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

Details 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 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

2011-05-10

arxiv.org/abs/1105.2038v2

Astronomy and Astrophysics

Astrophysics

Cosmology and Extragalactic Astrophysics

6 pages, 4 figures, 1 table, resubmitted to Astrophysical Journal Letters, after responding to referee report

Scientific paper

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 (<0.05 L*(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). Slopes alpha ~ -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. Adopting typical parameters, extrapolating the current LF evolution to z>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.

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