Abstract
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.
Highlights
Cosmic reionization was a major phase transition in the early history of the Universe, and great efforts have been made in the past decade to place constraints on when and how it occurred, as well as to identify the main sources of ionizing photons
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 contribute to the decreasing Lyman-α emitters (LAEs) fraction
The overall conclusion of these surveys is that a drop in the Lyman-α emitter (LAE) fraction among the Lyman break galaxy (LBG) population is observed from z ∼ 6 to z ∼ 7 (e.g., Fontana et al 2010; Ouchi et al 2010; Stark et al 2011; Pentericci et al 2011; Ono et al 2012; Ota et al 2012; Caruana et al 2014; Schenker et al 2014), leading to the conclusion that cosmic reionization ended between z ∼ 6 and z ∼ 7
Summary
Cosmic reionization was a major phase transition in the early history of the Universe, and great efforts have been made in the past decade to place constraints on when and how it occurred, as well as to identify the main sources of ionizing photons. Alternative explanations have been proposed, such as an increase in incidence of optically thick systems that would require a lower neutral fraction (Bolton & Haehnelt 2013), or a contribution from evolving galaxy properties (Mesinger et al 2015) It has been pointed out in several studies that the IGM might strongly affect Lyα visibility even in a fully ionized Universe. While we minimize the number of assumptions in our analysis, we verified that the models reproduce the observed properties, and reproduce the predicted properties for which we do not have empirical constraints at z ∼ 6, namely the [O iii]+Hβ emission line equivalent widths This increases the confidence in the derived physical parameters.
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