Lyman Continuum Leakage Research Articles

Extreme ionizing properties of a metal-poor, MUV ≃ −12 star complex in the first gigayear

We report the serendipitous discovery of a faint (MUV > −12.2), low-metallicity (Z ∼ 0.02 Z⊙) ionizing source, dubbed T2c, with a spectroscopic redshift of z = 6.146. T2c is part of a larger structure amplified by the Hubble Frontier Field galaxy cluster MACSJ0416 and was observed with the James Webb Space Telescope (JWST) NIRSpec integral field unit. Stacking the short-wavelength NIRCam data reveals no stellar continuum detection down to a magnitude limit of mUV ≃ 31.0 (3σ). However, prominent Hβ, [O III]λλ4959, 5007, and Hα emissions are detected, with equivalent widths exceeding 200 Å, 800 Å, and 1300 Å (3σ), respectively. The corresponding intrinsic (magnification-corrected ×23 ± 3) ultraviolet and optical rest-frame magnitudes exceed 34.4 and 33.9 (corresponding to MUV and Mopt fainter than −12.2 and −12.8 at λrest ∼ 2000 Å and ∼5000 Å, respectively), suggesting a stellar mass lower than a few 104 M⊙ under an instantaneous burst scenario. The inferred ionizing photon production efficiency (ξion) is high: ξion ≳ 26.08(25.86) 3(5)σ, assuming no dust attenuation and no Lyman continuum leakage. This indicates the presence of massive stars despite the low mass of the object. The very poor sampling of the initial mass function in such a low-mass star-forming complex suggests that the formation of very massive stars might be favored in very low-metallicity environments. T2c is surrounded by Balmer and weak oxygen emission on a spatial scale of a few hundred parsecs, after correcting for lensing effects. This system resembles a H II region potentially powered by currently undetected, extremely efficient, low-metallicity star complexes or clusters. We propose that massive O-type stars populate these low-mass, low-metallicity, high-redshift satellites, likely observed in an early and short formation phase, and contribute to the ionization of the surrounding medium.

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.

Lyman Continuum Leakers at z > 3 in the GOODS-S Field: Starburst or Not?

We investigate the star-forming properties of 23 Lyman continuum (LyC) leakers at z > 3 in the Great Observatories Origins Deep Survey-South field based on a systematic review of LyC observations from the literature. Using data from the Hubble Space Telescope and the James Webb Space Telescope, we construct the spectral energy distributions for these LyC leakers, covering the spectrum from rest-frame ultraviolet to near-infrared. Through the application of a unified modeling approach, we measure the ultraviolet slope (β), star formation rate, and stellar mass for these LyC leakers in a consistent manner. These high-redshift LyC leakers demonstrate statistically blue UV-continuum slopes (β), which is consistent with their high escape fraction of LyC photons. We find that these high-redshift LyC leakers span a wide range of specific star formation rate ( log(sSFR /yr) from −8.6 to −6.7). Ten of these LyC leakers are located on the star formation main sequence, instead of all being in the starburst mode. The results indicate that intense bursts of star formation are not necessarily required for the leakage of LyC photons for galaxies at z > 3.

MOSEL Survey: Spatially Offset Lyman-continuum Emission in a New Emitter at z = 3.088 Can Explain the Low Number Density of Observed LyC Leakers

We present the discovery of a unique Lyman-continuum (LyC) emitter at z = 3.088. The LyC emission was detected using the Hubble Space Telescope WFC3/UVIS F336W filter, covering a rest-frame wavelength range of 760–900 Å. The peak signal-to-noise ratio of LyC emission is 3.9 in an r = 0.″24 aperture and is spatially offset by 0.″29 ± 0.″04 (∼2.2 ± 0.3 kpc) from the peak of rest-UV emission (F606W). By combining imaging and spectroscopic data from the James Webb Space Telescope (JWST) JADES, FRESCO, and JEMS surveys, along with VLT/MUSE data from the MXDF survey, we estimate that the probability of random alignment with an interloper galaxy causing the LyC emission is less than 6 × 10−5. The interstellar medium (ISM) conditions in the galaxy are similar to those in other LyC emitters at high redshift ( 12+log(O/H)=7.79−0.05+0.06 , logU=−3.27−0.12+0.14 , O32 = 0.63 ± 0.03), although the single-peaked Lyα profile and lack of rest-UV emission lines suggest an optically thick ISM. Our observations indicate that LyC photons are leaking through a narrow cone of optically thin neutral ISM, most likely created by a past merger (as evidenced by medium-band F210M and F182M images). Using the constraints on escape fraction from individual leakers and a simple model, we estimate that the opening half-angle of ionization cones can be as small as 16° (2% ionized fraction) to reproduce some of the theoretical constraints on the average escape fraction for galaxies. The narrow opening angle required can explain the low number density of confirmed LyC leakers.

