Lyman-alpha Research Articles

A candidate quadruple AGN system at z∼3

Multiple galaxies hosting active galactic nuclei (AGNs) at kiloparsec separations from each other are exceedingly rare, and in fact, only one quadruple AGN is known so far. These extreme densities of AGNs are expected to pinpoint protocluster environments and therefore should be surrounded by large galaxy overdensities. In this letter, we present another quadruple AGN candidate at z ∼ 3 including two SDSS quasars at a separation of roughly 480 kpc. The brighter quasar is accompanied by two AGN candidates (a type 1 AGN and a likely type 2 quasar) at a close (∼ 20 kpc) separation identified through emission line ratios, line widths, and high ionization lines, such as The extended Lyα emission associated with the close triple system is more modest in extent and brightness compared to similar multiple AGN systems and could be caused by ram-pressure stripping of the type 2 quasar host during infall into the central dark matter halo. The predicted evolution of the system into a z=0 galaxy cluster with the AGN host galaxies forming the brightest cluster galaxy needs to be further tested by galaxy overdensity studies on large scales around the quadruple AGN candidate. If confirmed as a quadruple AGN with X-ray observations or rest-frame optical line ratios, this system would represent a second AGN quartet and be the highest-redshift multiplet and the closest high-redshift triplet known.

Direct high-resolution observation of feedback and chemical enrichment in the circumgalactic medium at redshift z ∼ 2.8

Context. The circumgalactic medium (CGM) plays a vital role in galaxy evolution, however, studying the emission from CGM is challenging due to its low surface brightness and the complexities involved in interpreting resonant lines such as Lyman-alpha (Lyα). Aims. The near-infrared coverage, unprecedented sensitivity, and high spatial resolution of the James Webb Space Telescope (JWST) enable us to study the optical strong lines associated with the extended Lyα “nebulae” at redshifts of 2−3. These lines serve as diagnostic tools to infer the physical conditions in the massive CGM gas reservoir of these systems. Methods. In deep medium-band images taken by the JWST, we serendipitously discovered the [O III] emission from the CGM surrounding a massive interacting galaxy system at a redshift of z ∼ 2.8, known to be embedded in a bright extended (100 kpc) Lyα “nebula”. Results. This is the first time that the [O III] lines have been detected from a Lyα “nebula”. The JWST images reveal that the CGM gas actually resides in narrow (∼2.5 kpc) filamentary structures with strong [O III] emission, tracing the same extent as the Lyα emission. An analysis of the [O III] suggests that the emitting CGM is fully ionized and is energetically dominated by mechanical heating. We also find that the inferred density and pressure are higher than those commonly predicted by simulations of the CGM. Conclusions. We conclude that the observed CGM emission originates from the gas expelled by the episodic feedback processes, cooling down and enriching the CGM, while traveling a distance of at least 60 kpc. These observations demonstrate how intensive feedback processes shape gas distribution and properties in the CGM around massive halos. While access to such deep, high-resolution imaging opens up a new discovery space for investigating the CGM, it also challenges numerical simulations with respect to explaining and reproducing the exquisitely complex structures revealed by the observations.

MUSEQuBES: Unveiling Cosmic Web Filaments at z ≈ 3.6 through Dual Absorption and Emission Line Analysis

Abstract According to modern cosmological models, galaxies are embedded within cosmic filaments, which supply a continuous flow of pristine gas, fueling star formation and driving their evolution. However, due to their low density, the direct detection of diffuse gas in cosmic filaments remains elusive. Here, we report the discovery of an extremely metal-poor ([X/H] ≈ −3.7), low-density ( log 10 n H / cm − 3 ≈ −4, corresponding to an overdensity of ≈5) partial Lyman limit system (pLLS) at z ≈ 3.577 along the quasar sightline Q1317–0507, probing cosmic filaments. Additionally, two other low-metallicity ([X/H] ≲ −2) absorption systems are detected at similar redshifts, one of which is also a pLLS. Very Large Telescope (VLT) MUSE observations reveal a significant overdensity of Lyα emitters (LAEs) associated with these absorbers. The spatial distribution of the LAEs strongly suggests the presence of an underlying filamentary structure. This is further supported by the detection of a large Lyα-emitting nebula with a surface brightness of ≥ 1 0 − 19 erg cm − 2 s − 1 arcsec − 2 , with a maximum projected linear size of ≈260 pkpc extending along the LAEs. This is the first detection of giant Lyα emission tracing cosmic filaments, linked to normal galaxies and likely powered by in situ recombination.

