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The UV Luminosity Function of star-forming galaxies via dropout selection at redshifts z ~ 7 and 8 from the 2012 Ultra Deep Field campaign

We present a catalog of high redshift star-forming galaxies selected to lie within the redshift range z ~ 7-8 using the Ultra Deep Field 2012 (UDF12), the deepest near-infrared (near-IR) exposures yet taken with the Hubble Space Telescope. As a result of the increased near-infrared exposure time compared to previous HST imaging in this field, we probe 0.65 (0.25) mag fainter in absolute UV magnitude, at z ~ 7 (8), which increases confidence in a measurement of the faint end slope of the galaxy luminosity function. Through a 0.7 mag deeper limit in the key F105W filter that encompasses or lies just longward of the Lyman break, we also achieve a much-refined color-color selection that balances high redshift completeness and a low expected contamination fraction. We improve the number of drop-out selected UDF sources to 47 at z ~ 7 and 27 at z ~ 8. Incorporating brighter archival and ground-based samples, we measure the z ~ 7 UV luminosity function to an absolute magnitude limit of M_UV = -17 and find a faint end Schechter slope of α= -1.87+/- 0.18. Using a similar color-color selection at z ~ 8 that takes account of our newly-added imaging in the F140W filter, and incorporating archival data from the HIPPIES and BoRG campaigns, we provide a robust estimate of the faint end slope at z ~ 8, α= -1.94 +/- 0.23. We briefly discuss our results in the context of earlier work and that derived using the same UDF12 data but with an independent photometric redshift technique (McLure et al 2012).