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.

Citation for published item:rohskD tF vier nd erkD tessi uF nd orsekD q¡ or nd rippD odd wF nd umlinsonD tson nd furhettD toseph xF nd poxD endrew tF nd pumglliD wihele nd vehnerD xiols nd eeplesD wolly F nd ejosD xiols @PHIUA 9he gyErlos survey X metlliities in the lowEredshift irumglti mediumF9D estrophysil journlFD VQU @PAF pF ITWF Further information on publisher's website: Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. AbstractWe analyze new far-ultraviolet spectra of 13quasars from thez 0.2 COS-Halos survey that cover the H I Lyman limit of 14circumgalactic medium (CGM) systems. These data yield precise estimates or more constraining limits than previous COS-Halos measurements on the H I column densities N H I . We then apply a Monte-Carlo Markov chain approach on 32systems from COS-Halos to estimate the metallicity of the cool (T 10 4 K) CGM gas that gives rise to low-ionization state metal lines, under the assumption of photoionization equilibrium with the extragalactic UV background. The principle results are: (1) the CGM of field L * galaxies exhibits a declining H I surface density with impact parameterR (at >99.5% confidence), (2) the transmission of ionizing radiation through CGM gas alone is 70±7%; (3) the metallicity distribution function of the cool CGM is unimodal with a median of - to >  Z 3 ; the incidence of metal-poor (<  Z 1 100 ) gas is low, implying any such gas discovered along quasar sightlines is typically unrelated to L * galaxies; (4) we find an unexpected increase in gas metallicity with declining N H I (at >99.9% confidence) and, therefore, also with increasingR ; the high metallicity at large radii implies early enrichment; and (5) a non-parametric estimate of the cool CGM gas mass is = ´ ( ) M M 9.2 4.3 10 CGM cool 10, which together with new mass estimates for the hot CGM may resolve the galactic missing baryons problem. Future analyses of halo gas should focus on the underlying astrophysics governing the CGM, rather than processes that simply expel the medium from the halo.

We present new measurements of the free-streaming of warm dark matter (WDM) from Lyman-α flux-power spectra. We use data from the medium resolution, intermediate redshift XQ-100 sample observed with the X-shooter spectrograph (z = 3−4.2) and the high-resolution, high-redshift sample used in obtained with the HIRES/MIKE spectrographs (z = 4.2 − 5.4). Based on further improved modelling of the dependence of the Lyman-α flux-power spectrum on the freestreaming of dark matter, cosmological parameters, as well as the thermal history of the intergalactic medium (IGM) with hydrodynamical simulations, we obtain the following limits, expressed as the equivalent mass of thermal relic WDM particles. The XQ-100 flux power spectrum alone gives a lower limit of 1.4 keV, the re-analysis of the HIRES/MIKE sample gives 4.1 keV while the combined analysis gives our best and significantly strengthened lower limit of 5.3 keV (all 2σ C.L.). The further improvement in the joint analysis is partly due to the fact that the two data sets have different degeneracies between astrophysical and cosmological parameters that are broken when the data sets are combined, and more importantly on chosen priors on the thermal evolution. These results all assume that the temperature evolution of the IGM can be modelled as a power law in redshift. Allowing for a non-smooth evolution of the temperature of the IGM with sudden temperature changes of up to 5000K reduces the lower limit for the combined analysis to 3.5 keV. A WDM with smaller thermal relic masses would require, however, a sudden temperature jump of 5000 K or more in the narrow redshift interval z = 4.6 − 4.8, in disagreement with observations of the thermal history based on high-resolution resolution Lyman-α forest data and expectations for photo-heating and cooling in the low density IGM at these redshifts.

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...

The ionising continuum from active galactic nuclei (AGN) is fundamental for interpreting their broad emission lines and understanding their impact on the surrounding gas. Furthermore, it provides hints on how matter accretes onto supermassive black holes. Using HST's Wide Field Camera 3 we have constructed the first stacked ultraviolet (rest-frame wavelengths 600-2500Å) spectrum of 53 luminous quasars at z 2.4, with a state-of-the-art correction for the intervening Lyman forest and Lyman continuum absorption. The continuum slope (f ν ∝ ν αν ) of the full sample shows a break at ∼912Å with spectral index α ν = −0.61 ± 0.01 at λ > 912Å and a softening at shorter wavelengths (α ν = −1.70 ± 0.61 at λ 912Å). Our analysis proves that a proper intergalactic medium absorption correction is required to establish the intrinsic continuum emission of quasars. We interpret our average ultraviolet spectrum in the context of photoionisation, accretion disk models, and quasar contribution to the ultraviolet background. We find that observed broad line ratios are consistent with those predicted assuming an ionising slope of α ion =−2.0, similar to the observed ionising spectrum in the same wavelength range. The continuum break and softening are consistent with accretion disk plus X-ray corona models when black hole spin is taken into account. Our spectral energy distribution yields a 30% increase to previous estimates of the specific quasar emissivity, such that quasars may contribute significantly to the total specific Lyman limit emissivity estimated from the Lyα forest at z < 3.2.