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The relationship between galaxy characteristics and the reionization of the universe remains elusive, mainly due to the observational difficulty in accessing the Lyman continuum (LyC) at these redshifts. It is thus important to identify low-redshift LyC-leaking galaxies that can be used as laboratories to investigate the physical processes that allow LyC photons to escape. The weakness of the [S ii ] nebular emission lines relative to typical star-forming galaxies has been proposed as a LyC predictor. In this paper, we show that the [S ii ] deficiency is an effective method to select LyC-leaking candidates using data from the Low-redshift LyC Survey, which has detected flux below the Lyman edge in 35 out of 66 star-forming galaxies with the Cosmic Origins Spectrograph on board the Hubble Space Telescope. We show that LyC leakers tend to be more [S ii ] deficient and that the fraction of their detections increases as [S ii ] deficiency becomes more prominent. Correlational studies suggest that [S ii ] deficiency complements other LyC diagnostics (such as strong Lyα emission and high [O iii ]/[O ii ]). Our results verify an additional technique by which reionization-era galaxies could be studied. [ABSTRACT FROM AUTHOR]

Understanding the escape of ionizing (Lyman continuum) photons from galaxies is vital for determining how galaxies contributed to reionization in the early universe. While directly detecting the Lyman continuum from high-redshift galaxies is impossible due to the intergalactic medium, low-redshift galaxies in principle offer this possibility but require observations from space. The first local galaxy for which Lyman continuum escape was found is Haro 11, a luminous blue compact galaxy at z = 0.02, where observations with the FUSE satellite revealed an escape fraction of 3.3%. However, the FUSE aperture covers the entire galaxy, and it is not clear from where the Lyman continuum is leaking out. Here we utilize Hubble Space Telescope/Cosmic Origins Spectrograph spectroscopy in the wavelength range 1100–1700 Å of the three knots (A, B, and C) of Haro 11 to study the presence of Lyα emission and the properties of intervening gas. We find that all knots have bright Lyα emission. UV absorption lines, originating in the neutral interstellar medium, as well as lines probing the ionized medium, are seen extending to blueshifted velocities of 500 km s−1 in all three knots, demonstrating the presence of an outflowing multiphase medium. We find that knots A and B have large covering fractions of neutral gas, making LyC escape along these sightlines improbable, while knot C has a much lower covering fraction (≲50%). Knot C also has the the highest Lyα escape fraction, and we conclude that it is the most likely source of the escaping Lyman continuum detected in Haro 11. [ABSTRACT FROM AUTHOR]

We study young star-forming clumps on physical scales of 10–500 pc in the Lyman-Alpha Reference Sample (LARS), a collection of low-redshift (z  = 0.03–0.2) UV-selected star-forming galaxies. In each of the 14 galaxies of the sample, we detect clumps for which we derive sizes and magnitudes in five UV-optical filters. The final sample includes ∼1400 clumps, of which ∼600 have magnitude uncertainties below 0.3 in all filters. The UV luminosity function for the total sample of clumps is described by a power law with slope |$\alpha =-2.03^{+0.11}_{-0.13}$|⁠. Clumps in the LARS galaxies have on average ΣSFR values higher than what is observed in H  ii regions of local galaxies and comparable to typical star formation rate (SFR) densities of clumps in z  = 1–3 galaxies. We derive the clumpiness as the relative contribution from clumps to the UV emission of each galaxy, and study it as a function of galactic-scale properties, i.e. ΣSFR and the ratio between rotational and dispersion velocities of the gas (v s/σ0). We find that in galaxies with higher ΣSFR or lower v s /σ0, clumps dominate the UV emission of their host systems. All LARS galaxies with Ly α escape fractions larger than 10% have more than 50% of the UV luminosity from clumps. We tested the robustness of these results against the effect of different physical resolutions. At low resolution, the measured clumpiness appears more elevated than if we could resolve clumps down to single clusters. This effect is small in the redshift range covered by LARS; thus, our results are not driven by the physical resolution. [ABSTRACT FROM AUTHOR]

