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The HUDF09 data are the deepest near-IR observations ever, reaching to 29.5mag. Luminosity functions (LF) from these new HUDF09 data for 132 z\sim7 andz\sim8 galaxies are combined with new LFs for z\sim5-6 galaxies and the earlierz\sim4 LF to reach to very faint limits (<0.05 L*(z=3)). The faint-end slopesalpha are steep: -1.79+/-0.12 (z\sim5), -1.73+/-0.20 (z\sim6), -2.01+/-0.21(z\sim7), and -1.91+/-0.32 (z\sim8). Slopes alpha\lesssim-2 lead to formallydivergent UV fluxes, though galaxies are not expected to form below \sim-10 ABmag. These results have important implications for reionization. The weightedmean slope at z\sim6-8 is -1.87+/-0.13. For such steep slopes, and a faint-endlimit of -10 AB mag, galaxies provide a very large UV ionizing photon flux.While current results show that galaxies can reionize the universe by z\sim6,matching the Thomson optical depths is more challenging. Extrapolating thecurrent LF evolution to z>8, taking alpha to be -1.87+/-0.13 (the mean value atz\sim6-8), and adopting typical parameters, we derive Thomson optical depths of0.061_{-0.006}^{+0.009}. However, this result will change if the faint-endslope alpha is not constant with redshift. We test this hypothesis and find aweak, though uncertain, trend to steeper slopes at earlier times(dalpha/dz\sim-0.05+/-0.04), that would increase the Thomson optical depths to0.079_{-0.017}^{+0.063}, consistent with recent WMAP estimates(tau=0.088+/-0.015). It may thus not be necessary to resort to extremeassumptions about the escape fraction or clumping factor. Nevertheless, theuncertainties remain large. Deeper WFC3/IR+ACS observations can furtherconstrain the ionizing flux from galaxies.

the astrophysical journal. letters

Lower-Luminosity Galaxies could reionize the Universe: Very Steep  Faint-End Slopes to the UV Luminosity Functions at z>=5-8 from the HUDF09  WFC3/IR Observations