A method for evaluating the stray-light suppression performance of internally occulted reflecting coronagraph in the Lyman alpha band

A method for evaluating the stray-light suppression performance of internally occulted reflecting coronagraph in the Lyman alpha band

Uranus’ hydrogen upper atmosphere: Insights from pre- and post-equinox HST Lyman-α images

We present the first spatially resolved images of Lyman-α (Lyα) emissions from Uranus taken by the Hubble Space Telescope (HST). The observations were carried out using HST’s Space Telescope Imaging Spectrograph instrument as part of two far-ultraviolet (FUV) observing campaigns in 1998 and 2011, before and after Uranus’ equinox in 2007. The average intensities (± uncertainties) on Uranus’ disk were 860 ± 6 and 725 ± 9 R, respectively. The images reveal widely extended emissions, detectable up to ~4 Uranus radii (RU). We performed simulations of the Lyα radiative transfer in the atmosphere, considering resonant scattering by H, Rayleigh scattering by H2, and absorption by CH4. We considered only solar Lyα fluxes at Uranus as the Lyα source for simulations. The effects of hydrogen in the interplanetary medium and Earth’s exosphere on Uranus’ Lyα emissions were taken into account. We find a good agreement between on-disk brightnesses from simulations and the HST observations assuming the (H, H2, and CH4) atmosphere profile derived from Voyager 2 measurements. Only slight adjustments of the H or H2 densities were required in some of the simulation cases, in particular, for the 1998 observations. To match the off-disk HST brightnesses in both years, a substantial exosphere of gravitationally bound hot H is required, which we modelled assuming the hot H number density has a Chapman profile. We find that compared to 1998, the hot H abundance required for 2011 is lower and the inferred hot H profiles seem to be more extended. This bound hot H is likely to be a persistent part of Uranus’ upper atmosphere and is distinct from the escaping hot H population derived from Voyager 2 observations. We discuss the possible production mechanisms involving solar EUV radiation and study the sensitivity of the modelled brightness to the parameters of the hot H profile. We find that solar EUV radiation is not a sufficient source to explain the hot H in the exosphere of Uranus.

Quantifying Lyman-α emissions from reionization fronts

Abstract During reionization, intergalactic ionization fronts (I-fronts) are sources of Lyα line radiation produced by collisional excitation of hydrogen atoms within the fronts. In principle, detecting this emission could provide direct evidence for a reionizing intergalactic medium (IGM). In this paper, we use a suite of high-resolution one-dimensional radiative transfer simulations run on cosmological density fields to quantify the parameter space of I-front Lyα emission. We find that the Lyα production efficiency — the ratio of emitted Lyα flux to incident ionizing flux driving the front — depends mainly on the I-front speed and the spectral index of the ionizing radiation. IGM density fluctuations on scales smaller than the typical I-front width produce scatter in the efficiency, but they do not significantly boost its mean value. The Lyα flux emitted by an I-front is largest if 3 conditions are met simultaneously: (1) the incident ionizing flux is large; (2) the incident spectrum is hard, consisting of more energetic photons; (3) the I-front is traveling through a cosmological over-density, which causes it to propagate more slowly. We present a convenient parameterization of the efficiency in terms of I-front speed and incident spectral index. We make these results publicly available as an interpolation table and we provide a simple fitting function for a representative ionizing background spectrum. Our results can be applied as a sub-grid model for I-front Lyα emissions in reionization simulations with spatial and/or temporal resolutions too coarse to resolve I-front structure. In a companion paper, we use our results to explore the possibility of directly imaging Lyα emission around neutral islands during the last phases of reionization.

