Context. The Lyman-α (Lyα) line of hydrogen is a well-known tracer of galaxies at high redshift. However, the connection between Lyα observables and galaxy properties has not been fully established, limiting the use of the line to probe the physics of galaxies. Aims. Here, we derive the global neutral hydrogen gas (Hi) properties of nearby Lyα-emitting galaxies to assess the impact of neutral gas on the Lyα output of galaxies. Methods. We observed the 21 cm line emission using the Karl G. Jansky Very Large Array in D-array configuration (∼5500 resolution, ∼38 kpc) for 37 star-forming galaxies with available Hubble Space Telescope (HST) Lyα imaging from the Lyman Alpha Reference Samples. Results. We detected 21 cm emission for 33 out of the 37 galaxies observed. We found no significant correlation of global Hi properties (including Hi mass, column density, gas fraction, depletion time, line width, or velocity shift between Hi and Lyα), with the Lyα luminosity, escape fractions, or equivalent widths (EW) derived with HST photometry. Additionally, both Lyα-emitters and weak or non-emitters are distributed evenly along the Hi parameter space of optically selected z = 0 galaxies. Around 74% of the sample is undergoing galaxy interaction, this fraction is higher for Lyα-emitters (83% for galaxies with EW ≥ 20 Å) than for weak or non-emitters (70%). Nevertheless, galaxies identified as interacting have Lyα and Hi properties statistically consistent with those of non-interacting galaxies. Conclusions. Our results show that global Hi properties (on scales >30kpc) have little direct impact on the Lyα output from galaxies. Instead, Hi likely regulates Lyα emission on small scales: statistical comparisons of Lyα and high angular resolution 21 cm observations are required to fully assess the role of Hi in Lyα radiative transfer. While our study indicates that major and minor galaxy mergers could play a role in the emission of Lyα photons in the Local Universe, especially for galaxies with high Hi fractions, the line of sight that a system is observed through ultimately determines the Lyα observables.

We measure the gas-phase abundances of the elements He, N, O, Ne, S, Ar, and Fe in an individual H ii region known to be leaking Lyman-continuum photons in the Sunburst Arc, a highly magnified galaxy at redshift z = 2.37. We detect the temperature-sensitive auroral lines [S ii] λλ4070, 4076, [O iii] λ4363, [S iii] λ6314, [O ii] λλ7320, 7330, and [Ne iii] λ3343 in a stacked spectrum of five multiple images of the Lyman-continuum emitter (LCE), from which we directly measure the electron temperature in the low-, intermediate-, and high-ionization zones. We also detect the density-sensitive doublets of [O ii] λλ3727, 3730, [S ii] λλ6718, 6733, and [Ar iv] λλ4713, 4741, which constrain the density in both the low- and high-ionization gas. With these temperature and density measurements, we measure gas-phase abundances with similar rigor as studies of local galaxies and H ii regions. We measure a gas-phase metallicity for the LCE of 12+log(O/H)=7.97±0.05, and find an enhanced nitrogen abundance log(N/O)=. This nitrogen abundance is consistent with enrichment from a population of Wolf–Rayet stars, additional signatures of which are reported in a companion paper. Abundances of sulfur, argon, neon, and iron are consistent with local low-metallicity H ii regions and low-redshift galaxies. This study represents the most complete chemical abundance analysis of an individual H ii region at Cosmic Noon to date, which enables direct comparisons between local H ii regions and those in the distant Universe.

We analyze the far-ultraviolet (1130−1770 Å rest frame) spectroscopy of 20 young (<50 Myr) and massive (>10⁴M_⊙) star clusters (YSCs) in 11 nearby star-forming galaxies. We probe the interstellar gas intervening along the line of sight, detecting several metal absorption lines of a wide range of ionization potentials, from 6.0 to 77.5 eV. Multiple-component Voigt fits to the absorption lines are used to study the kinematics of the gas. We find that nearly all targets in the sample feature gas outflowing from 30 up to 190 km s⁻¹, often in both the neutral and ionized phases. The outflow velocities correlate with the underlying stellar population properties directly linked to the feedback: the mass of the YSCs, the photon production rate, and the instantaneous mechanical luminosity produced by stellar winds and supernovae. We detect a neutral inflow in four targets, which we interpret as likely not associated with the star cluster but tracing larger-scale gas kinematics. A comparison between the outflows' energy and that produced by the associated young stellar populations suggests an average coupling efficiency of 10% with a broad scatter. Our results extend the relation found in previous works between galactic outflows and the host galaxy star formation rate to smaller scales, pointing toward the key role that clustered star formation and feedback play in regulating galaxy growth.