Disentangling the X-ray variability in the Lyman continuum emitter Haro 11

Context. Lyman break analogs in the local Universe serve as counterparts to Lyman break galaxies (LBGs) at high redshifts, which are widely regarded as major contributors to cosmic reionization in the early stages of the Universe. Aims. We studied XMM-Newton and Chandra observations of the nearby LBG analog Haro 11, which contains two X-ray-bright sources, X1 and X2. Both sources exhibit Lyman continuum (LyC) leakage, particularly X2. Methods. We analyzed the X-ray variability using principal component analysis (PCA) and performed spectral modeling of the X1 and X2 observations made with the Chandra ACIS-S instrument. Results. The PCA component, which contributes to the X-ray variability, is apparently associated with variable emission features, likely from ionized superwinds. Our spectral analysis of the Chandra data indicates that the fainter X-ray source, X2 (X-ray luminosity LX ∼ 4 × 1040 erg s−1), the one with higher LyC leakage, has a much lower absorbing column (NH ∼ 1.2 × 1021 cm−2) than the heavily absorbed luminous source X1 (LX ∼ 9 × 1040 erg s−1 and NH ∼ 11.5 × 1021 cm−2). Conclusions. We conclude that X2 is likely less covered by absorbing material, which may be a result of powerful superwinds clearing galactic channels and facilitating the escape of LyC radiation. Much deeper X-ray observations are required to validate the presence of potential superwinds and determine their implications for the LyC escape.

Characterizing the Average Interstellar Medium Conditions of Galaxies at z ∼ 5.6–9 with Ultraviolet and Optical Nebular Lines

Ultraviolet (UV; rest-frame ∼1200–2000 Å) spectra provide a wealth of diagnostics to characterize fundamental galaxy properties, such as their chemical enrichment, the nature of their stellar populations, and their amount of Lyman-continuum (LyC) radiation. In this work, we leverage publicly released JWST data to construct the rest-frame UV-to-optical composite spectrum of a sample of 63 galaxies at 5.6 < z < 9, spanning a wavelength range from 1500 to 5200 Å. Based on the composite spectrum, we derive an average dust attenuation E(B−V)gas=0.10−0.11+0.10 from Hβ/Hγ, an electron density ne=570−290+510 cm−3 from the [O ii] doublet ratio, an electron temperature Te=16700−1500+1500 K from the [O iii] λ4363/[O iii] λ5007 ratio, and an ionization parameter log(U)=−2.15−0.03+0.03 from the [O iii]/[O ii] ratio. Using a direct T e method, we calculate an oxygen abundance 12+log(O/H)=7.67±0.08 and a carbon-to-oxygen (C/O) abundance ratio log(C/O)=−0.86−0.10+0.13 . This C/O ratio is smaller than compared to z = 0 and z = 2–4 star-forming galaxies, albeit with moderate significance. This indicates the reionization-era galaxies might be undergoing a rapid buildup of stellar mass with high specific star formation rates. A UV diagnostic based on the ratios of C iii] λ λ1907, 1909/He ii λ1640 versus O iii] λ1666/He ii λ1640 suggests that the star formation is the dominant source of ionization, similar to the local extreme dwarf galaxies and z ∼ 2–4 He ii–detected galaxies. The [O iii]/[O ii] and C iv/C iii] ratios of the composite spectrum are marginally larger than the criteria used to select galaxies as LyC leakers, suggesting that some of the galaxies in our sample are strong contributors to the reionizing radiation.