The Lyman Alpha Reference Sample

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 (∼55″ 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.

Imaging reionization's last phases with I-front Lyman-α emissions

Long troughs observed in the z > 5.5 Lyα and Lyβ forests are thought to be caused by the last remaining neutral patches during the end phases of reionization — termed neutral islands. If this is true, then the longest troughs mark locations where we are most likely to observe the reionizing intergalactic medium (IGM). A key feature of the neutral islands is that they are bounded by ionization fronts (I-fronts) which emit Lyman series lines. In this paper, we explore the possibility of directly imaging the outline of neutral islands with a narrowband survey targeting Lyα. In a companion paper, we quantified the intensity of I-front Lyα emissions during reionization and its dependence on the spectrum of incident ionizing radiation and I-front speed. Here we apply those results to reionization simulations to model the emissions from neutral islands. We find that neutral islands would appear as diffuse structures that are tens of comoving Mpc across, with surface brightnesses in the range ≈ 1 - 5× 10-21 erg s-1 cm-2 arcsec-2. The islands are brighter if the spectrum of ionizing radiation driving the I-fronts is harder, and/or if the I-fronts are moving faster. We develop mock observations for current and futuristic observatories and find that, while extremely challenging, detecting neutral islands is potentially within reach of an ambitious observing program with wide-field narrowband imaging. Our results demonstrate the potentially high impact of low-surface brightness observations for studying reionization.

DESI 2024 IV: Baryon Acoustic Oscillations from the Lyman alpha forest

We present the measurement of Baryon Acoustic Oscillations (BAO) from the Lyman-α (Lyα) forest of high-redshift quasars with the first-year dataset of the Dark Energy Spectroscopic Instrument (DESI). Our analysis uses over 420 000 Lyα forest spectra and their correlation with the spatial distribution of more than 700 000 quasars. An essential facet of this work is the development of a new analysis methodology on a blinded dataset. We conducted rigorous tests using synthetic data to ensure the reliability of our methodology and findings before unblinding. Additionally, we conducted multiple data splits to assess the consistency of the results and scrutinized various analysis approaches to confirm their robustness. For a given value of the sound horizon (rd ), we measure the expansion at z eff = 2.33 with 2% precision, H(z eff) = ( 239.2 ± 4.8 ) (147.09 Mpc /rd ) km/s/Mpc. Similarly, we present a 2.4% measurement of the transverse comoving distance to the same redshift, DM (z eff) = ( 5.84 ± 0.14 ) (rd /147.09 Mpc) Gpc. Together with other DESI BAO measurements at lower redshifts, these results are used in a companion paper to constrain cosmological parameters.

HETDEX-LOFAR Spectroscopic Redshift Catalog∗ ∗Based on observations obtained with the Hobby–Eberly Telescope, which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximilians-Universität München, and Georg-August-Universität Göttingen.

We combine the power of blind integral field spectroscopy from the Hobby–Eberly Telescope (HET) Dark Energy Experiment (HETDEX) with sources detected by the Low Frequency Array (LOFAR) to construct the HETDEX-LOFAR Spectroscopic Redshift Catalog. Starting from the first data release of the LOFAR Two-metre Sky Survey, including a value-added catalog with photometric redshifts, we extracted 28,705 HETDEX spectra. Using an automatic classifying algorithm, we assigned each object a star, galaxy, or quasar label along with a velocity/redshift, with supplemental classifications coming from the continuum and emission-line catalogs of the internal, fourth data release from HETDEX (HDR4). We measured 9087 new redshifts; in combination with the value-added catalog, our final spectroscopic redshift sample is 9710 sources. This new catalog contains the highest substantial fraction of LOFAR galaxies with spectroscopic redshift information; it improves archival spectroscopic redshifts and facilitates research to determine the [O ii] emission properties of radio galaxies from 0.0 < z < 0.5, and the Lyα emission characteristics of both radio galaxies and quasars from 1.9 < z < 3.5. Additionally, by combining the unique properties of LOFAR and HETDEX, we are able to measure star formation rates (SFRs) and stellar masses. Using the Visible Integral-field Replicable Unit Spectrograph, we measure the emission lines of [O iii], [Ne iii], and [O ii] and evaluate line-ratio diagnostics to determine whether the emission from these galaxies is dominated by active galactic nuclei or star formation and fit a new SFR–L 150MHz relationship.

