (1998) (Ap.J.(Letters),Vol. 498, pp. L107--L110) worked out the shape of the mass in the galaxy cluster 0024+1654 by gravitational lensing. Happily, this cluster produces several gravitationally lensed images of the same background galaxy, and they used these images to solve for the shape of the mass in the cluster. Most of the mass is smoothly distributed throughout the cluster, although individual galaxies show up as sharp spikes atop that smooth background. An important feature of the background mass (which is most of the mass in the cluster) is that it is smoothly rounded at the center, which differs from expectations according to CDM. X-ray maps of many non-cD clusters also show rounded centers. We are looking into other processes that might form clusters without cuspy centers. The first, and most obvious, process is to start from material with appreciable velocity dispersion. Systematic properties of clusters formed from these initial conditions are being studied: core radius, velocity dispersion, degree of central concentration, and the time required to reach a steady state, to list a few. Properties seem to scale with the ratio D/J, where D is the mean near neighbor distance and J is the Jeans length in the initial condition. Perhaps the most interesting property of these configurations is that they show long lived normal-mode oscillations. The present status of the investigation will be reported with emphasis on the character of the oscillatory modes.
- Publication:
-
AAS/Division of Dynamical Astronomy Meeting
- Pub Date:
- September 1999
- Bibcode:
- 1999DDA....30.0512S