The great escape: understanding the connection between Ly α emission and LyC escape in simulated JWST analogues

ABSTRACT Constraining the escape fraction of Lyman Continuum (LyC) photons from high-redshift galaxies is crucial to understanding reionization. Recent observations have demonstrated that various characteristics of the Ly$\, \alpha$ emission line correlate with the inferred LyC escape fraction ($f_{\rm esc}^{\rm LyC}$) of low-redshift galaxies. Using a data set of 9600 mock Ly$\, \alpha$ spectra of star-forming galaxies at $4.64 \le z \le 6$ from the SPHINX$^{20}$ cosmological radiation hydrodynamical simulation, we study the physics controlling the escape of Ly$\, \alpha$ and LyC photons. We find that our mock Ly$\, \alpha$ observations are representative of high-redshift observations and that typical observational methods tend to overpredict the Ly$\, \alpha$ escape fraction ($f_{\rm esc}^{\rm Ly\, \alpha }$) by as much as 2 dex. We investigate the correlations between $f_{\rm esc}^{\rm LyC}$ and $f_{\rm esc}^{\rm Ly\, \alpha }$, Ly$\, \alpha$ equivalent width ($W_{\lambda }({\rm Ly\, \alpha })$), peak separation ($v_{\rm sep}$), central escape fraction ($f_{\rm cen}$), and red peak asymmetry ($A_f^{\rm red}$). We find that $f_{\rm esc}^{\rm Ly\, \alpha }$ and $f_{\rm cen}$ are good diagnostics for LyC leakage, selecting for galaxies with lower neutral gas densities and less UV attenuation that have recently experienced supernova feedback. In contrast, $W_{\lambda }({\rm Ly\, \alpha })$ and $v_{\rm sep}$ are found to be necessary but insufficient diagnostics, while $A_f^{\rm red}$ carries little information. Finally, we use stacks of Ly$\, \alpha$, H$\, \alpha$, and F150W mock surface brightness profiles to find that galaxies with high $f_{\rm esc}^{\rm LyC}$ tend to have less extended Ly$\, \alpha$ and F150W haloes but larger H$\, \alpha$ haloes than their non-leaking counterparts. This confirms that Ly$\, \alpha$ spectral profiles and surface brightness morphology can be used to better understand the escape of LyC photons from galaxies during the epoch of reionization.

Comparing the VANDELS Sample to a Zoom-in Radiative Hydrodynamical Simulation: Using the Si ii and C ii Line Spectra as Tracers of Galaxy Evolution and Lyman Continuum Leakage

We compare mock ultraviolet C ii and Si ii absorption and emission line features generated using a ∼109 M ⊙ virtual galaxy with observations of 131 z ∼ 3 galaxies from the vandels survey. We find that the mock spectra reproduce reasonably well a large majority (83%) of the vandels spectra (χ 2 < 2), but do not resemble the most massive objects (⪆1010 M ⊙), which exhibit broad absorption features. Interestingly, the best-matching mock spectra originate from periods of intense star formation in the virtual galaxy, where its luminosity is 4 times higher than in periods of relative quiescence. Furthermore, for each galaxy, we predict the Lyman continuum (LyC) escape fractions ( fesc(pred)LyC ) using the environment of the virtual galaxy. We derive an average fesc(pred)LyC of 0.01 ± 0.02, consistent with other estimates from the literature. The fesc(pred)LyC are tightly correlated with the Lyα escape fractions and highly consistent with observed empirical trends. Additionally, galaxies with larger fesc(pred)LyC exhibit bluer β slopes, more Lyα flux, and weaker low-ionization absorption lines. Building upon the good agreement between fesc(pred)LyC and observationally established LyC diagnostics, we examine the LyC leakage mechanisms in the simulation. We find that LyC photon leakage is enhanced in directions where the observed flux dominantly emerges from compact regions depleted of neutral gas and dust, mirroring the scenario inferred from observational data. In general, this study further highlights the potential of high-resolution radiation hydrodynamics simulations in analyzing UV absorption and emission line features and providing valuable insights into the LyC leakage of star-forming galaxies.