Signals from the Early Universe

It is proposed to account for the time-dependent partial thermalization of the Ly α lines emitted during cosmic recombination of electrons and protons in the early Universe based on an analytically solvable nonlinear diffusion model. The amplitude of the partially thermalized and redshifted Lyα line is found to be too low to be visible in the cosmic microwave spectrum, in accordance with previous numerical models and Planck observations. New space missions with more sensitive spectrometers are required to detect Ly α-remnants from recombination as frequency fluctuations in the cosmic microwave background. (An extended version of this article has previously appeared in Scientific Reports).

A resolved Lyman α profile with doubly peaked emission at z ∼ 7

Context. The epoch of reionization is a landmark in structure formation and galaxy evolution. How it happened is still not clear, especially regarding which population of objects was responsible for contributing the bulk of ionizing photons to this process. Doubly peaked Lyman-alpha profiles in this epoch are of particular interest since they hold information about the escape of ionizing radiation and the environment surrounding the source. Aims. We wish to understand the escape mechanisms of ionizing radiation in Lyα emitters during this time and the origin of a doubly peaked Lyman-alpha profile. We also wish to estimate the size of a potential ionized bubble. Methods. Using radiative transfer models, we fit the line profile of a bright Lyα emitter at z ∼ 6.9 using various gas geometries. The line modeling reveals significant radiation escape from this system. Results. The studied source shows significant escape (fesc(Lyα) ∼ 0.8, as predicted by the best fitting radiative transfer model) and appears to inhabit an ionized bubble of radius Rb ≈ 0.8−0.3+0.5 pMpc(tage/108)1/3. Radiative transfer modeling predicts the line to be completely redward of the systemic redshift. We suggest the line morphology is produced by inflows, by multiple components emitting Lyα, or by an absorbing component in the red wing. Conclusions. We propose that CDFS-1’s profile has two red peaks produced by winds within the system. Its high fesc(Lyα) and the low-velocity offset from the systemic redshift suggest that the source is an active ionizing agent. Future observations will reveal whether a peak is present blueward of the systemic redshift or if multiple components produce the profile.

The Second Case of a Major Merger Triggering a Starburst in a Green Pea Galaxy

We have used the Karl G. Jansky Very Large Array (VLA) to map H i 21 cm emission from the Green Pea galaxy GP J1148+2546 at z ≈ 0.0451, only the second measurement of the H i spatial distribution of a Green Pea. The VLA H i 21 cm image, the DECaLS optical image, and Sloan Digital Sky Survey spectroscopy show that GP J1148+2546 has two neighbors, the nearer of which is only ≈17.5 kpc away, and that the H i 21 cm emission extends in an inverted “C” shape around the Green Pea and its companions, with the highest H i column density between the two neighboring galaxies. The starburst in GP J1148+2546 is likely to have been triggered by the ongoing merger with its neighbors, although the velocity field and velocity dispersion images do not show clear merger signatures at the Green Pea location. The H i mass of the Green Pea and its immediate surroundings is (3.58 ± 0.37) × 109 M ⊙, a factor of ≈7.4 lower than the total H i mass of the system of three interacting galaxies, while the H i depletion timescale of GP J1148+2546 is ≈0.69 Gyr, much shorter than that of typical galaxies at z ≈ 0. We detect damped Lyα absorption and Lyα emission from the Green Pea in a Hubble Space Telescope Cosmic Origins Spectrograph spectrum, obtaining a high H i column density, ≈2.0 × 1021 cm−2, and a low Lyα escape fraction, ≈0.8%, consistent with the relatively low value (≈5.4) of the ratio O32 ≡ [O iii]λ5007 + λ4959/[O ii]λ3727,3729.

