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

The HUDF09 data are the deepest near-IR observations ever, reaching to 29.5 mag. Luminosity functions (LFs) from these new HUDF09 data for 132 z similar to 7 and z similar to 8 galaxies are combined with new LFs for z similar to 5-6 galaxies and the earlier z similar to 4 LF to reach to very faint limits (<0.05 L-z=3*). The faint-end slopes alpha are steep: -1.79 +/- 0.12 (z similar to 5), -1.73 +/- 0.20 (z similar to 6), -2.01 +/- 0.21 (z similar to 7), and -1.91 +/- 0.32 (z similar to 8). Slopes alpha less than or similar to -2 lead to formally divergent UV fluxes, though galaxies are not expected to form below similar to -10 AB mag. These results have important implications for reionization. The weighted mean slope at z similar to 6-8 is -1.87 +/- 0.13. For such steep slopes, and a faint-end limit 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 similar to 6, matching the Thomson optical depths is more challenging. Extrapolating the current LF evolution to z > 8, taking alpha to be -1.87 +/- 0.13 (the mean value at z similar to 6-8), and adopting typical parameters, we derive Thomson optical depths of 0.061(-0.006)(+0.009). However, this result will change if the faint-end slope alpha is not constant with redshift. We test this hypothesis and find a weak, though uncertain, trend to steeper slopes at earlier times (d alpha/dz similar to -0.05 +/- 0.04) that would increase the Thomson optical depths to 0.079(-0.017)(+0.063), consistent with recent WMAP estimates (tau = 0.088 +/- 0.015). It may thus not be necessary to resort to extreme assumptions about the escape fraction or clumping factor. Nevertheless, the uncertainties remain large. Deeper WFC3/IR+ACS observations can further constrain the UV ionizing flux from faint galaxies.