The UDF05 Follow-up of the HUDF: II. Constraints on Reionization from z-dropout Galaxies

Abstract

[Abridged] We detect three (plus one less certain) z-dropout sources in two separate fields of our UDF05 HST NICMOS images. These z~7 Lyman-Break Galaxy (LBG) candidates allow us to constrain the Luminosity Function (LF) of the star forming galaxy population at those epochs. By assuming a change in only M* and adopting a linear evolution in redshift, anchored to the measured values at z~6, the best fit evolution coefficient is found to be 0.43+-0.19 mag per unit redshift (0.36+-0.18, if including all four candidates), which provides a value of M*(z=7.2)=-19.7+-0.3. This implies a steady evolution for the LBG LF out to z~7, at the same rate that is observed throughout the z~3 to 6 period. This puts a strong constraint on the star-formation histories of z~6 galaxies, whose ensemble star-formation rate density must be lower by a factor 2 at ~170 Myr before the epoch at which they are observed. In particular, a large fraction of stars in the z~6 LBG population must form at redshifts well above z~7. Extrapolating this steady evolution of the LF out to higher redshifts, we estimate that galaxies would be able to reionize the universe by z~6, provided that the faint-end slope of the z>7 LF steepens to alpha~-1.9, and that faint galaxies, with luminosities below the current detection limits, contribute a substantial fraction of the required ionizing photons. This scenario gives however an integrated optical depth to electron scattering that is ~2sigma below the WMAP-5 measurement. Therefore, altogether, our results indicate that, should galaxies be the primary contributors to reionization, either the currently detected evolution of the galaxy population slows down at z>7, or the LF evolution must be compensated by a decrease in metallicity and a corresponding increase in ionization efficiency at these early epochs.

Accepted for publication in ApJ, 7 pages, 4 figures; minor revisions to match accepted version

References in corpus (21)

• The Hubble Ultra Deep Field
• UV Luminosity Functions at z~4, 5, and 6 from the HUDF and other Deep HST ACS Fields: Evolution and Star Formation History
• Galaxies in the Hubble Ultra Deep Field: I. Detection, Multiband Photometry, Photometric Redshifts, and Morphology
• The observed ionization rate of the intergalactic medium and the ionizing emissivity at z >5: Evidence for a photon starved and extended epoch of reionization
• The Direct Detection of Lyman-Continuum Emission from Star-forming Galaxies at z~3
• A galaxy at a redshift z = 6.96
• z~7-10 Galaxies in the HUDF and GOODS fields, and their UV Luminosity Functions
• New Constraints on the Lyman Continuum Escape Fraction at z~1.3
• Discovery of a Very Bright Strongly-Lensed Galaxy Candidate at z ~ 7.6
• The Stellar Mass Density at z~6 from Spitzer Imaging of i-drop Galaxies
• Lyman-break galaxies at z~5 -I. First significant stellar mass assembly in galaxies that are not simply z~3 LBGs at higher redshift
• A Hubble & Spitzer Space Telescope Survey for Gravitationally-Lensed Galaxies: Further Evidence for a Significant Population of Low Luminosity Galaxies beyond Redshift Seven
• The UDF05 Follow-up of the HUDF: I. The Faint-End Slope of the Lyman-Break Galaxy Population at z~5
• Rapid Evolution in the Most Luminous Galaxies During the First 900 Million Years
• Constraining the population of 6 < z < 10 star-forming galaxies with deep near-IR images of lensing clusters
• Differential Evolution of the UV Luminosity Function of Lyman Break Galaxies from z~5 to 3
• Luminosity Functions of Lyman-Break Galaxies at z~4 and 5 in the Subaru Deep Field
• Evidence for strong evolution of the cosmic star formation density at high redshift
• Constraints on First-Light Ionizing Sources from Optical Depth of the Cosmic Microwave Background
• Are There Enough Ionizing Photons to Reionize the Universe by z=6?
• A Limit on the Number Density of Bright z~7 Galaxies

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• Keeping the Universe ionised: Photo-heating and the clumping factor of the high-redshift intergalactic medium
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• Galaxy UV-luminosity function and reionization constraints on axion dark matter
• The Luminosity Function at z~8 from 97 Y-band dropouts: Inferences About Reionization
• Metal-Free Gas Supply at the Edge of Reionization: Late-Epoch Population III Star Formation
• The Effect of Galactic Properties on the Escape Fraction of Ionizing Photons
• Constraining the galaxy mass content in the core of A383 using velocity dispersion measurements for individual cluster members
• Galaxy formation in the PLANCK cosmology - III. The high-redshift universe
• Starburst and post-starburst high-redshift protogalaxies: The feedback impact of high energy cosmic rays
• The influence of metagalactic ultra-violet background fluctuations on the high-redshift Ly$\alpha$ forest
• Diversity of the Lyman continuum escape fractions of high-$z$ galaxies and its origins
• A quantitative assessment of completeness correction methods and public release of a versatile simulation code