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 ∼ 7 and z ∼ 8 galaxies are combined with new LFs for z ∼ 5–6 galaxies and the earlier z ∼ 4 LF to reach to very faint limits (<0.05 L * z = 3 ). The faint-end slopes α are steep: −1.79 ± 0.12 ( z ∼ 5), −1.73 ± 0.20 ( z ∼ 6), −2.01 ± 0.21 ( z ∼ 7), and −1.91 ± 0.32 ( z ∼ 8). Slopes α ≲ −2 lead to formally divergent UV fluxes, though galaxies are not expected to form below ∼ − 10 AB mag. These results have important implications for reionization. The weighted mean slope at z ∼ 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 ∼ 6, matching the Thomson optical depths is more challenging. Extrapolating the current LF evolution to z > 8, taking α to be −1.87 ± 0.13 (the mean value at z ∼ 6-8), and adopting typical parameters, we derive Thomson optical depths of 0.061 +0.009 − 0.006 . However, this result will change if the faint-end slope α is not constant with redshift. We test this hypothesis and find a weak, though uncertain, trend to steeper slopes at earlier times ( d α/ dz ∼ −0.05 ± 0.04) that would increase the Thomson optical depths to 0.079 +0.063 − 0.017 , consistent with recent WMAP estimates (τ = 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.