Black Pits in Cluster Cores

R. D. Blandford and Larry Smarr. Theoretical Astrophysics, California Institute of Technology, Pasadena, CA 91125. Department of Astronomy and Physics, University of Illinois Urbana, Illinois 61801

ABSTRACT
The relative radial velocities of multiple-nuclei galaxies, together with recent X-ray observations, suggest that the dark matter in the centers of rich clusters can have core radii ranging from the conventionally assumed value of∼200 kpc down to as small as 10 kpc. We present a two-parameter(core radius and velocity dispersion) model of the cluster gravitational potential, describing a family of potentials corresponding to the two-parameter observed morphology of X-ray clusters. Because the dark matter can dominate the luminous matter down to such small scales, we consider a cluster of galaxies as a “black pit” in which the galaxies move as test particles. Using this model potential, we interpret the unusual optical morphology, often observed for brightest cluster member galaxies (Morgan’s cD, D, and db classes), as a natural consequence of galaxy formation, tidal stripping, and merging in a pre-existing small core radius black pit. Dynamical friction is less efficacious in these small core radius pits, as observations seem to require. The central concentration of dark matter in cluster cores may explain the phenomenon of wide-angle-tail radio sources associated with brightest cluster members. The amount of gas and stars in a pit potential may vary widely, producing the richness variation observed in clusters. Optically under-illuminated pits (such as the Yerkes poor clusters) can still have their observed cD galaxies, small X-ray cores, and wide-angle tails caused by the dark matter. The most likely cause of the high density of dark matter in some cluster cores is the fragmentation of the cosmological dark matter. We make a number of predictions for observational tests of the black pit model.