A search for star-forming galaxies at z~7-10 in deep HST NICMOS imaging data around massive galaxy clusters

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Abstract
We conduct a search for z>~7 dropout galaxies over 21 arcmin**2 of deep HST NICMOS, ACS, and WFPC2 data behind 11 massive lensing clusters. In total, we find only one robust z~7 z-dropout candidate, previously reported over Abell 1689, over this entire area. Three other weaker z-dropout and J-dropout candidates are also found. The nature of the three weaker candidates could not be precisely determined due to the limited depth of the available optical data, but detailed simulations suggest that all three could be low-redshift interlopers. We compare these numbers with what we would might expect using the z~7 UV LF determined from field searches. We predict 2.7 z~7 z-dropouts and 0.3 z~9 J-dropouts over our cluster search area in reasonable agreement, given the small numbers, with our observational results. The number of z>~7 candidates we find in the present search are much lower than have been reported in several previous studies of the prevalence of z>~7 galaxies behind lensing clusters. To understand these differences, we examined z>~7 candidates in previous selections and conclude that only a small fraction are likely to be z>~7 galaxies. Our findings support models that show that gravitational lensing from clusters is of the most value for finding galaxies at brighter magnitudes (H<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 to be of less value due to uncertainties in modelling both the gravitational lensing and incompleteness, as well as a shallower effective slope expected to the LF at fainter magnitudes.

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