Aims. The development of a general code for 3D Lyα radiation transfer in galaxies to understand the diversity of Lyα line profiles observed in star-forming galaxies and related objects. Methods. Using a Monte Carlo technique, we developed a 3D Lyα radiation transfer code that allows for prescribed arbitrary hydrogen density, ionisation, temperature structures, dust distributions, arbitrary velocity fields, and UV photon sources. As a first test and application we examined the Lyα line profiles predicted for several simple geometrical configurations and their dependence on the main input parameters. Results. Overall, we find line profiles reaching from doubly peaked symmetric emission to symmetric Voigt (absorption) in static configurations with increasing dust content, and asymmetric red-(blue-) shifted emission lines with a blue (red) counterpart ranging from absorption to emission (with increasing line/continuum strength) in expanding (infalling) media. In particular we find the following results to be interesting for the interpretation of Lyα profiles from galaxies. 1) Standard Lyα absorption line fitting of global spectra of galaxies may lead to an underestimation of the true hydrogen column density in certain geometrical conditions; 2) Normal (inverted) P-Cygni-like Lyα profiles can be obtained in expanding (infalling) media from objects without any intrinsic Lyα emission, as a natural consequence of radiation transfer effects; 3) The formation and the detailed shape of Lyα profiles resulting from expanding shells has been thoroughly revised. In particular we find that, for sufficiently large column densities (N H > ∼ 10 20 cm −2 ), the position of the main Lyα emission peak is quite generally redshifted by approximately twice the expansion velocity. This is in excellent agreement with the observations of z ∼ 3 LBGs, which show that Lyα is redshifted by ∼2V exp , where V exp is the expansion velocity measured from the interstellar absorption lines blueshifted with respect to the stellar redshift. This finding also indicates that large-scale, fairly symmetric shell structures must be a good description of the outflows in LBGs.

Direct Lyα imaging of intergalactic gas atz 2 has recently revealed giant cosmological structures around quasars, e.g., the Slug Nebula. Despite their high luminosity, the detection rate of such systems in narrow-band and spectroscopic surveys is less than 10%, possibly encoding crucial information on the distribution of gas around quasars and the quasar emission properties. In this study, we use the MUSE integral-field instrument to perform a blind survey for giant a Ly nebulae around 17 bright radio-quiet quasars at < < z 3 4 that does not suffer from most of the limitations of previous surveys. After data reduction and analysis performed with specifically developed tools, we found that each quasar is surrounded by giant a Ly nebulae with projected sizes larger than 100 physical kiloparsecs and, in some cases, extending up to 320 kpc. The circularly averaged surface brightness profiles of the nebulae appear to be very similar to each other despite their different morphologies and are consistent with power laws with slopes»-1.8. The similarity between the properties of all these nebulae and the Slug Nebula suggests a similar origin for all systems and that a large fraction of gas around bright quasars could be in a relatively "cold" (T ∼ 10 4 K) and dense phase. In addition, our results imply that such gas is ubiquitous within at least 50 kpc from bright quasars at < < z 3 4 independently of the quasar emission opening angle, or extending up to 200 kpc for quasar isotropic emission.

Aims. The aim of our study is to understand the variety of observed Lyα line profiles and strengths in Lyman Break Galaxies (LBGs) and Lyα emitters (LAEs), the physical parameters governing them, and hence derive constraints on the gas and dust content and stellar populations of these objects. Methods. Using our 3D Lyα radiation transfer code including gas and dust, MCLya, we fit 11 LBGs from the FORS Deep Field with redshifts between 2.8 and 5. A simple geometry of a spherically expanding shell of H i is adopted. Results. The variety of observed Lyα profiles is successfully reproduced. Most objects show outflow velocities of V exp ∼ 150−200 km s −1 ; two objects are most likely quasi-static. The radial H i column density ranges from N HI ∼ 2 × 10 19 to 7 × 10 20 cm −2. Our Lyα profile fits yield values of E(B − V) ∼ 0.05−0.2 for the gas extinction. We find indications for a dust-togas ratio higher than the Galactic value, and for a substantial scatter. The escape fraction of Lyα photons is found to be determined primarily by the extinction, and a simple fit formula is proposed. In this case a measurement of EW(Lyα) obs can yield E(B − V), if the intrinsic Lyα equivalent width is known (or assumed). Intrinsic EW(Lyα) int ∼ 50−100 Å are found for 8/11 objects, as expected for stellar populations forming constantly over long periods (> ∼ 10−100 Myr). In three cases we found indications of younger populations. Our model results also allow us to understand observed correlations between EW(Lyα) obs and other observables such as FWHM(Lyα), E(B − V), SFR(UV) etc. We suggest that most observed trends of Lyα, both in LBGs and LAEs, are driven by variations of N HI and the accompanying variation of the dust content. Ultimately, the main parameter responsible for these variations may be the galaxy mass. We also show that there is a clear overlap between LBGs and LAEs: at z

