UV Luminosity Functions from 113 z~7 and z~8 Lyman-Break Galaxies in the ultra-deep HUDF09 and wide-area ERS WFC3/IR Observations

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  • 28 June 2010
Abstract
(abridged) Using a Lyman-Break technique, we identify 66 z~7 and 47 z~8 candidate galaxies in the ultra-deep (~29 AB mag) WFC3/IR observations over the HUDF and two nearby HUDF09 fields (14 arcmin**2) and the deep (~27.5 AB mag), wide-area (~40 arcmin**2) ERS observations. This 26-29 AB mag sample of 113 galaxies in the reionization epoch is the largest currently available. A contamination rate of <=14% is found after thoroughly assessing the impact of lower redshift sources, photometric scatter, low mass stars, spurious sources, and transients on our selection. Carefully modelling the selection volumes for each of our search fields, we derive luminosity functions for galaxies at z~7 and z~8. The faint-end slopes we find at z~7 and z~8 are alpha = -1.94+/-0.24 and alpha=-2.00+/-0.33, respectively. This provides increasingly strong evidence that the UV LF at z>~6 is at least as steep as at z~4 (alpha=-1.73+/-0.05), with alpha<~-1.7, and that lower luminosity galaxies dominate the galaxy luminosity density during the epoch of reionization. Luminosity densities and SFR densities derived from these UV LFs are compared to those derived from recent stellar mass density determinations at z>~4. We find reasonable consistency, with the SFR densities implied from the stellar mass densities being only ~40% higher at z<7. This suggests that (1) the stellar mass densities inferred from the Spitzer IRAC photometry are reasonably accurate and (2) that the IMF at very high redshift may not be that different from later times. Extrapolating these LFs to lower luminosities and higher redshifts show that galaxies contribute substantially to the reionization. Thomson optical depths of 0.056 (0.075), within 2sigma (1sigma) of the latest WMAP values, are obtained for escape fractions of 20% (60%) and clumping factors of 3.

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