Modelling the star-formation activity and ionizing properties of high-redshift galaxies

Early results from the JWST observations have reported a surprisingly high number of UV-bright galaxies at z ≥ 10, which appears to challenge the theoretical predictions from standard galaxy formation models in the ΛCDM framework at these redshifts. To alleviate this tension, several cosmological and astrophysical interpretations have been advanced. However, all of these proposed scenarios carry noteworthy consequences for other large-scale processes in the early Universe, particularly cosmic reionization, since high-redshift galaxies are believed to be the primary ionizing sources during the Epoch of Reionization (EoR). To investigate this, we introduce a semi-analytical model of galaxy formation and evolution that explains the evolving galaxy UV luminosity function (UVLF) over 6 ≲ z ≲ 15, and also jointly tracks the time evolution of the globally averaged neutral hydrogen fraction in the intergalactic medium. The model self-consistently accounts for the suppression of star formation in low-mass galaxies due to reionization feedback and is constrained by comparing the model predictions with various observational probes like the UVLF data from HST and JWST, recent measurements of the neutral hydrogen fraction, and the CMB scattering optical depth. Our analysis confirms that a rapid enhancement in the star-formation rate efficiency and/or UV luminosity per stellar mass formed is necessary for consistency with the JWST UVLF estimates at z ≥ 10. We further find that it is possible to jointly satisfy the current reionization constraints when the escape fraction is assumed to be halo-mass dependent, requiring higher Lyman-continuum leakage from low-mass galaxies. We also examine the relative contribution of galaxies with different UV luminosities towards the ionizing photon budget for the EoR and investigate the large-scale bias of high-z galaxies.

Linking Mg II and [O II] spatial distribution to ionizing photon escape in confirmed LyC leakers and non-leakers

The geometry of the neutral gas in and around galaxies is a key regulator of the escape of ionizing photons. We present the first statistical study aimed at linking the neutral and ionized gas distributions to the Lyman continuum (LyC) escape fraction (fescLyC) in a sample of 22 confirmed LyC leakers and non-leakers atz ≈ 0.35 using the Keck Cosmic Web Imager (Keck/KCWI) and the Low Resolution Spectrograph 2 (HET/LRS2). Our integral field unit data enable the detection of neutral and low-ionization gas, as traced by Mg II, and ionized gas, as traced by [O II], extending beyond the stellar continuum for seven and ten objects, respectively. All but one object with extended Mg IIemission also show extended [O II] emission; in this case, Mg IIemission is always more extended than [O II] by a factor 1.2 on average. Most of the galaxies with extended emission are non or weak LyC leakers (fescLyC &lt; 5%), but we find a large diversity of neutral and low-ionization gas configurations around these weakly LyC-emitting galaxies. Conversely, the strongest leakers (fescLyC &gt; 5%) appear uniformly compact in both Mg IIand [O II] with exponential scale lengths ≲1 kpc. Most are unresolved at the resolution of our data. We also find a trend betweenfescLyCand the spatial offsets of the nebular gas and the stellar continuum emission. Moreover, we find significant anticorrelations between the spatial extent of the neutral and/or low-ionization gas and the [O III]/[O II] ratio, and Hβequivalent width, as well as positive correlations with metallicity and UV size, suggesting that galaxies with more compact neutral and/or low-ionization gas sizes are more highly ionized. The observations suggest that strong LyC emitters do not have extended neutral and/or low-ionization gas halos and ionizing photons may be emitted in many directions. Combined with high ionization diagnostics, we propose that the Mg II, and potentially [O II], spatial compactness are indirect indicators of LyC emitting galaxies at high redshift.

