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

Abstract

The HUDF09 data are the deepest near-IR observations ever, reaching to 29.5 mag. Luminosity functions (LF) 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 $z\sim4$ LF to reach to very faint limits (8, and taking alpha to be -1.87+/-0.13 (the mean value at z~6-8), we derive Thomson optical depths of 0.061. However, this result will change if the faint-end slope alpha is not constant with redshift. We test this hypothesis and find a weak, but still very uncertain, trend to steeper slopes at earlier times (dalpha/dz ~ -0.05+/-0.04), that would increase the Thomson optical depths to 0.079, in excellent agreement 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 are large. We show that deeper WFC3/IR+ACS observations can substantially lessen the uncertainties in the z~5-8 slopes and further constrain this key parameter in determining the ionizing flux from galaxies.