Aims. We propose to infer the output of the ionising continuum-leaking properties of galaxies based upon their Lyα line profiles. Methods. We carried out Lyα radiation transfer calculations in two models of H ii regions. These models are porous to ionising continuum escape: 1) we define Lyman-continuum (LyC) optically thin star clusters, in which massive stars produce enough ionising photons to keep the surrounding interstellar medium transparent to the ionising continuum, in other words, almost totally ionised; and 2) we define riddled ionisation-bounded media that are surrounded by neutral interstellar medium, but have holes, which results in a covering fraction lower than unity. Results. The Lyα spectra that emerge from these configurations have distinctive features: 1) a classical asymmetric redshifted profile in the first case, but with a small shift of the profile maximum compared to the systemic redshift (v peak ≤ 150 km s −1 ); 2) a main peak at the systemic redshift in the second case (v peak = 0), with a non-zero Lyα flux bluewards of the systemic redshift as a consequence. If in a galaxy that leaks ionising photons the Lyα component that emerges from the leaking star cluster(s) is assumed to dominate the total Lyα spectrum, the Lyα shape may be used as a pre-selection tool for detecting LyC-leaking galaxies in objects with high spectral resolution Lyα spectra (R ≥ 4000). Our predictions are corroborated by examination of a sample of ten local starbursts with high-resolution HST/COS Lyα spectra that are known in the literature as LyC leakers or leaking candidates. Conclusions. Observations of Lyα profiles at high resolution are expected to show definite signatures revealing the escape of Lyman-continuum photons from star-forming galaxies.

We report the detection of extended Lyα haloes around 145 individual star-forming galaxies at redshifts 3 ≤ z ≤ 6 in the Hubble Ultra Deep Field observed with the Multi-Unit Spectroscopic Explorer (MUSE) at ESO-VLT. Our sample consists of continuum-faint (−15 ≥ M UV ≥ −22) Lyα emitters (LAEs). Using a 2D, two-component (continuum-like and halo) decomposition of Lyα emission assuming circular exponential distributions, we measure scale lengths and luminosities of Lyα haloes. We find that 80% of our objects having reliable Lyα halo measurements show Lyα emission that is significantly more extended than the UV continuum detected by HST (by a factor ≈4 to >20). The median exponential scale length of the Lyα haloes in our sample is ≈4.5 kpc with a few haloes exceeding 10 kpc. By comparing the maximal detected extent of the Lyα emission with the predicted dark matter halo virial radii of simulated galaxies, we show that the detected Lyα emission of our selected sample of Lyα emitters probes a significant portion of the cold circum-galactic medium of these galaxies (>50% in average). This result therefore shows that there must be significant HI reservoirs in the circum-galactic medium and reinforces the idea that Lyα haloes are ubiquitous around high-redshift Lyα emitting galaxies. Our characterization of the Lyα haloes indicates that the majority of the Lyα flux comes from the halo (≈65%) and that their scale lengths seem to be linked to the UV properties of the galaxies (sizes and magnitudes). We do not observe a significant Lyα halo size evolution with redshift, although our sample for z > 5 is very small. We also explore the diversity of the Lyα line profiles in our sample and we find that the Lyα lines cover a large range of full width at half maximum (FWHM) from 118 to 512 km s −1 . While the FWHM does not seem to be correlated to the Lyα scale length, most compact Lyα haloes and those that are not detected with high significance tend to have narrower Lyα profiles (<350 km s −1 ). Finally, we investigate the origin of the extended Lyα emission but we conclude that our data do not allow us to disentangle the possible processes, i.e. scattering from star-forming regions, fluorescence, cooling radiation from cold gas accretion, and emission from satellite galaxies.

We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of five star-forming galaxies at redshifts z in the range 0.2993 -0.4317 and with high emission-line flux ratios O 32 = [O iii]λ5007/[O ii]λ3727 ∼ 8 -27 aiming to detect the Lyman continuum (LyC) emission. We detect LyC emission in all galaxies with the escape fractions f esc (LyC) in a range of 2 -72 per cent. A narrow Lyα emission line with two peaks in four galaxies and with three peaks in one object is seen in medium-resolution COS spectra with a velocity separation between the peaks V sep varying from ∼153 km s −1 to ∼ 345 km s −1 . We find a general increase of the LyC escape fraction with increasing O 32 and decreasing stellar mass M ⋆ , but with a large scatter of f esc (LyC). A tight anti-correlation is found between f esc (LyC) and V sep making V sep a good parameter for the indirect determination of the LyC escape fraction. We argue that one possible source driving the escape of ionizing radiation is stellar winds and radiation from hot massive stars.