Lyα emission in galaxies at z ≃ 5−6: new insight from JWST into the statistical distributions of Lyα properties at the end of reionization

ABSTRACT JWST has recently sparked a new era of Lyα spectroscopy, delivering the first measurements of the Lyα escape fraction and velocity profile in typical galaxies at z ≃ 6−10. These observations offer new prospects for insight into the earliest stages of reionization. But to realize this potential, we need robust models of Lyα properties in galaxies at z ≃ 5−6 when the IGM is mostly ionized. Here, we use new JWST observations from the JADES and FRESCO surveys combined with VLT/MUSE and Keck/DEIMOS data to characterize statistical distributions of Lyα velocity offsets, escape fractions, and EWs in z ≃ 5−6 galaxies. We find that galaxies with large Lyα escape fractions (&amp;gt;0.2) are common at z ≃ 5−6, comprising 30 per cent of Lyman break selected samples. Comparing to literature studies, our census suggests that Lyα becomes more prevalent in the galaxy population towards higher redshift from z ∼ 3 to z ∼ 6, although we find that this evolution slows considerably between z ∼ 5 and z ∼ 6, consistent with modest attenuation from residual H i in the mostly ionized IGM at z ≃ 5−6. We find significant evolution in Lyα velocity profiles between z ≃ 2−3 and z ≃ 5−6, likely reflecting the influence of resonant scattering from residual intergalactic H i on the escape of Lyα emission near line centre. This effect will make it challenging to use Lyα peak offsets as a probe of Lyman continuum leakage at z ≃ 5−6. We use our z ≃ 5−6 Lyα distributions to make predictions for typical Lyα properties at z ≳ 8 and discuss implications of a recently discovered Lyα emitter at z ≃ 8.5 with a small peak velocity offset (156 km s−1).

Lyman continuum leaker candidates at z ∼ 3–4 in the HDUV based on a spectroscopic sample of MUSE LAEs

Context. In recent years, a number of Lyman continuum (LyC) leaker candidates have been found at intermediate redshifts, providing insight into how the Universe was reionised at early cosmic times. There are now around 100 known LyC leakers at all redshifts, which enables us to analyse their properties statistically. Aims. Here, we identify new LyC leaker candidates at z ≈ 3 − 4.5 and compare them to objects from the literature to get an overview of the different observed escape fractions and their relation to the properties of the Lyman α (Lyα) emission line. The aim of this work is to test the indicators (or proxies) for LyC leakage suggested in the literature and to improve our understanding of the kinds of galaxies from which LyC radiation can escape. Methods. We used data from the Hubble Deep Ultraviolet (HDUV) legacy survey to search for LyC emission based on a sample of ≈2000 Lyα emitters (LAEs) detected previously in two surveys with the Multi-Unit Spectroscopic Explorer (MUSE), namely MUSE-Deep and MUSE-Wide. Based on the redshifts and positions of the LAEs, we look for potential LyC leakage in the WFC3/UVIS F336W band of the HDUV survey. The escape fractions are measured and compared in different ways, including spectral energy distribution (SED) fitting performed using the CIGALE software. Results. We add 12 objects to the sample of known LyC leaker candidates (5 highly likely leakers and 7 potential ones), 1 of which was previously known, and compare their Lyα properties to their escape fractions. We find escape fractions of between ∼20% and ∼90%, assuming a high transmission in the intergalactic medium (IGM). We present a method whereby the number of LyC leaker candidates we find is used to infer the underlying average escape fraction of galaxies, which is ≈12%. Conclusion. Based on their Lyα properties, we conclude that LyC leakers are not very different from other high-z LAEs and suggest that most LAEs could be leaking LyC even if this cannot always be detected because of the direction of emission and the transmission properties of the IGM.

Multiple Emission Lines of Hα Emitters at z ∼ 2.3 from the Broad- and Medium-band Photometry in the ZFOURGE Survey

