Counts and colours of faint galaxies in the U and R bands

Ground-based counts and colours of faint galaxies in the U and R bands in one field at high Galactic latitude are presented. Integrated over flux, a total of 1.2 × 105 source deg−2 are found to U=25.5 mag and 6.3 × 105 source deg−2 to R = 27 mag, with d logN/dm ∼ 0.5 in the U band and d log N/dm ∼ 0.3 in the R band. Consistent with these number-magnitude curves, sources become bluer with increasing magnitude to median U— R= 0.6 mag at 24<U<25 mag and U — R = 1.2 mag at 25 <R < 26 mag. Because the Lyman break redshifts into the U band at z ≈ 3, at least 1.2 × 105 source deg−2 must be at redshifts z < 3. Measurable U-band fluxes of 73 per cent of the 6.3 × 105 source deg−2 suggest that the majority of these also lie at z < 3. These results require an enormous space density of objects in any cosmological model.