FAST H i 21 cm Study of Blueberry Galaxies

Green Peas (GPs) and blueberry galaxies (BBs) are thought to be local analogs (z < 0.1) of high redshift Lyα emitters. H i study of these can help us understand the star formation in the primordial Universe. In this Letter, we present the results of H i 21 cm study of 28 high specific star formation rate (sSFR ≳ 10−8yr −1) BBs at z ≲ 0.05 with the Five-hundred-meter Aperture Spherical radio Telescope. We report significant H i detection towards two BBs, namely J1026+0426 and J1132+0809, and discuss possible H i contribution from neighboring galaxies. The median 3σ upper limit of ∼2.0 × 108 M ⊙ was obtained on H i mass for galaxies with nondetections. We find BBs tend to have lower H i-to-stellar-mass ratio or gas fraction (f H i ) than expected from f H i –sSFR and f H i –M * relations for main-sequence galaxies. The BBs also have a median 3σ upper limit on H i gas depletion timescale (τ H i ) ∼ 0.5 Gyr, about 1 order of magnitude lower than τ H i for local main-sequence galaxies. We find a significantly low H i detection rate of 2/28 (7.1 −4.6+9.4 %) towards these galaxies, which is similar to previous H i studies of low redshift GPs of high ionization parameter indicator, O32 ≡ O[iii]λ5007/O[ii]λ3727 ratios ≳10.

The Lyα Dependence on Nebular Properties from the HETDEX and MOSDEF Surveys

Investigating the impact of galaxy properties on emergent Lyα emission is crucial for reionization studies, given the sensitivity of Lyα to neutral hydrogen. This study presents an analysis of the physical characteristics of 155 star-forming galaxies, 29 with Lyα detected, and 126 with Lyα not detected with Lyα EW < 20 Å, at z = 1.9–3.5, drawn from the MOSFIRE Deep Evolution Field survey, that have overlapping observations from the Hobby–Eberly Telescope Dark Energy Experiment survey. To unravel the interstellar medium (ISM) conditions in our sample, we developed a custom nebular line modeling algorithm based on the MAPPINGS V photoionization model grid and the emcee framework. Combining nebular-based ISM properties with photometry-based global properties, constrained via Bagpipes, we explore distinctions in the stellar and gas properties between Lyα-detected and Lyα-nondetected galaxies. Our analysis reveals statistically significant differences between the two samples in terms of stellar mass and dust attenuation (A V ) at >2σ significance, as determined via a Kolmogorov–Smirnov test. Moreover, there are weaker (≲1σ significance) indications that the ionization parameter and metallicity differ between the two samples. Our results demonstrate that the escape fraction of Lyα ( fescLyα ) is inversely correlated with stellar mass, star formation rate, and dust attenuation, while it is positively correlated with the ionization parameter, with significance levels exceeding 2σ. Our findings suggest that the interstellar environments of Lyα-detected galaxies, characterized by low mass, low dust, low gas-phase metallicity, and high ionization parameters, play a pivotal role in promoting the escape of Lyα radiation.