We present a multiple emission line study of ∼1300 Hα emitters (HAEs) at z ∼ 2.3 in the ZFOURGE survey. In contrast to the traditional spectroscopic method, our sample is selected based on the flux excess in the ZFOURGE K s broadband data relative to the best-fit stellar continuum. Using the same method, we also extract the strong diagnostic emission lines for these individual HAEs: [O iii] λ λ4959, 5007 and [O ii] λ λ3726, 3729. Our measurements demonstrate good consistency with those obtained from spectroscopic surveys. We investigate the relationship between the equivalent widths (EWs) of these emission lines and various galaxy properties, including stellar mass, stellar age, star formation rate, specific star formation rate, and ionization state (O32). We have identified a discrepancy between HAEs at z ∼ 2.3 and typical local star-forming galaxies observed in the Sloan Digital Sky Survey, suggesting the evolution of lower gas-phase metallicity (Z) and higher ionization parameters (U) with redshift. Notably, we have observed a significant number of low-mass HAEs exhibiting exceptionally high EW[O iii]. Their galaxy properties are comparable to those of extreme objects, such as extreme O3 emitters and Lyα emitters at z ≃ 2–3. Considering that these characteristics may indicate potential strong Lyman continuum leakage, higher-redshift analogs of the low-mass HAEs could be significant contributors to the cosmic reionization. Further investigations of this particular population are required to gain a clearer understanding of galaxy evolution and cosmic reionization.

Lyman Continuum Leakers in the AstroSat Ultraviolet Deep Field: Extreme-ultraviolet Emitters at the Cosmic Noon

We report the direct detection of Lyman continuum (LyC) emission from nine galaxies and one active galactic nucleus at z ∼ 1.1–1.6 in the GOODS-North field using deep observations from the Ultraviolet Imaging Telescope (UVIT) on board AstroSat. The absolute escape fraction of the sources estimated from the far-ultraviolet and Hα-line luminosities using Monte Carlo analysis of two intergalactic medium models span a range ∼10%–55%. The rest-frame UV wavelength of the sources falls in the extreme-ultraviolet regime ∼550–700 Å, the shortest LyC wavelength range probed so far. This redshift range remains devoid of direct detections of LyC emission due to the instrumental limitations of previously available facilities. With UVIT having very low detector noise, each of these sources is detected with an individual signal-to-noise ratio (S/N) > 3, while for the stack of six sources, we achieve an S/N ∼ 7.4. The LyC emission is seen to be offset from the optical centroids and extended beyond the UVIT point-spread function of 1.″6 in most of the sources. This sample fills an important niche between GALEX and Cosmic Origins Spectrograph at low z and Hubble Space Telescope's Wide Field Camera 3 at high z and is crucial in understanding the evolution of LyC leakers.

Using Mg ii Doublet to Predict the Lyman Continuum Escape Fraction from 14 HETDEX Galaxies

Indirect diagnostics of Lyman continuum (LyC) escape are needed to constrain which sources reionized the universe. We used Mg ii to predict the LyC escape fraction () in 14 galaxies selected from the Hobby-Eberly Telescope Dark Energy Experiment solely based upon their Mg ii properties. Using the Low Resolution Spectrograph on HET, we identified 7 and 5 possible LyC leakers depending on the method, with ranging from 3% to 80%. Interestingly, our targets display diverse [O iii]/[O ii] ratios (O32), with strong inferred LyC candidates showing lower O32 values than previous confirmed LyC leaker samples. Additionally, a correlation between dust and was identified. Upcoming Hubble Space Telescope/Cosmic Origins Spectrograph LyC observations of our sample will test if Mg ii and dust are predictors of , providing insights for future JWST studies of high-redshift galaxies.

CLASSY VII Lyα Profiles: The Structure and Kinematics of Neutral Gas and Implications for LyC Escape in Reionization-era Analogs* * Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