Context. Among the different observational techniques used to select high-redshift galaxies, the hydrogen recombination line Lymanalpha (Lya) is of particular interest because it gives access to the measurement of cosmological quantities such as the star formation rate (SFR) of distant galaxy populations. However, interpreting this line and calibrating such observables are still subject to serious uncertainties. Aims. In this context, it important to understand the mechanisms responsible for the attenuation of Lya emission, and under what conditions the Lya emission line can be used as a reliable star formation diagnostic tool. Methods. We used a sample of 24 Lya emitters at z ~ 0.3 with an optical spectroscopic follow-up to calculate the Lya escape fraction and its dependence upon different physical properties. We also examined the reliability of Lya as a SFR indicator. We combined these observations with a compilation of Lya emitters selected at z = 0-0.3 from the literature to assemble a larger sample. Results. We confirm that the Lya escape fraction clearly depends on the dust extinction following the relation fesc(Lya) = CLya × 10-0.4 E(B-V) kLya where kLya ~ 6.67 and CLya = 0.22. However, the correlation does not follow the expected curve for a simple dust attenuation. A higher attenuation can be attributed to a scattering process, while fesc(Lya) values that are clearly above the continuum extinction curve can be the result of various mechanisms that can lead to an enhancement of the Lya output. We also observe that the strength of Lya and the escape fraction appear unrelated to the galaxy metallicity. Regarding the reliability of Lya as a SFR indicator, we show that the deviation of SFR(Lya) from the true SFR (as traced by the UV continuum) is a function of the observed SFR(UV), which can be seen as the decrease in fesc(Lya) with increasing UV luminosity. Moreover, we observe redshift dependence of this relationship, revealing the underlying evolution of fesc(Lya) with redshift. [ABSTRACT FROM AUTHOR]

The Lyman-α (Lyα) emission line is the primary observational signature of star-forming galaxies at the highest redshifts, and has enabled the compilation of large samples of galaxies with which to study cosmic evolution. The resonant nature of the line, however, means that Lyα photons scatter in the neutral interstellar medium of their host galaxies, and their sensitivity to absorption by interstellar dust may therefore be greatly enhanced. This implies that the Lyα luminosity may be significantly reduced, or even completely suppressed. Hitherto, no unbiased empirical test of the escaping fraction (fesc) of Lyα photons has been performed at high redshifts. Here we report that the average fesc from star-forming galaxies at redshift z = 2.2 is just 5 per cent by performing a blind narrowband survey in Lyα and Hα. This implies that numerous conclusions based on Lyα-selected samples will require upwards revision by an order of magnitude and we provide a benchmark for this revision. We demonstrate that almost 90 per cent of star-forming galaxies emit insufficient Lyα to be detected by standard selection criteria. Both samples show an anti-correlation of fesc with dust content, and we show that Lyα- and Hα-selection recovers populations that differ substantially in dust content and fesc. [ABSTRACT FROM AUTHOR]

The long-duration gamma-ray burst (GRB) GRB 030329, associated with the supernova (SN) SN 2003dh, occurred inside a star-forming dwarf galaxy at redshift . The low redshift, and a rich set of archival Hubble Space Telescope ( HST ) images, makes this GRB well suited for a detailed study of the stellar population in the immediate vicinity of the explosion. Since the lifetime of a star is directly tied to its mass, the age of the stellar population can be used to put constraints on the GRB and SN progenitor mass. From the HST images, we extract the colours of the precise site from which the GRB originated, and find that the colours are significantly different from those of the overall host galaxy and the surrounding starburst environment. We have used spectral evolutionary models, including nebular emission, to carefully constrain the age of the stellar population, and hence the progenitor, at the very explosion site. For instantaneous burst models, we find that a population age of ∼5 Myr best matches the data, suggesting a very massive star as the progenitor, with an upper limit of 8 Myr . For more extended star formation scenarios, the inferred progenitor age is in most cases still very young (<7 Myr, implying , with an upper limit of 20 Myr . These age estimates are an order of magnitude lower than the ages inferred from the overall host galaxy colours, indicating that progenitor mass estimates based on data for spatially unresolved GRB host galaxies will in general be of limited use. Our results are consistent with the collapsar scenario. [ABSTRACT FROM AUTHOR]

