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 CL0 024+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. Thi s is the first time that depletion is detected in the U band. This gives in dependent evidence for a break in the slope of the U-band luminosity functi on at faint magnitudes. The radially averaged R-band depletion curve is bro ader and deeper than in the U band. The differences can be attributed to th e 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 mo del using maximum-likelihood analysis. Adopting a slope of the number count s of alpha =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=1.67 5 seen as four giant arcs around 30 arcsec from the cluster centre, these v alues indicate a median redshift in the range <z(S)> approximate to0.7 to 1 .1 for the U(AB)greater than or equal to 24 mag and R(AB)greater than or eq ual to 24 mag populations.