We report the detection of extended Lyα emission around individual star-forming galaxies at redshifts z = 3−6 in an ultradeep exposure of the Hubble Deep Field South obtained with MUSE on the ESO-VLT. The data reach a limiting surface brightness (1σ) of ∼1 × 10 −19 erg s −1 cm −2 arcsec −2 in azimuthally averaged radial profiles, an order of magnitude improvement over previous narrowband imaging. Our sample consists of 26 spectroscopically confirmed Lyα-emitting, but mostly continuum-faint (m AB > ∼ 27) galaxies. In most objects the Lyα emission is considerably more extended than the UV continuum light. While five of the faintest galaxies in the sample show no significantly detected Lyα haloes, the derived upper limits suggest that this is due to insufficient S/N. Lyα haloes therefore appear to be ubiquitous even for low-mass (∼10 8 −10 9 M ) star-forming galaxies at z > 3. We decompose the Lyα emission of each object into a compact component tracing the UV continuum and an extended halo component, and infer sizes and luminosities of the haloes. The extended Lyα emission approximately follows an exponential surface brightness distribution with a scale length of a few kpc. While these haloes are thus quite modest in terms of their absolute sizes, they are larger by a factor of 5−15 than the corresponding rest-frame UV continuum sources as seen by HST. They are also much more extended, by a factor ∼5, than Lyα haloes around low-redshift star-forming galaxies. Between ∼40% and > ∼ 90% of the observed Lyα flux comes from the extended halo component, with no obvious correlation of this fraction with either the absolute or the relative size of the Lyα halo. Our observations provide direct insights into the spatial distribution of at least partly neutral gas residing in the circumgalactic medium of low to intermediate mass galaxies at z > 3.

One of the key questions in observational cosmology is the identification of the sources responsible for ionisation of the Universe after the cosmic Dark Ages, when the baryonic matter was neutral. The currently identified distant galaxies are insufficient to fully reionise the Universe by redshift z ∼ 6 1-3 , but low-mass star-forming galaxies are thought to be responsible for the bulk of the ionising radiation 4-6 . Since direct observations at high redshift are difficult for a variety of reasons, one solution is to identify local proxies of this galaxy population. However, starburst galaxies at low redshifts are generally opaque to their ionising radiation 7-9 . This radiation with small escape fractions of ∼1-3 % is directly detected only in three low-redshift galaxies 10, 11 . Here we present far-ultraviolet observations of a nearby low-mass star-forming galaxy, J0925+1403, selected for its compactness and high excitation. The galaxy is leaking ionising radiation, with an escape fraction of ∼ 8%. The total number of photons emitted during the starburst phase is sufficient to ionize intergalactic medium material, which is about 40 times more massive than the stellar mass of the galaxy.So-called "Green Peas" (GP), low-mass compact galaxies with very active star formation 12-15 , may be promising candidates for escaping ionising radiation. The GP galaxy J0925+1403 was selected from the Sloan Digital Sky Survey (SDSS) according to the following properties (Methods section): 1) a compact structure; 2) the presence of emission lines with high equivalent widths in its SDSS spectrum, suggesting active ongoing star formation and numerous hot O stars producing ionising Lyman continuum (LyC) radiation; 3) sufficiently bright in the far-ultraviolet (FUV) with a magnitude of 20.7 mag and redshifted enough (z = 0.301) to allow direct LyC observations with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST); and 4) a high O 32 = [OIII]λ5007/[OII]λ3727 flux ratio of 5 (see Fig. 1), which may indicate the presence of density-bounded HII regions 16 , i.e. escaping LyC radiation. We first derive some general properties of the galaxy, using the emission-line fluxes measured from the SDSS optical spectrum. After correction for the Milky Way extinction of A V,MW = 0.084 mag, we obtain an internal extinction A V,int = 0.36 mag, and a low oxygen abundance 12 + logO/H = 7.91±0.03, or less than 0.2 solar. The details of these determinations are given in the Methods 1 section. Everywhere in the paper the errors are 1σ errors.The same SDSS spectrum is used to fit a spectral energy distribution (SED) to derive the galaxy's global parameters, including the stellar mass and the age of the present burst of star formation (see Methods section). We obtain a starburst age of 2.6±0.2 Myr, a young stellar mass of (2.4±0.3)×108 M ⊙ , and a total galaxy stellar mass of (8.2±0.7)×10 8 M ⊙ . The star-formation rate is 52.2 M ⊙ yr −1 , as determined from the extinction-corrected Hβ line flux. With its low mass, low metall...