Between the Extremes: A JWST Spectroscopic Benchmark for High-redshift Galaxies Using ∼500 Confirmed Sources at z ≥ 5

The exceptional spectra of the most luminous z > 10 sources observed so far have challenged our understanding of early galaxy evolution, requiring a new observational benchmark for meaningful interpretation. As such, we construct spectroscopic templates representative of high-redshift, star-forming populations, using 482 confirmed sources at z = 5.0−12.9 with JWST/NIRSpec prism observations, and report on their average properties. We find z = 5−11 galaxies are dominated by blue UV continuum slopes (β = −2.3 to −2.7) and reduced Balmer indices, characteristic of dust-poor and young systems, with a shift towards bluer slopes and younger ages with redshift. The evolution is mirrored by ubiquitous C iii] detections across all redshifts (rest-frame equivalent widths of 5−14 Å), which increase in strength towards early times. Rest-frame optical lines reveal elevated ratios (O32 = 7–31, R23 = 5–8, and Ne3O2 = 1−2) and subsolar metallicities (log(O/H) = 7.3−7.9), typical of ionization conditions and metallicities rarely observed in z ∼ 0 populations. Within our sample, we identify 57 Lyα emitters, which we stack and compare to a matched sample of nonemitters. The former are characterized by more extreme ionizing conditions with enhanced C iii], C iv, and He ii + [O iii] line emission, younger stellar populations from Balmer jumps, and a more pristine interstellar medium seen through bluer UV slopes and elevated rest-frame optical line ratios. The novel comparison illustrates important intrinsic differences between the two populations, with implications for Lyα visibility. The spectral templates derived here represent a new observational benchmark with which to interpret high-redshift sources, lifting our constraints on their global properties to unprecedented heights and extending out to the earliest of cosmic times.

MAMMOTH-Subaru. II. Diverse Populations of Circumgalactic Lyα Nebulae at Cosmic Noon

Circumgalactic Lyα nebulae are gaseous halos around galaxies exhibiting luminous extended Lyα emission. This work investigates Lyα nebulae from deep imaging of ∼12 deg2 sky, targeted by the MAMMOTH-Subaru survey. Utilizing the wide-field capability of Hyper Suprime-Cam, we present one of the largest blind Lyα nebula selections, including QSO nebulae, Lyα blobs, and radio galaxy nebulae down to the typical 2σ Lyα surface brightness of (5-10)×10−18ergs−1cm−2arcsec−2 . The sample contains 117 nebulae with Lyα sizes of 40–400 kpc, and the most gigantic one spans about 365 kpc, and is referred to as the Ivory Nebula. Combining multiwavelength data, we investigate diverse nebula populations and associated galaxies. We find a small fraction of Lyα nebulae have QSOs (∼7%), luminous infrared galaxies (LIRGs; ∼1%), and radio galaxies (∼2%). Remarkably, among the 28 enormous Lyα nebulae (ELANe) exceeding 100 kpc, about 80% are associated with UV-faint galaxies (M UV > −22), and are categorized as Type II ELANe. We underscore that Type II ELANe constitute the majority but remain largely hidden in current galaxy and QSO surveys. Dusty starburst and obscured AGN activity are proposed to explain the nature of Type II ELANe. The spectral energy distribution of stacking all Lyα nebulae also reveals signs of massive dusty star-forming galaxies with obscured AGNs. We propose a model to explain the dusty nature where the diverse populations of Lyα nebulae capture massive galaxies at different evolutionary stages undergoing violent assembly. Lyα nebulae provide critical insights into the formation and evolution of today’s massive cluster galaxies at cosmic noon.

JWST/NIRSpec Observations of Lyman α Emission in Star-forming Galaxies at 6.5 ≲ z ≲ 13