We investigate the Lyα and Lyman continuum (LyC) properties of the Sunburst Arc, a z = 2.37 gravitationally lensed galaxy with a multiply imaged, compact region leaking LyC and a triple-peaked Lyα profile indicating direct Lyα escape. Non-LyC-leaking regions show a redshifted Lyα peak, a redshifted and central Lyα peak, or a triple-peaked Lyα profile. We measure the properties of the Lyα profile from different regions of the galaxy using R ∼ 5000 Magellan/Magellan Echellette spectra. We compare the Lyα spectral properties to LyC and narrowband Lyα maps from Hubble Space Telescope imaging to explore the subgalactic Lyα−LyC connection. We find strong correlations (Pearson correlation coefficient r > 0.6) between the LyC escape fraction and Lyα (1) peak separation v_sep, (2) ratio of the minimum flux density between the redshifted and blueshifted Lyα peaks to continuum flux density f_min/f_cont, and (3) equivalent width. We favor a complex H i geometry to explain the Lyα profiles from non-LyC-leaking regions and suggest two H i geometries that could diffuse and/or rescatter the central Lyα peak from the LyC-leaking region into our sight line across transverse distances of several hundred parsecs. Our results emphasize the complexity of Lyα radiative transfer and its sensitivity to the anisotropies of H i gas on subgalactic scales. Large differences in the physical scales on which we observe spatially variable direct-escape Lyα, blueshifted Lyα, and escaping LyC photons in the Sunburst Arc underscore the importance of resolving the physical scales that govern Lyα and LyC escape.

As the nearest confirmed Lyman continuum (LyC) emitter, Haro 11 is an exceptional laboratory for studying LyC escape processes crucial to cosmic reionization. Our new Hubble Space Telescope/Cosmic Origins Spectrograph G130M/1055 observations of its three star-forming knots now reveal that the observed LyC originates in Knots B and C, with 903–912 Å luminosities of 1.9 ± 1.5 × 10⁴⁰ erg s⁻¹ and 0.9 ± 0.7 × 10⁴⁰ erg s⁻¹, respectively. We derive local escape fractions f_esc,912 = 3.4% ± 2.9% and 5.1% ± 4.3% for Knots B and C, respectively. Our Starburst99 modeling shows dominant populations on the order of ∼1–4 Myr and 1–2 × 10⁷M_⊙ in each knot, with the youngest population in Knot B. Thus, the knot with the strongest LyC detection has the highest LyC production. However, LyC escape is likely less efficient in Knot B than in Knot C due to higher neutral gas covering. Our results therefore stress the importance of the intrinsic ionizing luminosity, and not just the escape fraction, for LyC detection. Similarly, the Lyα escape fraction does not consistently correlate with LyC flux, nor do narrow Lyα red peaks. High observed Lyα luminosity and low Lyα peak velocity separation, however, do correlate with higher LyC escape. Another insight comes from the undetected Knot A, which drives the Green Pea properties of Haro 11. Its density-bounded conditions suggest highly anisotropic LyC escape. Finally, both of the LyC-leaking Knots, B and C, host ultraluminous X-ray sources (ULXs). While stars strongly dominate over the ULXs in LyC emission, this intriguing coincidence underscores the importance of unveiling the role of accretors in LyC escape and reionization.

We report the detection of a population of Wolf-Rayet (WR) stars in the Sunburst Arc, a strongly gravitationally lensed galaxy at redshift z = 2.37. As the brightest known lensed galaxy, the Sunburst Arc has become an important cosmic laboratory for studying star and cluster formation, Lyman α (Lyα) radiative transfer, and Lyman continuum (LyC) escape. Here, we present the first results of JWST/NIRSpec IFU observations of the Sunburst Arc, focusing on a stacked spectrum of the 12-fold imaged Sunburst LyC-emitting (LCE) cluster. In agreement with previous studies, we find that the Sunburst LCE cluster is a very massive, compact star cluster with M_dyn = (9 ± 1)×10⁶ M_⊙. Our age estimate of 4.2–4.5 Myr is much larger than the crossing time of t_cross = 183 ± 9 kyr, indicating that the cluster is dynamically evolved and consistent with it being gravitationally bound. We find a significant nitrogen enhancement of the low ionization state interstellar medium (ISM), with log(N/O) =  − 0.74 ± 0.09, which is ≈0.8 dex above typical values for H II regions of a similar metallicity in the local Universe. We find broad stellar emission complexes around He IIλ4686 and C IVλ5808 with associated nitrogen emission; this is the first time WR signatures have been directly observed at redshifts above ∼0.5. The strength of the WR signatures cannot be reproduced by stellar population models that only include single-star evolution. While models with binary evolution better match the WR features, they still struggle to reproduce the nitrogen-enhanced WR features. JWST reveals the Sunburst LCE cluster to be a highly ionized proto-globular cluster with low oxygen abundance and extreme nitrogen enhancement that hosts a population of WR stars, likely including a previously suggested population of very massive stars (VMSs), which together are rapidly enriching the surrounding medium.

