CLASH: A CENSUS OF MAGNIFIED STAR-FORMING GALAXIES AT z ∼ 6-8

We utilize 16 band Hubble Space Telescope (HST) observations of 18 lensing clusters obtained as part of the Cluster Lensing And Supernova survey with Hubble (CLASH) Multi-Cycle Treasury program to search for z similar to 6-8 galaxies. We report the discovery of 204, 45, and 13 Lyman-break galaxy candidates at z similar to 6, z similar to 7, and z similar to 8, respectively, identified from purely photometric redshift selections. This large sample, representing nearly an order of magnitude increase in the number of magnified star-forming galaxies at z similar to 6-8 presented to date, is unique in that we have observations in four WFC3/UVIS UV, seven ACS/WFC optical, and all five WFC3/IR broadband filters, which enable very accurate photometric redshift selections. We construct detailed lensing models for 17 of the 18 clusters to estimate object magnifications and to identify two new multiply lensed z greater than or similar to 6 candidates. The median magnifications over the 17 clusters are 4, 4, and 5 for the z similar to 6, z similar to 7, and z similar to 8 samples, respectively, over an average area of 4.5 arcmin(2) per cluster. We compare our observed number counts with expectations based on convolving blank field UV luminosity functions through our cluster lens models and find rough agreement down to similar to 27 mag, where we begin to suffer significant incompleteness. In all three redshift bins, we find a higher number density at brighter observed magnitudes than the field predictions, empirically demonstrating for the first time the enhanced efficiency of lensing clusters over field surveys. Our number counts also are in general agreement with the lensed expectations from the cluster models, especially at z 6, where we have the best statistics.