z ∼ 7-10 GALAXIES BEHIND LENSING CLUSTERS: CONTRAST WITH FIELD SEARCH RESULTS

We conduct a search for z similar to 7 dropout galaxies behind 11 massive lensing clusters using 21 arcmin(2) of deep Hubble Space Telescope NICMOS, ACS, and WFPC2 image data. In total, over this entire area, we find only one robust z similar to 7 z-dropout candidate (previously reported around Abell 1689). Four less robust z-dropout and J-dropout candidates are also found. The nature of the four weaker candidates could not be precisely determined due to the limited depth of the available optical data, but detailed simulations suggest that all four are likely to be low-redshift interlopers. By contrast, we estimate that our robust candidate A1689-zD1 has <0.2% probability of being a low-redshift interloper. We compare these numbers with what we might expect using the z similar to 7 UV luminosity function (LF) determined from field searches. We predict 2.7 z similar to 7 z- dropouts and 0.3 z similar to 9 J-dropouts over our cluster search area, in reasonable agreement with our observational results, given the small numbers. The number of z greater than or similar to 7 candidates we find in the present search is much lower than that which has been reported in several previous studies of the prevalence of z greater than or similar to 7 galaxies behind lensing clusters. To understand these differences, we examined z greater than or similar to 7 candidates in other studies and conclude that only a small fraction are likely to be z greater than or similar to 7 galaxies. Our findings supportmodels that show that gravitational lensing from clusters is of the most value for detecting galaxies at magnitudes brighter than L* (H less than or similar to 27) where the LF is expected to be very steep. Use of these clusters to constrain the faint-end slope or determine the full LF is likely of less value due to the shallower effective slope measured for the LF at fainter magnitudes, as well as significant uncertainties introduced from modeling both the gravitational lensing and incompleteness.