Lensing the background population of galaxies.

Deep CCD surveys to 29 mag from .32 to .9 micron wavelength reveal an isotropic population of very blue galaxies. There are over 200,000 of these objects per square degree per mag. The redshift of this population appears to be in the range 1 - 3. Systematic alignment of many of these faint background galaxies has been detected, centered on foreground galaxy clusters. The background galaxy population is selected using its anomalously blue B-R color. At a limiting surface brightness of 29 Bj mag arcsec sup (-2) there are 30 background galaxies per square arcmin, which is sufficient to map statistically the dark matter distribution in the foreground cluster.