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 similar or equal to 7-8 using the Ultra Deep Field 2012 (UDF12), the deepest near-infrared (near-IR) exposures yet taken with the Hubble Space Telescope (HST). As a result of the increased near-IR exposure time compared to previous HST imaging in this field, we probe similar to 0.65 (0.25) mag fainter in absolute UV magnitude, at z similar to 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 dropout-selected UDF sources to 47 at z similar to 7 and 27 at z similar to 8. Incorporating brighter archival and ground-based samples, we measure the z similar or equal to 7 UV luminosity function to an absolute magnitude limit of M-UV = -17 and find a faint end Schechter slope of alpha= -1.87(-0.17)(+0.18). Using a similar color-color selection at z similar or equal to 8 that takes our newly added imaging in the F140W filter into account, and incorporating archival data from the HIPPIES and BoRG campaigns, we provide a robust estimate of the faint end slope at z similar or equal to 8, alpha = -1.94(-0.24)(+0.21). 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.