Depletion of background galaxies owing to the cluster lens CL0024+1654: U- and R-band observations

We have obtained U- and R-band observations of the depletion of background galaxies resulting from the gravitational lensing of the galaxy cluster CL0024+1654 (z = 0.39). The radial depletion curves show a significant depletion in both bands within a radius of 40-70 arcsec from the cluster centre. This is the first time that depletion is detected in the U band. This gives independent evidence for a break in the slope of the U-band luminosity function at faint magnitudes. The radially averaged R-band depletion curve is broader and deeper than in the U band. The differences can be attributed to the wavelength dependence of the slope of the luminosity function and to the different redshift distribution of the objects probed in the two bands. We estimate the Einstein radius, r_E, of a singular isothermal sphere lens model using maximum-likelihood analysis. Adopting a slope of the number counts of α = 0.2 and using the background density found beyond r= 150 arcsec, we find r_E = 17 ± 3 and 25 ± 3 arcsec in the U and R bands, respectively. When combined with the redshift of the single background galaxy at z = 1.675 seen as four giant arcs around 30 arcsec from the cluster centre, these values indicate a median redshift in the range <z_S> ≈ 0.7 to 1.1 for the U_AB ≥ 24 mag and R_AB ≥ 24 mag populations.