Lyα line profiles are a powerful probe of interstellar medium (ISM) structure, outflow speed, and Lyman-continuum escape fraction. In this paper, we present the Lyα line profiles of the Cosmic Origins Spectrograph (COS) Legacy Archive Spectroscopic SurveY, a sample rich in spectroscopic analogs of reionization-era galaxies. A large fraction of the spectra show a complex profile, consisting of a double-peaked Lyα emission profile in the bottom of a damped, Lyα absorption trough. Such profiles reveal an inhomogeneous ISM. We successfully fit the damped Lyα absorption and the Lyα emission profiles separately, but with complementary covering factors, a surprising result because this approach requires no Lyα exchange between high-N H i and low-N H i paths. The combined distribution of column densities is qualitatively similar to the bimodal distributions observed in numerical simulations. We find an inverse relation between Lyα peak separation and the [O iii]/[O ii] flux ratio, confirming that the covering fraction of Lyman-continuum-thin sightlines increases as the Lyα peak separation decreases. We combine measurements of Lyα peak separation and Lyα red peak asymmetry in a diagnostic diagram, which identifies six Lyman-continuum leakers in the COS Legacy Archive Spectrocopy SurveY (CLASSY) sample. We find a strong correlation between the Lyα trough velocity and the outflow velocity measured from interstellar absorption lines. We argue that greater vignetting of the blueshifted Lyα peak, relative to the redshifted peak, is the source of the well-known discrepancy between shell-model parameters and directly measured outflow properties. The CLASSY sample illustrates how scattering of Lyα photons outside the spectroscopic aperture reshapes Lyα profiles because the distances to these compact starbursts span a large range.

Abundances of CNO elements in z ∼ 0.3–0.4 Lyman continuum leaking galaxies

ABSTRACT We present observations with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope (HST) of 11 Lyman continuum (LyC) leaking galaxies at redshifts, z, in the range 0.29–0.43, with oxygen abundances 12+log(O/H) = 7.64–8.16, stellar masses M⋆ ∼ 107.8–109.8 M⊙, and O32 = [O iii] λ5007/[O ii] λ3727 of ∼ 5–20 aiming to detect the C iii] λ1908 emission line. We combine these observations with the optical Sloan Digital Sky Survey (SDSS) spectra for the determination of carbon, nitrogen, and oxygen abundances. Our sample was supplemented by 31 galaxies from the literature, for which carbon, nitrogen, and oxygen abundances can be derived from the HST and SDSS spectra. These additional galaxies, however, do not have LyC observations. We find that log(C/O) for the entire sample at 12+log(O/H) &amp;lt; 8.1 does not depend on metallicity, with a small dispersion of ∼0.13 dex around the average value of ∼−0.75 dex. On the other hand, the log(N/O) in galaxies at z &amp;gt; 0.1, including LyC leakers, is systematically higher compared to the rest of the sample with lower metallicity. We find that log(C/O) slightly decreases with increasing M⋆ from ∼ −0.65 at M⋆ = 106 M⊙ to ∼ −0.80 at M⋆ = 109–1010 M⊙, whereas log(N/O) is considerably enhanced at M⋆ &amp;gt; 108 M⊙. The origin of these trends remains basically unknown. A possible solution would be to assume that the upper mass limit of the stellar initial mass function in more massive galaxies is higher. This would result in a higher production of oxygen and a larger fraction of massive stars with stellar wind polluting the interstellar medium with nitrogen.

Closing in on the sources of cosmic reionization: First results from the GLASS-JWST program

The escape fraction of Lyman-continuum (LyC) photons (fesc) is a key parameter for determining the sources of cosmic reionization at z ≥ 6. At these redshifts, owing to the opacity of the intergalactic medium, the LyC emission cannot be measured directly. However, LyC leakers during the epoch of reionization could be identified using indirect indicators that have been extensively tested at low and intermediate redshifts. These include a high [O III]/[O II] flux ratio, high star-formation surface density, and compact sizes. In this work, we present observations of 29 4.5 ≤ z ≤ 8 gravitationally lensed galaxies in the Abell 2744 cluster field. From a combined analysis of JWST-NIRSpec and NIRCam data, we accurately derived their physical and spectroscopic properties: our galaxies have low masses (log(M⋆)∼8.5), blue UV spectral slopes (β ∼ −2.1), compact sizes (re ∼ 0.3 − 0.5 kpc), and high [O III]/[O II] flux ratios. We confirm that these properties are similar to those characterizing low-redshift LyC leakers. Indirectly inferring the fraction of escaping ionizing photons, we find that more than 80% of our galaxies have predicted fesc values larger than 0.05, indicating that they would be considered leakers. The average predicted fesc value of our sample is 0.12, suggesting that similar galaxies at z ≥ 6 have provided a substantial contribution to cosmic reionization.

