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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

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 {approx} 7 and z {approx} 8 galaxies are combined with new LFs for z {approx} 5-6 galaxies and the earlier z {approx} 4 LF to reach to very faint limits (<0.05 L*{sub z=3}). The faint-end slopes {alpha} are steep: -1.79 {+-} 0.12 (z {approx} 5), -1.73 {+-} 0.20 (z {approx} 6), -2.01 {+-} 0.21 (z {approx} 7), and -1.91 {+-} 0.32 (z {approx} 8). Slopes {alpha} {approx}< -2 lead to formally divergent UV fluxes, though galaxies are not expected to form below {approx} - 10 AB mag. These results have important implications for reionization. The weighted mean slope at z {approx} 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 {approx} 6, matching the Thomson optical depths is more challenging. Extrapolating the current LF evolution to z > 8, taking {alpha} to be -1.87 {+-} 0.13 (the mean value at z {approx} 6-8), and adopting typical parameters,more » we derive Thomson optical depths of 0.061{sup +0.009}{sub -0.006}. However, this result will change if the faint-end slope {alpha} is not constant with redshift. We test this hypothesis and find a weak, though uncertain, trend to steeper slopes at earlier times (d{alpha}/dz {approx} -0.05 {+-} 0.04) that would increase the Thomson optical depths to 0.079{sup +0.063}{sub -0.017}, consistent 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 remain large. Deeper WFC3/IR+ACS observations can further constrain the UV ionizing flux from faint galaxies.« less

Authors:
; ;  [1]; ; ;  [2];  [3];  [4];  [5];  [6]
  1. Leiden Observatory, Leiden University, NL-2300 RA Leiden (Netherlands)
  2. UCO/Lick Observatory, University of California, Santa Cruz, CA 95064 (United States)
  3. Center for Astrophysics and Space Astronomy, University of Colorado, 389-UCB, Boulder, CO 80309 (United States)
  4. Space Telescope Science Institute, Baltimore, MD 21218 (United States)
  5. Institute for Astronomy, ETH Zurich, 8092 Zurich (Switzerland)
  6. Department of Astronomy, Yale University, New Haven, CT 06520 (United States)
Publication Date:
OSTI Identifier:
22047768
Resource Type:
Journal Article
Journal Name:
Astrophysical Journal Letters
Additional Journal Information:
Journal Volume: 752; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 2041-8205
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASTRONOMY; ASTROPHYSICS; COSMIC PHOTONS; COSMIC RAY FLUX; EVOLUTION; GALAXIES; HYPOTHESIS; LUMINOSITY; NEAR INFRARED RADIATION; RED SHIFT; ULTRAVIOLET RADIATION; UNIVERSE

Citation Formats

Bouwens, R. J., Labbe, I., Franx, M., Illingworth, G. D., Oesch, P. A., Magee, D., Trenti, M., Stiavelli, M., Carollo, C. M., and Van Dokkum, P. 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. United States: N. p., 2012. Web. doi:10.1088/2041-8205/752/1/L5.
Bouwens, R. J., Labbe, I., Franx, M., Illingworth, G. D., Oesch, P. A., Magee, D., Trenti, M., Stiavelli, M., Carollo, C. M., & Van Dokkum, P. 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. United States. doi:10.1088/2041-8205/752/1/L5.
Bouwens, R. J., Labbe, I., Franx, M., Illingworth, G. D., Oesch, P. A., Magee, D., Trenti, M., Stiavelli, M., Carollo, C. M., and Van Dokkum, P. Sun . "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". United States. doi:10.1088/2041-8205/752/1/L5.
@article{osti_22047768,
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},
author = {Bouwens, R. J. and Labbe, I. and Franx, M. and Illingworth, G. D. and Oesch, P. A. and Magee, D. and Trenti, M. and Stiavelli, M. and Carollo, C. M. and Van Dokkum, P.},
abstractNote = {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 {approx} 7 and z {approx} 8 galaxies are combined with new LFs for z {approx} 5-6 galaxies and the earlier z {approx} 4 LF to reach to very faint limits (<0.05 L*{sub z=3}). The faint-end slopes {alpha} are steep: -1.79 {+-} 0.12 (z {approx} 5), -1.73 {+-} 0.20 (z {approx} 6), -2.01 {+-} 0.21 (z {approx} 7), and -1.91 {+-} 0.32 (z {approx} 8). Slopes {alpha} {approx}< -2 lead to formally divergent UV fluxes, though galaxies are not expected to form below {approx} - 10 AB mag. These results have important implications for reionization. The weighted mean slope at z {approx} 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 {approx} 6, matching the Thomson optical depths is more challenging. Extrapolating the current LF evolution to z > 8, taking {alpha} to be -1.87 {+-} 0.13 (the mean value at z {approx} 6-8), and adopting typical parameters, we derive Thomson optical depths of 0.061{sup +0.009}{sub -0.006}. However, this result will change if the faint-end slope {alpha} is not constant with redshift. We test this hypothesis and find a weak, though uncertain, trend to steeper slopes at earlier times (d{alpha}/dz {approx} -0.05 {+-} 0.04) that would increase the Thomson optical depths to 0.079{sup +0.063}{sub -0.017}, consistent 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 remain large. Deeper WFC3/IR+ACS observations can further constrain the UV ionizing flux from faint galaxies.},
doi = {10.1088/2041-8205/752/1/L5},
journal = {Astrophysical Journal Letters},
issn = {2041-8205},
number = 1,
volume = 752,
place = {United States},
year = {2012},
month = {6}
}