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.

We investigate the morphology of the [Cii] emission in a sample of "normal" star-forming galaxies at 5 < z < 7.2 in relation to their UV (rest-frame) counterpart. We use new ALMA observations of galaxies at z ∼ 6−7, as well as a careful re-analysis of archival ALMA data. In total 29 galaxies were analysed, 21 of which are detected in [Cii]. For several of the latter the [Cii] emission breaks into multiple components. Only a fraction of these [Cii] components, if any, is associated with the primary UV systems, while the bulk of the [Cii] emission is associated either with fainter UV components, or not associated with any UV counterpart at the current limits. By taking into account the presence of all these components, we find that the L [CII] -SFR relation at early epochs is fully consistent with the local relation, but it has a dispersion of 0.48±0.07 dex, which is about two times larger than observed locally. We also find that the deviation from the local L [CII] -SFR relation has a weak anti-correlation with the EW(Lyα). The morphological analysis also reveals that [Cii] emission is generally much more extended than the UV emission. As a consequence, these primordial galaxies are characterised by a [Cii] surface brightness generally much lower than expected from the local Σ [CII] − Σ SFR relation. These properties are likely a consequence of a combination of different effects, namely: gas metallicity, [Cii] emission from obscured star-forming regions, strong variations of the ionisation parameter, and circumgalactic gas in accretion or ejected by these primeval galaxies.

Aims. We analyse the spectral energy distributions (SEDs) of the most distant galaxies discovered with the Hubble Space telescope and from the COSMOS survey and determine their physical properties, such as stellar age and mass, dust attenuation, and star-formation rate. Methods. We use our SED fitting tool including the effects of nebular emission to analyse three samples of z ∼ 6−8 galaxies with observed magnitudes J AB ∼ 23 to 29. Our models cover a wide parameter space. Results. We find that the physical parameters of most galaxies cover a wide range of acceptable values. Stellar ages, in particular, are not strongly constrained, even for objects detected longward of the Balmer break. As already pointed out earlier, the effects of nebular lines significantly affect the age determinations of star-forming galaxies at z ∼ 6−8. We find no need for stellar populations with extreme metallicities or other non-standard assumptions (IMF, escape fraction) to explain the observed properties of faint z-dropout galaxies. Albeit with large uncertainties, our fit results show indications of dust attenuation in some of the z ≈ 6−8 galaxies, which have best-fit values of A V up to ∼1. Furthermore, we find a possible trend of increasing dust attenuation with galaxy mass, and a relatively large scatter in specific star-formation rates, SFR/M . Conclusions. The physical parameters of very high-z galaxies may be more uncertain than indicated by previous studies. Dust attenuation seems also to be present in some z ≈ 6−8 galaxies, and may be correlated with galaxy mass, as is also the case for SFR.

Context. To gain insight on the mass assembly and place constraints on the star formation history (SFH) of Lyman break galaxies (LBGs), it is important to accurately determine their properties. Aims. We estimate how nebular emission and different SFHs affect parameter estimation of LBGs.Methods. We present a homogeneous, detailed analysis of the spectral energy distribution (SED) of ∼1700 LBGs from the GOODS-MUSIC catalogue with deep multi-wavelength photometry from the U band to 8 μm to determine stellar mass, age, dust attenuation, and star formation rate. Using our SED fitting tool, which takes into account nebular emission, we explore a wide parameter space. We also explore a set of different star formation histories. Results. Nebular emission is found to significantly affect the determination of the physical parameters for the majority of z ∼ 3−6 LBGs. We identify two populations of galaxies by determining the importance of the contribution of emission lines to broadband fluxes. We find that ∼65% of LBGs show detectable signs of emission lines, whereas ∼35% show weak or no emission lines. This distribution is found over the entire redshift range. We interpret these groups as actively star-forming and more quiescent LBGs, respectively. We find that it is necessary to considerer SED fits with very young ages (<50 Myr) to reproduce some colours affected by strong emission lines. Other arguments favouring episodic star formation and relatively short star formation timescales are also discussed. Considering nebular emission generally leads to a younger age, lower stellar mass, higher dust attenuation, higher star formation rate, and a large scatter in the SFR-M relation. Our analysis yields a trend of increasing specific star formation rate with redshift, as predicted by recent galaxy evolution models. Conclusions. The physical parameters of approximately two thirds of high redshift galaxies are significantly modified when we account for nebular emission. The SED models, which include nebular emission shed new light on the properties of LBGs with numerous important implications.

We report on five compact, extremely young (< 10 Myr) and blue (β U V < −2.5, F λ = λ β ) objects observed with VLT/MUSE at redshift 3.1169, 3.235, in addition to three objects at z = 6.145. These sources are magnified by the Hubble Frontier Field galaxy clusters MACS J0416 and AS1063. Their de-lensed half light radii (R e ) are between 16 to 140 pc, the stellar masses are 1 − 20 × 10 6 M , the magnitudes are m U V = 28.8 − 31.4 (−17 < M U V < −15) and specific star formation rates can be as large as ∼ 800 Gyr −1 . Multiple images of these systems are widely separated in the sky (up to 50 ) and individually magnified by factors 3-40. Remarkably, the inferred physical properties of two objects are similar to those expected in some globular cluster formation scenarios, representing the best candidate proto-globular clusters (proto-GC) discovered so far. Rest-frame optical high dispersion spectroscopy of one of them at z = 3.1169 yields a velocity dispersion σ v 20 km s −1 , implying a dynamical mass dominated by the stellar mass. Another object at z = 6.145, with de-lensed31.4), shows a stellar mass and a star-formation rate surface density consistent with the values expected from popular GC formation scenarios. An additional star-forming region at z = 6.145, with de-lensed m U V 32, a stellar mass of 0.5 ×10 6 M and a star formation rate of 0.06 M yr −1 is also identified. These objects currently represent the faintest spectroscopically confirmed star-forming systems at z > 3, elusive even in the deepest blank fields. We discuss how proto-GCs might contribute to the ionization budget of the universe and augment Lyα visibility during reionization. This work underlines the crucial role of JWST in characterizing the restframe optical and near-infrared properties of such low-luminosity high−z objects.