SIMBAD references

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 (dα/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.