Abstract
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−2 there are 30 background galaxies per square arcmin, which is sufficient to map statistically the dark matter distribution in the foreground cluster. Automated pattern recognition software generates a 2-d mass density map. There is evidence for a soft core of about 100 kpc radius for the dark matter in the clusters studied so far. Independent of other redshift constraints, the existence of such a large percentage of lens-distorted faint blue galaxies sets a lower mean redshift limit to this background galaxy population of z > 1.
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