A redshift limit for the faint blue galaxy population from deep U band imaging

We have obtained a definitive upper limit for the redshift of the population of faint blue galaxies found in deep imaging surveys. The U- B₃_, and particularly the B₃_-R, colors of these objects show a blueing trend toward fainter magnitudes. A typical galaxy at R = 26 has colors that are only slightly redder than a flat (constant Fᵥ_) spectrum. For any reasonable Lyman limit break, this constrains 93% (a conservative lower limit) or more of the galaxies to under z ~ 3, beyond which the break gets redshifted through the U band. The galaxies appear to be undergoing relatively recent evolution with rest frame spectra that are approximately flat down to the Lyman limit.