%0 Generic %1 Bouwens2011 %A Bouwens, R. J. %A Illingworth, G. D. %A Oesch, P. A. %A Trenti, M. %A Labbe, I. %A Franx, M. %A Stiavelli, M. %A Carollo, C. M. %A van Dokkum, P. %A Magee, D. %D 2011 %K fuv high-z lumfunc reionization %T 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 %U http://arxiv.org/abs/1105.2038 %X 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 <0.05L*(z=3). The faint-end slopes alpha 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). The results are consistent with a slope that increases systematically to earlier times, with slopes close to -2 at z~7-8. Slopes alpha ~-2 lead to formally divergent UV fluxes, though galaxies are not expected to form below ~-10 AB mag. These results have major implications for reionization. For such steep slopes, and a faint-end limit of -10 AB mag, galaxies provide a very large UV ionizing photon flux. Adopting typical parameters and extrapolating the current LF and faint-end slope evolution from z~4-8 to z>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.

@misc{Bouwens2011, abstract = { 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 <0.05L*(z=3). The faint-end slopes alpha 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). The results are consistent with a slope that increases systematically to earlier times, with slopes close to -2 at z~7-8. Slopes alpha ~-2 lead to formally divergent UV fluxes, though galaxies are not expected to form below ~-10 AB mag. These results have major implications for reionization. For such steep slopes, and a faint-end limit of -10 AB mag, galaxies provide a very large UV ionizing photon flux. Adopting typical parameters and extrapolating the current LF and faint-end slope evolution from z~4-8 to z>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. }, added-at = {2011-05-12T21:04:08.000+0200}, author = {Bouwens, R. J. and Illingworth, G. D. and Oesch, P. A. and Trenti, M. and Labbe, I. and Franx, M. and Stiavelli, M. and Carollo, C. M. and van Dokkum, P. and Magee, D.}, biburl = {https://www.bibsonomy.org/bibtex/288e8963baee5808a218d0d4b8b2d1380/miki}, description = {[1105.2038] 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}, interhash = {2d1e9ad47bea08c147ad556a57bd1bce}, intrahash = {88e8963baee5808a218d0d4b8b2d1380}, keywords = {fuv high-z lumfunc reionization}, note = {cite arxiv:1105.2038 Comment: 6 pages, 4 figures, 1 table, submitted to Astrophysical Journal Letters}, timestamp = {2011-05-12T21:04:08.000+0200}, title = {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}, url = {http://arxiv.org/abs/1105.2038}, year = 2011 }