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
(23 votes over all institutions)
The HUDF09 data are the deepest near-IR observations ever, reaching to ~29.5 mag. The recent sample of 132 z~7 and z~8 galaxies from the HUDF09 data are complemented here by new large samples at z~5 and z~6 galaxies (507 and 203, respectively) and prior results at z~4. The luminosity functions we derive at these redshifts reach to very faint limits of 8, we derive Thomson optical depths of 0.079, in excellent agreement with the seven-year 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 and depend critically on the trend to steeper faint-end slopes at z>=6. We show that deeper, and realistic, WFC3/IR+ACS observations can substantially lessen the uncertainties on the z~5-8 slopes and further constrain this key parameter in determining the UV ionizing flux from galaxies.
