Volume 608, December 2017
|Number of page(s)||13|
|Published online||15 December 2017|
VLT/FORS2 view at z ~ 6: Lyman-α emitter fraction and galaxy physical properties at the edge of the epoch of cosmic reionization
1 Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
2 INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
3 INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio, Italy
4 School of Physics & Astronomy, The University of Nottingham, University Park, Nottingham NG7 2RD, UK
5 Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211, USA
6 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
7 INAF–Osservatorio Astronomico di Trieste, via G.B. Tiepolo, 11, 34143 Trieste, Italy
8 National Optical Astronomy Observatory, 950 North Cherry Ave, Tucson, AZ 85719, USA
9 Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA
10 Cavendish Astrophysics, University of Cambridge, Cambridge CB3 0HE, UK
11 Kavli Institute for Cosmology, University of Cambridge, Cambridge CB3 0HE, UK
Received: 29 June 2017
Accepted: 4 October 2017
The fraction of Lyman-α emitters (LAEs) among the galaxy population has been found to increase from z ~ 0 to z ~ 6 and drop dramatically at z> 6. This drop has been interpreted as an effect of an increasingly neutral intergalactic medium (IGM) with increasing redshift, while a Lyman continuum escape fraction evolving with redshift and/or a sudden change of galaxy physical properties can also contribute to the decreasing LAE fraction. We report the result of a large VLT/FORS2 program aiming to confirm spectroscopically a large galaxy sample at z ≥ 6 that has been selected in several independent fields through the Lyman break technique. Combining those data with archival data, we create a large and homogeneous sample of z ~ 6 galaxies (N = 127), complete in terms of Lyα detection at > 95% for Lyα equivalent width EW(Lyα) ≥ 25 Å. We use this sample to derive a new measurement of the LAE fraction at z ~ 6 and derive the physical properties of these galaxies through spectral energy distribution (SED) fitting. We find a median LAE fraction at z ~ 6 lower than in previous studies, while our sample exhibits typical properties for z ~ 6 galaxies in terms of UV luminosity and UVβ slope. The comparison of galaxy physical properties between LAEs and non-LAEs is comparable to results at lower redshift: LAEs with the largest EW(Lyα) exhibit bluer UV slopes, are slightly less massive and less star-forming. The main difference between LAEs and non-LAEs is that the latter are significantly dustier. Using predictions of our SED fitting code accounting for nebular emission, we find an effective Lyα escape fraction fesceff(Lyα) = 0.23-0.17+0.36 remarkably consistent with the value derived by comparing UV luminosity function with Lyα luminosity function. We conclude that the drop in the LAE fraction from z ~ 6 to z> 6 is less dramatic than previously found and the effect of an increasing IGM neutral fraction is possibly observed at 5 <z< 6. The processes driving the escape of Lyα photons at z ~ 6 are similar to those at lower redshifts and based on our derived fesceff(Lyα), we find that the IGM has a relatively small impact on Lyα photon visibility at z ~ 6, with a lower limit for the IGM transmission to Lyα photons, TIGM ≳ 0.20, likely due to the presence of outflows.
Key words: galaxies: high-redshift / galaxies: evolution / dark ages, reionization, first stars
© ESO, 2017
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