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Keck spectroscopy of faint 3 < z < 7 Lyman break galaxies – I. New constraints on cosmic reionization from the luminosity and redshift-dependent fraction of Lyman α emission

Stark, Daniel P. and Ellis, Richard S. and Chiu, Kuenley and Ouchi, Masami and Bunker, Andrew (2010) Keck spectroscopy of faint 3 < z < 7 Lyman break galaxies – I. New constraints on cosmic reionization from the luminosity and redshift-dependent fraction of Lyman α emission. Monthly Notices of the Royal Astronomical Society, 408 (3). pp. 1628-1648. ISSN 0035-8711. http://resolver.caltech.edu/CaltechAUTHORS:20101110-110651236

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Abstract

We present the first results of a new Keck spectroscopic survey of UV faint Lyman break galaxies in the redshift range 3 < z < 7. Combined with earlier Keck and published European Southern Observatory (ESO) VLT data, our spectroscopic sample contains more than 600 dropouts offering new insight into the nature of sub-L* sources typical of those likely to dominate the cosmic reionization process. In this first paper, in a series discussing these observations, we characterize the fraction of strong Lyα emitters within the continuum-selected dropout population. By quantifying how the ‘Lyα fraction’, x_(Lyα), varies with redshift, we seek to constrain changes in Lyα transmission associated with reionization. In order to distinguish the effects of reionization from other factors which affect the Lyα fraction [e.g. dust, interstellar medium (ISM) kinematics], we study the luminosity and redshift-dependence of the Lyα fraction over 3 ≲ z ≲ 6, when the intergalactic medium (IGM) is known to be ionized. These results reveal that low-luminosity galaxies show strong Lyα emission much more frequently (x_(Lyα)= 0.47 ± 0.16 at M_(UV)=−19) than luminous systems (x_(Lyα)= 0.08 ± 0.02 at MUV=−21), and that at fixed luminosity, the prevalence of strong Lyman α emission increases moderately with redshift over 3 < z < 6 (d x_(Lyα)/d z= 0.05 ± 0.03). Based on the bluer mean UV slopes of the strong Lyα emitting galaxies in our data set (〈β〉_(Lyα)−〈β〉_(no Lyα)=−0.33 ± 0.09 at MUV=−20.5) we argue that the Lyα fraction trends are governed by redshift and luminosity-dependent variations in the dust obscuration, with likely additional contributions from trends in the kinematics and covering fraction of neutral hydrogen. Using the limited infrared spectroscopy of candidate z ≃ 7 galaxies, we find a tentative decrease in the Lyα fraction by a factor of > 1.9 with respect to the predicted z ≃ 7 value, a result which, if confirmed with future surveys, would suggest an increase in the neutral fraction by this epoch. Given the abundant supply of z and Y drops now available from deep Hubble WFC3/IR surveys, we show it will soon be possible to significantly improve estimates of the Lyα fraction using optical and near-infrared multi-object spectrographs, thereby extending the study conducted in this paper to 7 ≲ z ≲ 8.


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http://dx.doi.org/10.1111/j.1365-2966.2010.17227.xDOIUNSPECIFIED
http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2966.2010.17227.x/abstractPublisherUNSPECIFIED
Additional Information:© 2010 The Authors. Journal compilation © 2010 RAS. Accepted 2010 June 21. Received 2010 June 21; in original form 2010 April 1. Article first published online: 14 Sep 2010. DPS acknowledges financial support from a postdoctoral fellowship from the Science Technology and Research Council and a Schlumberger Interdisciplinary Research Fellow at Darwin College. RSE acknowledges financial support from the Royal Society. We thank our referee, Simon Morris, for very thoughtful comments which improved the quality of the paper. We acknowledge useful conversations with George Becker, Jim Dunlop, Anna Quider, Ross McLure, Johan Richard, Elizabeth Stanway and Stephen Wilkins, and we thank Sebastiano Cantalupo for comments after reading an earlier version of the manuscript.
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Science Technology and Research Council UNSPECIFIED
Darwin College Schlumberger Interdisciplinary Research FellowUNSPECIFIED
Royal SocietyUNSPECIFIED
Subject Keywords:galaxies: evolution; galaxies: formation; galaxies: high-redshift; cosmology: observations
Record Number:CaltechAUTHORS:20101110-110651236
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20101110-110651236
Official Citation:Stark, D. P., Ellis, R. S., Chiu, K., Ouchi, M. and Bunker, A. (2010), Keck spectroscopy of faint 3 < z < 7 Lyman break galaxies – I. New constraints on cosmic reionization from the luminosity and redshift-dependent fraction of Lyman α emission. Monthly Notices of the Royal Astronomical Society, 408: 1628–1648. doi: 10.1111/j.1365-2966.2010.17227.x
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:20743
Collection:CaltechAUTHORS
Deposited By: Jason Perez
Deposited On:11 Nov 2010 16:12
Last Modified:25 Mar 2014 19:54

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