We present a new flexible Bayesian framework for directly inferring the fraction of neutral hydrogen in the intergalactic medium (IGM) during the Epoch of Reionization (EoR, z ∼ 6 − 10) from detections and non-detections of Lyman Alpha (Lyα) emission from Lyman Break galaxies (LBGs). Our framework combines sophisticated reionization simulations with empirical models of the interstellar medium (ISM) radiative transfer effects on Lyα. We assert that the Lyα line profile emerging from the ISM has an important impact on the resulting transmission of photons through the IGM, and that these line profiles depend on galaxy properties. We model this effect by considering the peak velocity offset of Lyα lines from host galaxies' systemic redshifts, which are empirically correlated with UV luminosity and redshift (or halo mass at fixed redshift). We use our framework on the sample of LBGs presented in Pentericci et al. (2014) and infer a global neutral fraction at z ∼ 7 of x hi = 0.59 +0.11 −0.15 , consistent with other robust probes of the EoR and confirming reionization is on-going ∼ 700 Myr after the Big Bang. We show that using the full distribution of Lyα equivalent width detections and upper limits from LBGs places tighter constraints on the evolving IGM than the standard Lyα emitter fraction, and that larger samples are within reach of deep spectroscopic surveys of gravitationally lensed fields and JWST NIRSpec.

Aims. We examine the influence of nebular continuous and line emission in high redshift star forming galaxies on determinations of their age, formation redshift and other properties from SED fits. Methods. We include nebular emission consistently with the stellar emission in our SED fitting tool and analyse differentially a sample of 10 z ≈ 6 galaxies in the GOODS-S field studied earlier by Eyles et al. (2007). Results. We find that the apparent Balmer/4000 Å breaks observed in a number of z ≈ 6 galaxies detected at > ∼ 3.6 µm with IRAC/Spitzer can be mimicked by the presence of strong restframe optical emission lines, implying in particular younger ages than previously thought. Applying these models to the small sample of z ≈ 6 galaxies, we find that this effect may lead to a typical downward revision of their stellar ages by a factor ∼ 3. In consequence their average formation redshift may drastically be reduced, and these objects may not have contributed to cosmic reionisation at z > 6. Extinction and stellar mass estimates may also be somewhat modified, but to a lesser extent. Conclusions. Careful SED fits including nebular emission and treating properly uncertainties and degeneracies are necessary for more accurate determinations of the physical parameters of high-z galaxies.

The Lyman-alpha (Lyalpha) emission line is the primary observational signature of star-forming galaxies at the highest redshifts, and has enabled the compilation of large samples of galaxies with which to study cosmic evolution. The resonant nature of the line, however, means that Lyalpha photons scatter in the neutral interstellar medium of their host galaxies, and their sensitivity to absorption by interstellar dust may therefore be greatly enhanced. This implies that the Lyalpha luminosity may be significantly reduced, or even completely suppressed. Hitherto, no unbiased empirical test of the escaping fraction (f(esc)) of Lyalpha photons has been performed at high redshifts. Here we report that the average f(esc) from star-forming galaxies at redshift z = 2.2 is just 5 per cent by performing a blind narrowband survey in Lyalpha and Halpha. This implies that numerous conclusions based on Lyalpha-selected samples will require upwards revision by an order of magnitude and we provide a benchmark for this revision. We demonstrate that almost 90 per cent of star-forming galaxies emit insufficient Lyalpha to be detected by standard selection criteria. Both samples show an anti-correlation of f(esc) with dust content, and we show that Lyalpha- and Halpha-selection recovers populations that differ substantially in dust content and f(esc).

Star-forming galaxies are considered to be the leading candidate sources that dominate the cosmic reionization at z > 7, and the search for analogs at moderate redshift showing Lyman continuum (LyC) leakage is currently a active line of research. We have observed a star-forming galaxy at z = 3.2 with Hubble/WFC3 in the F336W filter, corresponding to the 730-890Å rest-frame, and detect LyC emission. This galaxy is very compact and also has large Oxygen ratio [OIII]λ5007/[OII]λ3727 ( 10). No nuclear activity is revealed from optical/nearinfrared spectroscopy and deep multi-band photometry (including the 6Ms X-ray Chandra). The measured escape fraction of ionizing radiation spans the range 50-100%, depending on the IGM attenuation. The LyC emission is detected with m F336W = 27.57 ± 0.11 (S/N=10) and it is spatially unresolved, with effective radius R e < 200pc. Predictions from photoionization and radiative transfer models are in line with the properties reported here, indicating that stellar winds and supernova explosions in a nucleated star-forming region can blow cavities generating density-bounded conditions compatible with optically thin media. Irrespective to the nature of the ionizing radiation, spectral signatures of these sources over the entire electromagnetic spectrum are of central importance for their identification during the epoch of reionization, when the LyC is unobservable. Intriguingly, the Spitzer/IRAC photometric signature of intense rest-frame optical emissions ([O III]λλ4959, 5007 + Hβ) observed recently at z 7.5 − 8.5 is similar to what is observed in this galaxy. Only the James Webb Space Telescope will measure optical line ratios at z > 7 allowing a direct comparison with lower redshift LyC emitters, as reported here.