Early Results from GLASS-JWST. XVI. Discovering a Bluer z ∼ 4–7 Universe through UV Slopes

We use the GLASS-JWST Early Release Science NIRCam parallel observations to provide a first view of the UV continuum properties of NIRCam/F444W selected galaxies at 4 < z < 7. By combining multiwavelength NIRCam observations, we constrain the UV continuum slope for a sample of 401 galaxies with stringent quality controls. We find that >99% of the galaxies are blue star-forming galaxies with very low levels of dust (Av β ∼ 0.01 ± 0.33). We find no statistically significant correlation for UV slope with redshift or UV magnitude. However, we find that in general galaxies at higher redshifts and fainter UV magnitudes have steeper UV slopes. We find a statistically significant correlation for UV slope with stellar mass, with galaxies with higher stellar mass showing shallower UV slopes. Individual fits to some of our galaxies reach the bluest UV slopes of β ∼ −3.1 allowed by stellar population models used in this analysis. Therefore, it is likely that stellar population models with a higher amount of Lyman continuum leakage, active galactic nucleus effects, and/or Population III contributions are required to accurately reproduce the rest-UV and optical properties of some of our bluest galaxies. This dust-free early view confirms that our current cosmological understanding of gradual mass + dust buildup of galaxies with cosmic time is largely accurate to describe the ∼0.7–1.5 Gyr age window of the universe. The abundance of a large population of UV faint dust-poor systems may point to a dominance of low-mass galaxies at z > 6 playing a vital role in cosmic reionization.

Lyman continuum leaker candidates among highly ionised, low-redshift dwarf galaxies selected from He II

Context. Contemporary research suggests that the reionisation of the intergalactic medium (IGM) in the early Universe was predominantly realised by star-forming (proto-)galaxies (SFGs). Due to observational constraints, our knowledge on the origins of sufficient amounts of ionising Lyman continuum (LyC) photons and the mechanisms facilitating their transport into the IGM remains sparse. Recent efforts have thus focussed on the study of local analogues to these high-redshift objects. Aims. We aim to acquire a set of very low-redshift SFGs that exhibit signs of a hard radiation field being present. A subsequent analysis of their emission line properties is intended to shed light on how the conditions prevalent in these objects compare to those predicted to be present in early SFGs that are thought to be LyC emitters (LCEs). Methods. We used archival spectroscopic SDSS DR12 data to select a sample of low-redshift He II 4686 emitters and restricted it to a set of SFGs with an emission line diagnostic sensitive to the presence of an active galactic nucleus, which serves as our only selection criterion. We performed a population spectral synthesis with FADO to reconstruct these galaxies’ star-formation histories (SFHs). Utilising the spectroscopic information at hand, we constrained the predominant ionisation mechanisms in these galaxies and inferred information on interstellar medium (ISM) conditions relevant for the escape of LyC radiation. Results. Our final sample consists of eighteen ionised, metal-poor galaxies (IMPs). These low-mass (6.2 ≤ log(M⋆/M⊙) ≤ 8.8), low-metallicity (7.54 ≤ log(O/H) + 12 ≤ 8.13) dwarf galaxies appear to be predominantly ionised by stellar sources. We find large [OIII] 5007/[OII] 3727 ratios and [SII] 6717,6731/Hα deficiencies, which provide strong indications for these galaxies to be LCEs. At least 40% of these objects are candidates for featuring cosmologically significant LyC escape fractions ≳10%. The IMPs’ SFHs exhibit strong similarities and almost all galaxies appear to contain an old (&gt; 1 Gyr) stellar component, while also harbouring a young, two-stage (∼10 Myr and &lt; 1 Myr) starburst, which we speculate might be related to LyC escape. Conclusions. The properties of the compact emission line galaxies presented here align well with those observed in many local LCEs. In fact, our sample may prove as an extension to the rather small catalogue of local LCEs, as the extreme ISM conditions we find are assumed to facilitate LyC leakage. Notably, all of our eighteen candidates are significantly closer (z &lt; 0.1) than most established LCEs. If the inferred LyC photon loss is genuine, this demonstrates that selecting SFGs from He II 4686 is a powerful selection criterion in the search for LCEs.