UVUDF: ULTRAVIOLET IMAGING OF THE HUBBLE ULTRA DEEP FIELD WITH WIDE-FIELD CAMERA 3

Abstract

We present an overview of a 90-orbit Hubble Space Telescope treasury program\nto obtain near ultraviolet imaging of the Hubble Ultra Deep Field using the\nWide Field Camera 3 UVIS detector with the F225W, F275W, and F336W filters.\nThis survey is designed to: (i) Investigate the episode of peak star formation\nactivity in galaxies at 1<z<2.5; (ii) Probe the evolution of massive galaxies\nby resolving sub-galactic units (clumps); (iii) Examine the escape fraction of\nionizing radiation from galaxies at z~2-3; (iv) Greatly improve the reliability\nof photometric redshift estimates; and (v) Measure the star formation rate\nefficiency of neutral atomic-dominated hydrogen gas at z~1-3. In this overview\npaper, we describe the survey details and data reduction challenges, including\nboth the necessity of specialized calibrations and the effects of charge\ntransfer inefficiency. We provide a stark demonstration of the effects of\ncharge transfer inefficiency on resultant data products, which when\nuncorrected, result in uncertain photometry, elongation of morphology in the\nreadout direction, and loss of faint sources far from the readout. We agree\nwith the STScI recommendation that future UVIS observations that require very\nsensitive measurements use the instrument's capability to add background light\nthrough a "post-flash". Preliminary results on number counts of UV-selected\ngalaxies and morphology of galaxies at z~1 are presented. We find that the\nnumber density of UV dropouts at redshifts 1.7, 2.1, and 2.7 is largely\nconsistent with the number predicted by published luminosity functions. We also\nconfirm that the image mosaics have sufficient sensitivity and resolution to\nsupport the analysis of the evolution of star-forming clumps, reaching 28-29th\nmagnitude depth at 5 sigma in a 0.2 arcsecond radius aperture depending on\nfilter and observing epoch.\n