We present an analysis of JWST Lyα spectroscopy of z ≳ 6.5 galaxies, using observations in the public archive covering galaxies in four independent fields: Great Observatories Origins Deep Survey (GOODS)-N, GOODS-S, A2744, and the Extended Groth Strip (EGS). We measure the Lyα emission line properties for a sample of 210 z ≃ 6.5–13 galaxies, with redshifts confirmed independently of Lyα in all cases. We present three new detections of Lyα emission in JWST spectra, including a large equivalent width (EW; =143 Å) Lyα emitter (LAE) with strong C iv emission (EW = 21 Å) at z = 7.1 in GOODS-N. We measure the redshift-dependent Lyα EW distribution across our sample. We find that strong Lyα emission (EW > 25 Å) becomes increasingly rare at earlier epochs, suggesting that the transmission of Lyα photons decreases by 4× between z ≃ 5 and z ≃ 9. We describe potential implications for the intergalactic medium neutral fraction. There is significant field-to-field variance in the LAE fraction. In contrast to the three other fields, the EGS shows no evidence for reduced transmission of Lyα photons at z ≃ 7–8, suggesting a significantly ionized sight line may be present in the field. We use available NIRCam grism observations from the First Reionization Epoch Spectroscopically Complete Observations survey to characterize overdensities on large scales around known LAEs in the GOODS fields. The strongest overdensities appear linked with extremely strong Lyα detections (EW > 50 Å) in most cases. Future Lyα spectroscopy with JWST has the potential to constrain the size of ionized regions around early galaxy overdensities, providing a new probe of the reionization process.

The stellar population of a z ∼ 3.25 Lyα-emitting group associated with a damped Lyα absorber

We present near-infrared observations, acquired with the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope (HST), of a Lyα-emitting double-clumped nebula at z ≈ 3.25 associated with a damped Lyα absorber (DLA). With the WFC3/F160W data we observe the stellar continuum around 3600 Å in the rest frame of a galaxy embedded in the west clump of the nebula, GW, for which we estimate a star formation rate (SFR) of SFRGW = 5.0 ± 0.4 M⊙ yr−1 and a maximum stellar mass MGW &lt; 9.9 ± 0.7 × 109 M⊙. With the enhanced spatial resolution of HST, we discover the presence of an additional faint source, GE, in the center of the east clump, with SFRGE = 0.70 ± 0.20 M⊙ yr−1 and a maximum stellar mass MGE &lt; 1.4 ± 0.4 × 109 M⊙. We show that the Lyα emission in the two clumps can be explained by recombination following in situ photoionization by the two galaxies, assuming escape fractions of ionizing photons of ≲0.24 for GW and ≲0.34 for GE. The fact that GW is offset by ≈8 kpc from the west clump means we cannot fully rule out the presence of additional fainter star-forming sources, which would further contribute to the photon budget inside this ≈1012 M⊙ galaxy group that extends over a region of 30 × 50 kpc.

A public grid of radiative transfer simulations for Lyα and metal lines in idealised galactic outflows

The vast majority of star-forming galaxies are surrounded by large reservoirs of gas ejected from the interstellar medium. Ultraviolet absorption and emission lines represent powerful diagnostics to constrain the cool phase of these outflows, through resonant transitions of hydrogen and metal ions. The interpretation of these observations is often remarkably difficult as it requires detailed modelling of the propagation of the continuum and emission lines in the gas. To this aim, we present a large public grid of ≈20 000 simulated spectra that includes H I Lyα and five metal transitions associated with Mg II, C II, Si II, and Fe II which is accessible online. The spectra have been computed with the RASCAS Monte Carlo radiative transfer code for 5760 idealised spherically symmetric configurations surrounding a central point source emission, and characterised by their column density, Doppler parameter, dust opacity, wind velocity, as well as various density and velocity gradients. Designed to predict and interpret Lyα and metal line profiles, our grid exhibits a wide diversity of resonant absorption and emission features, as well as fluorescent lines. We illustrate how it can help better constrain the wind properties by performing a joint modelling of observed Lyα, C II, and Si II spectra. Using CLOUDY simulations and virial scaling relations, we also show that Lyα is expected to be a faithful tracer of the gas at T ≈ 104 − 105 K, even if the medium is highly ionised. While C II is found to probe the same range of temperatures as Lyα, other metal lines merely trace cooler phases (T ≈ 104 K). As their gas opacity strongly depends on gas temperature, incident radiation field, metallicity and dust depletion, we caution that optically thin metal lines do not necessarily originate from low H I column densities and may not accurately probe Lyman continuum leakage.