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Context. When studying the production and escape of Lyman Continuum from galaxies, it is standard to rely on array of indirect observational tracers in preselection of candidate leakers. Aims. In this work, we investigate how much ionizing radiation might be missed due to these selection criteria by completely removing them and performing a search selected purely from rest-frame LyC emission; and how that affects our estimates of the ionizing background. Methods. We invert the conventional method and perform a bottom-up search for Lyman-continuum leaking galaxies at redshifts 2 z 3.5. Using archival data from HST and VLT/MUSE, we run source finding software on UV-filter HST images from the HUDF, and subject all detected sources to a series of tests to eliminate those that are inconsistent with being ionizing sources. Results. We find 6 new and one previously identified candidate leakers with absolute escape fractions ranging from 36% to ∼ 100%. Our filtering criteria eliminate one object previously reported as a candidate ionizing emitter in the literature, while we report nondetection in the rest frame Lyman continuum of two other previously reported sources. We find that our candidates make a contribution to the metagalactic ionizing field of log 10 ( ν ) = 25.32 +0.25 −0.21 and 25.29 +0.27 −0.22 erg s −1 Hz −1 cMpc −3 for the full set of candidates and for the 4 strongest candidates only; both values are higher than but consistent with other recent figures in the literature. Conclusions. Our findings suggest that galaxies that do not meet the usual selection criteria may make a non-negligible contribution to the cosmic ionizing field. We recommend that similar searches be carried out on a larger scale in well-studied fields with both UV and large ancillary data coverage, for example in the full set of CANDELS fields.

We demonstrate how the stellar and nebular conditions in star-forming galaxies modulate the emission and spectral profile of H Ly𝛼 emission line. We examine the net Ly𝛼 output, kinematics, and in particular emission of blue-shifted Ly𝛼 radiation, using spectroscopy from with the Cosmic Origins Spectrograph on HST, giving a sample of 87 galaxies at redshift 𝑧 = 0.05 − 0.44. We contrast the Ly𝛼 spectral measurements with properties of the ionized gas (from optical spectra) and stars (from stellar modeling). We demonstrate correlations of unprecedented strength between the Ly𝛼 escape fraction (and equivalent width) and the ionization parameter (𝑝 ≈ 10 −15 ). The relative contribution of blue-shifted emission to the total Ly𝛼 also increases from ≈ 0 to ≈ 40 % over the range of O 32 ratios (𝑝 ≈ 10 −6 ). We also find particularly strong correlations with estimators of stellar age and nebular abundance, and weaker correlations regarding thermodynamic variables. Low ionization stage absorption lines suggest the Ly𝛼 emission and line profile are predominantly governed by the column of absorbing gas near zero velocity. Simultaneous multi-parametric analysis over many variables shows we can predict 80 % of the variance on Ly𝛼 luminosity, and ∼ 50 % on the EW. We determine the most crucial predictive variables, finding that for tracers of the ionization state and H𝛽 luminosity dominate the luminosity prediction whereas the Ly𝛼 EW is best predicted by H𝛽 EW and the H𝛼/H𝛽 ratio. We discuss our results with reference to high redshift observations, focussing upon the use of Ly𝛼 to probe the nebular conditions in high-𝑧 galaxies and cosmic reionization.

We present Lyα imaging of 45 low-redshift star-forming galaxies observed with the Hubble Space Telescope. The galaxies have been selected to have moderate to high star formation rates (SFRs) using far-ultraviolet (FUV) luminosity and Hα equivalent width criteria, but no constraints on Lyα luminosity. We employ a pixel stellar continuum fitting code to obtain accurate continuum-subtracted Lyα, Hα, and Hβ maps. We find that Lyα is less concentrated than FUV and optical line emission in almost all galaxies with significant Lyα emission. We present global measurements of Lyα and other quantities measured in apertures designed to capture all of the Lyα emission. We then show how the escape fraction of Lyα relates to a number of other measured quantities (mass, metallicity, star formation, ionization parameter, and extinction). We find that the escape fraction is strongly anticorrelated with nebular and stellar extinction, weakly anticorrelated with stellar mass, but no conclusive evidence for correlations with other quantities. We show that Lyα escape fractions are inconsistent with common dust extinction laws, and discuss how a combination of radiative transfer effects and clumpy dust models can help resolve the discrepancies. We present an SFR calibration based on Lyα luminosity, where the equivalent width of Lyα is used to correct for nonunity escape fraction, and show that this relation provides a reasonably accurate SFR estimate. We also show stacked growth curves of Lyα for the galaxies that can be used to find aperture loss fractions at a given physical radius.