Lyman α (Lyα) is one of the dominant tools used to probe the star-forming galaxy population at high redshift ( z). However, astrophysical interpretations of data drawn from Lyα alone hinge on the Lyα escape fraction which, due to the complex radiative transport, may vary greatly. Here, we map the Lyα emission from the local luminous blue compact galaxy Haro 11, a known emitter of Lyα and the only known candidate for low- z Lyman continuum emission. To aid in the interpretation, we perform a detailed ultraviolet and optical multiwavelength analysis and model the stellar population, dust distribution, ionizing photon budget, and star-cluster population. We use archival X-ray observations to further constrain properties of the starburst and estimate the neutral hydrogen column density. The Lyα morphology is found to be largely symmetric around a single young star-forming knot and is strongly decoupled from other wavelengths. From general surface photometry, only very slight correlation is found between Lyα and , and the age of the stellar population. Only around the central Lyα bright cluster do we find the Lyα/Hα ratio at values predicted by the recombination theory. The total Lyα escape fraction is found to be just 3 per cent. We compute that ∼90 per cent of the Lyα photons that escape do so after undergoing multiple resonance scattering events, masking their point of origin. This leads to a largely symmetric distribution and, by increasing the distance that photons must travel to escape, decreases the escape probability significantly. While dust must ultimately be responsible for the destruction of Lyα, it plays a little role in governing the observed morphology, which is regulated more by interstellar medium kinematics and geometry. We find tentative evidence for local Lyα equivalent width in the immediate vicinity of star clusters being a function of cluster age, consistent with hydrodynamic studies. We estimate the intrinsic production of ionizing photons and put further constraints of ∼9 per cent on the escaping fraction of photons at 900 Å. [ABSTRACT FROM AUTHOR]

Using photometric redshifts we determine the galaxy population of the clusters of galaxies Cl0016+16 at , Cl1600+41 at , Cl1601+42 at and MS1008–1224 at . Comparing the clusters, we find no evidence for a universal shape of the total luminosity function (LF) at these redshifts. When dividing the LFs into spectral types, we find that the LF of the early-type galaxies alone can be described by a Gaussian, while the LF of the late-type galaxies is well fitted by a Schechter function, suggesting that the separate LFs for different populations may be universal. The difference in the total LFs can mainly be attributed to the varying relative normalization of these populations, implying that clusters with an abundant population of late-type galaxies also have steeper faint-end slopes. In MS1008–1224 we detect a faint blue population that dominates over a population with colours consistent with dwarf ellipticals, opposite to clusters at lower redshift. Compared to low-redshift clusters, we find that a general fading of the late-type population by ∼2 mag and the early-type population by ∼1 mag describes the evolution from to well. As a consequence of the different early-type and late-type LFs and their dependence on cluster radius, the fraction of blue cluster galaxies, as measured by the Butcher–Oemler effect, differs between the clusters and depends on limiting magnitude and radius. We find a correlation between the dwarf-to-giant ratio and the surface density, indicating that the high-density environment in the cluster cores is hostile to dwarf galaxies. [ABSTRACT FROM AUTHOR]

Abstract: This discussion session started off with a quick review of some key results regarding Lyα emitters (LAEs) at low redshift: some illustrative images were shown, some intriguing results highlighted, and some comparison with high redshift LAEs made. Then followed an open discussion for about 1h which touched upon questions such as: how similar are low- and high-z objects, Lyα escape fractions, what observations are needed to go from interesting to understandable results on low-z objects, how do we understand the complexity of Lyα in local galaxies and what does it imply for high-z objects? [Copyright &y& Elsevier]

We present new H I imaging and spectroscopy of the 14 UV-selected star-forming galaxies in the Lyman Alpha Reference Sample (LARS), aimed for a detailed study of the processes governing the production, propagation, and escape of Lyα photons. New H I spectroscopy, obtained with the 100 m Green Bank Telescope (GBT), robustly detects the H I spectral line in 11 of the 14 observed LARS galaxies (although the profiles of two of the galaxies are likely confused by other sources within the GBT beam); the three highest redshift galaxies are not detected at our current sensitivity limits. The GBT profiles are used to derive fundamental H I line properties of the LARS galaxies. We also present new pilot H I spectral line imaging of five of the LARS galaxies obtained with the Karl G. Jansky Very Large Array (VLA). This imaging localizes the H I gas and provides a measurement of the total H I mass in each galaxy. In one system, LARS 03 (UGC 8335 or Arp 238), VLA observations reveal an enormous tidal structure that extends over 160 kpc from the main interacting systems and that contains >109M☼ of H I. We compare various H I properties with global Lyα quantities derived from Hubble Space Telescope measurements. The measurements of the Lyα escape fraction are coupled with the new direct measurements of H I mass and significantly disturbed H I velocities. Our robustly detected sample reveals tentative correlations between the total H I mass and linewidth, and key Lyα tracers. Further, on global scales, these data support a complex coupling between Lyα propagation and the H I properties of the surrounding medium. [ABSTRACT FROM AUTHOR]