IR Observations (original) (raw)

Abstract

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.

Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.