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We present the results of a systematic search for gravitationally-lensedcontinuum Lyman break `drop-outs' beyond a redshift 7 conducted via very deepimaging through six foreground clusters undertaken with the Hubble and SpitzerSpace Telescopes. The survey has yielded 10 z-band and 2 J-band drop-outcandidates to photometric limits of J_110~=26.2 AB (5sigma). Taking intoaccount the magnifications afforded by our clusters (1-4 magnitudes), we probethe presence of z>7 sources to unlensed limits of J_{110}~=30 AB, fainter thanthose charted in the Hubble Ultradeep Field. To verify the fidelity of ourcandidates we conduct a number of tests for instrumental effects which wouldlead to spurious detections, and carefully evaluate the likelihood offoreground contamination by considering photometric uncertainties in thedrop-out signature, the upper limits from stacked IRAC data and the statisticsof multiply-imaged sources. Overall, we conclude that we can expect about halfof our sample of z-band drop-outs are likely to be at high redshift. Anambitious infrared spectroscopic campaign undertaken with the NIRSPECspectrograph at the WM Keck Observatory for seven of the most promisingcandidates failed to detect any Lyman-alpha emission highlighting the challengeof making further progress in this field. While the volume density of highredshift sources will likely remain uncertain until more powerful facilitiesare available, our data provides the first potentially interesting constraintson the UV luminosity function at z~=7.5 at intrinsically faint limits. Wediscuss the implications of our results in the context of the hypothesis thatthe bulk of the reionizing photons in the era 7<z<12 arise in low luminositygalaxies undetected by conventional surveys.

astrophysical journal/the astrophysical journal

A Hubble & Spitzer Space Telescope Survey for Gravitationally-Lensed  Galaxies: Further Evidence for a Significant Population of Low Luminosity  Galaxies beyond Redshift Seven