Anthony Tyson (then in the physics division of AT&T Bell Laboratories) and his colleagues noticed the systematic alignment of a few dozen faint blue galaxies
behind two massive foreground galaxy clusters, Abell 1689 and CL 1409+52.
They associate the faint red population with faint blue galaxies
that have become satellite galaxies.
One possible explanation was that the faint blue galaxies were dwarfs, and that the universe once had hosted many more of those tiny galaxies than it does today.
Times change, and one of Ferguson's original motivations for undertaking the HDF--a supposed overabundance of faint blue galaxies in the early cosmos--has been rendered all but moot with a model of the universe now gaining wide acceptance.
The redshifts we obtained for the faint blue galaxies revealed that they are indeed members of distant clusters and not just field interlopers.
Deep CCD images were used to identify a swell in the number of faint blue galaxies at deeper apparent magnitudes.
A puzzle for astronomers in the last few years has been the overabundance of "faint blue galaxies
" at distances of several billion light-years, as revealed in very deep CCD images taken with large telescopes.