Detection of Systematic Gravitational Lens Galaxy Image Alignments: Mapping Dark Matter in Galaxy Clusters

A gravitational lens distorts most background galaxies by stretching along a circle centered on the lens. This systematic alignment of 20-60 faint background galaxy images has been detected, centered on foreground galaxy clusters of high velocity dispersion. The background galaxy population is selected by its extreme blue B-R color. At a limiting surface brightness of 29 B mag arcsec^-2^, there are over 30 background galaxies arcmin^-2^ mag^-1^ anywhere in the sky, which is sufficient to map statistically the dark matter distribution in a foreground cluster. Pattern recognition software generates a two-dimensional lens distortion map. Initial results for the high-velocity dispersion clusters A1689 and CL 1409+52 are presented here. The dark matter is apparently correlated (center and radial extent) with the cluster red light, suggestive of a baryonic origin or dissipative coupling. The existence of a high percentage of lens-distorted faint blue galaxies sets a lower limit of approximately 0.9 to this background galaxy population mean redshift.