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.

Aims. We have recently reported the discovery of five low redshift Lyman continuum (LyC) emitters (LCEs, hereafter) with absolute escape fractions f LyC esc ranging from 6 to 13%, higher than previously found, and which more than doubles the number of low redshift LCEs. We use these observations to test theoretical predictions about a link between the characteristics of the Lyman-alpha (Lyα) line from galaxies and the escape of ionising photons. Methods. We analyse the Lyα spectra of eight LCEs of the local Universe observed with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope (our five leakers and three galaxies from the litterature), and compare their strengths and shapes to the theoretical criteria and comparison samples of local galaxies: the Lyman Alpha Reference Survey, Lyman Break Analogs, Green Peas, and the high-redshift strong LyC leaker Ion2. Results. Our LCEs are found to be strong Lyα emitters, with high equivalent widths, EW(Lyα)> 70 Å, and large Lyα escape fractions, f Lyα esc > 20%. The Lyα profiles are all double-peaked with a small peak separation, in agreement with our theoretical expectations. They also have no underlying absorption at the Lyα position. All these characteristics are very different from the Lyα properties of typical star-forming galaxies of the local Universe. A subset of the comparison samples (2-3 Green Pea galaxies) share these extreme values, indicating that they could also be leaking. We also find a strong correlation between the star formation rate surface density and the escape fraction of ionising photons, indicating that the compactness of star-forming regions plays a role in shaping low column density paths in the interstellar medium of LCEs. Conclusions. The Lyα properties of LCEs are peculiar: Lyα can be used as a reliable tracer of LyC escape from galaxies, in complement to other indirect diagnostics proposed in the literature.

We report the detection of the Lyman continuum (LyC) radiation of the compact starforming galaxy (SFG) J1154+2443 observed with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope. This galaxy, at a redshift of z = 0.3690, is characterized by a high emission-line flux ratio O 32 = [O iii]λ5007/[O ii]λ3727 = 11.5. The escape fraction of the LyC radiation f esc (LyC) in this galaxy is 46 per cent, the highest value found so far in low-redshift SFGs and one of the highest values found in galaxies at any redshift. The narrow double-peaked Lyα emission line is detected in the spectrum of J1154+2443 with a separation between the peaks V sep of 199 km s −1 , one of the lowest known for Lyα-emitting galaxies, implying a high f esc (Lyα). Comparing the extinction-corrected Lyα/Hβ flux ratio with the case B value we find f esc (Lyα) = 98 per cent. Our observations, combined with previous detections in the literature, reveal an increase of O 32 with increasing f esc (LyC). We also find a tight anticorrelation between f esc (LyC) and V sep . The surface brightness profile derived from the COS acquisition image reveals a bright star-forming region in the centre and an exponential disc in the outskirts with a disc scale length α = 1.09 kpc. J1154+2443, compared to other known low-redshift LyC leakers, is characterized by the lowest metallicity, 12+logO/H = 7.65±0.01, the lowest stellar mass M = 10 8.20 M , a similar star formation rate SFR = 18.9 M yr −1 and a high specific SFR of 1.2×10 −7 yr −1 .

We report upon new results regarding the Lyα output of galaxies, derived from the Lyman alpha Reference Sample (LARS), focusing on Hubble Space Telescope imaging. For 14 galaxies we present intensity images in Lyα, Hα, and UV, and maps of Hα/Hβ, Lyα equivalent width (EW), and Lyα/Hα. We present Lyα and UV light profiles and show they are well-fitted by Sérsic profiles, but Lyα profiles show indices systematically lower than those of the UV (n ≈ 1 − 2 instead of 4). This reveals a general lack of the central concentration in Lyα that is ubiquitous in the UV. Photometric growth curves increase more slowly for Lyα than the FUV, showing that small apertures may underestimate the EW. For most galaxies, however, flux and EW curves flatten by radii ≈ 10 kpc, suggesting that if placed at high-z , only a few of our galaxies would suffer from large flux losses. We compute global properties of the sample in large apertures, and show total luminosities to be independent of all other quantities. Normalized Lyα throughput, however, shows significant correlations: escape is found to be higher in galaxies of lower star formation rate, dust content, mass, and several quantities that suggest harder ionizing continuum and lower metallicity. Eight galaxies could be selected as high-z Lyα emitters, based upon their luminosity and EW. We discuss the results in the context of high-z Lyα and UV samples. A few galaxies have EWs above 50Å, and one shows f Lyα esc of 80%; such objects have not previously been reported at low-z.

We report on new imaging observations of the Lyman alpha emission line (Lyα), performed with the Hubble Space Telescope, that comprise the backbone of the Lyman alpha Reference Sample (LARS). We present images of 14 starburst galaxies at redshifts 0.028 < z < 0.18 in continuum-subtracted Lyα, Hα, and the far ultraviolet continuum. We show that Lyα is emitted on scales that systematically exceed those of the massive stellar population and recombination nebulae: as measured by the Petrosian 20 percent radius, R P20 , Lyα radii are larger than those of Hα by factors ranging from 1 to 3.6, with an average of 2.4. The average ratio of Lyα-to-FUV radii is 2.9. This suggests that much of the Lyα light is pushed to large radii by resonance scattering. Defining the Relative Petrosian Extension of Lyα compared to Hα, ξ Lyα = R Lyα P20 /R Hα P20 , we find ξ Lyα to be uncorrelated with total Lyα luminosity. However ξ Lyα is strongly correlated with quantities that scale with dust content, in the sense that a low dust abundance is a necessary requirement (although not the only one) in order to spread Lyα photons throughout the interstellar medium and drive a large extended Lyα halo.