We demonstrate how the stellar and nebular conditions in star-forming galaxies modulate the emission and spectral profile of H I Ly α emission line. We examine the net Ly α output, kinematics, and in particular emission of blueshifted Ly α radiation, using spectroscopy from with the Cosmic Origins Spectrograph on Hubble Space Telescope (HST), giving a sample of 87 galaxies at redshift z = 0.05−0.44. We contrast the Ly α spectral measurements with properties of the ionized gas (from optical spectra) and stars (from stellar modelling). We demonstrate correlations of unprecedented strength between the Ly α escape fraction (and equivalent width) and the ionization parameter (p ≈ 10⁻¹⁵). The relative contribution of blueshifted emission to the total Ly α also increases from ≈0 to ≈40 per cent over the range of O₃₂ 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 Ly α emission and line profile are predominantly governed by the column of absorbing gas near zero velocity. Simultaneous multiparametric analysis over many variables shows we can predict 80 per cent of the variance on Ly α luminosity, and ∼50 per cent on the EW. We determine the most crucial predictive variables, finding that for tracers of the ionization state and H β luminosity dominate the luminosity prediction whereas the Ly α EW is best predicted by H β EW and the H α/H β ratio. We discuss our results with reference to high-redshift observations, focussing upon the use of Ly α 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.

Context. When studying the production and escape of Lyman continuum (LyC) from galaxies, it is standard to rely on an array of indirect observational tracers in the 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 inverted the conventional method and performed a bottom-up search for LyC leaking galaxies at redshifts 2 ≲ z ≲ 3.5. Using archival data from HST and VLT/MUSE, we ran source finding software on UV-filter HST images from the Hubble Ultra Deep Field (HUDF), and subjected all detected sources to a series of tests to eliminate those that are inconsistent with being ionizing sources.

Results. We find six 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, and we report non-detections 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 log₁₀(ϵν) = 25.32_−0.21^+0.25 and 25.29_−0.22^+0.27 erg s⁻¹ Hz⁻¹ cMpc⁻³ for the full set of candidates and for the four 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.

The Lyα emission line is one of the main observables of galaxies at high redshift, but its output depends strongly on the neutral gas distribution and kinematics around the star-forming regions where UV photons are produced. We present observations of Lyα and 21 cm H ı emission at comparable scales with the goal to qualitatively investigate how the neutral interstellar medium (ISM) properties impact Lyα transfer in galaxies. We have observed 21 cm H ı at the highest possible angular resolution (≈3'' beam) with the Very Large Array in two local galaxies from the Lyman Alpha Reference Sample. We compare these data with Hubble Space Telescope Lyα imaging and spectroscopy, and Multi Unit Spectroscopic Explorer and Potsdam MultiAperture Spectrophotometer ionized gas observations. In LARS08, high-intensity Lyα emission is cospatial with high column density H ı where the dust content is the lowest. The Lyα line is strongly redshifted, consistent with a velocity redistribution that allows Lyα escape from a high column density neutral medium with a low dust content. In eLARS01, high-intensity Lyα emission is located in regions of low column density H ı, below the H ı data sensitivity limit ( < 2 × 10²⁰ cm⁻²). The perturbed ISM distribution with low column density gas in front of the Lyα emission region plays an important role in the escape. In both galaxies, the faint Lyα emission (∼1×10⁻¹⁶ erg s⁻¹cm⁻² arcsec⁻²) traces intermediate Hα emission regions where H ı is found, regardless of the dust content. Dust seems to modulate, but not prevent, the formation of a faint Lyα halo. This study suggests the existence of scaling relations between dust, Hα, H ı, and Lyα emission in galaxies.