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 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.

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...

Following our first detection reported in , we present the detection of Lyman continuum (LyC) radiation of four other compact star-forming galaxies observed with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST). These galaxies, at redshifts of z ∼ 0.3, are characterized by high emission-line flux ratios5. The escape fractions of the LyC radiation f esc (LyC) in these galaxies are in the range of ∼ 6 % -13 %, the highest values found so far in low-redshift star-forming galaxies. Narrow double-peaked Lyα emission lines are detected in the spectra of all four galaxies, compatible with predictions for Lyman continuum leakers. We find escape fractions of Lyα, f esc (Lyα) ∼ 20% -40%, among the highest known for Lyα emitters (LAEs). Surface brightness profiles produced from the COS acquisition images reveal bright star-forming regions in the center and exponential discs in the outskirts with disc scale lengths α in the range ∼ 0.6 -1.4 kpc. Our galaxies are characterized by low metallicity, ∼ 1/8 − 1/5 solar, low stellar mass ∼ (0.2 -4) × 10 9 M , high star formation rates SFR ∼ 14 -36 M yr −1 , and high SFR densities Σ ∼ 2 -35 M yr −1 kpc −2 . These properties are comparable to those of high-redshift star-forming galaxies. Finally, our observations, combined with our first detection reported in , reveal that a selection for compact star-forming galaxies showing high [O iii]λ5007/[O ii]λ3727 ratios appears to pick up very efficiently sources with escaping Lyman continuum radiation: all five of our selected galaxies are LyC leakers.