Lyα Escape Fraction Research Articles

The Lyα Nondetection by JWST NIRSpec of a Strong Lyα Emitter at z = 5.66 Confirmed by MUSE

The detections of Lyα emission in galaxies with redshifts above 5 are of utmost importance for constraining the cosmic reionization timeline, yet such detections are usually based on slit spectroscopy. Here we investigate the significant bias induced by slit placement on the estimate of Lyα escape fraction ( fescLyα ) by presenting a galaxy (dubbed A2744-z6Lya) at z = 5.66, where its deep JWST/NIRSpec prism spectroscopy completely misses the strong Lyα emission detected in the MUSE data. A2744-z6Lya exhibits a pronounced UV continuum with an extremely steep spectral slope of β=−2.640−0.008+0.008 , and it has a stellar mass of ∼108.75 M ⊙, a star formation rate of ∼5.95 M ⊙ yr−1, and a gas-phase metallicity of 12+log(O/H)∼7.90 . The observed flux and rest-frame equivalent width of its Lyα from MUSE spectroscopy are 1.2 × 10−16 erg s−1 cm−2 and 75 Å, equivalent to fescLyα=42%±1% . However, its Lyα nondetection from JWST NIRSpec gives a 5σ upper limit of <5%, in stark contrast to that derived from MUSE. To explore the reasons for this bias, we perform spatially resolved stellar population analysis of A2744-z6Lya using the JWST NIRCam and HST imaging data to construct 2D maps of star formation rate, dust extinction, and neutral hydrogen column density. We find that the absence of Lyα in the slit regions probably stems from both the resonance scattering effect of neutral hydrogen and dust extinction. Through analyzing an extreme case in detail, this work highlights the important caveat of inferring fescLyα from slit spectroscopy, particularly when using the JWST multiplexed NIRSpec microshutter assembly.

Multivariate Predictors of Lyman Continuum Escape. I. A Survival Analysis of the Low-redshift Lyman Continuum Survey* * Based on observations made with the NASA/ESA Hubble Space Telescope, obtained 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. These observations are associated with programs GO-15626, GO-13744,

To understand how galaxies reionized the Universe, we must determine how the escape fraction of Lyman continuum (LyC) photons (f esc) depends on galaxy properties. Using the z ∼ 0.3 Low-redshift Lyman Continuum Survey (LzLCS), we develop and analyze new multivariate predictors of f esc. These predictions use the Cox proportional hazards model, a survival analysis technique that incorporates both detections and upper limits. Our best model predicts the LzLCS f esc detections with an rms scatter of 0.31 dex, better than single-variable correlations. According to ranking techniques, the most important predictors of f esc are the equivalent width (EW) of Lyman-series absorption lines and the UV dust attenuation, which track line-of-sight absorption due to H i and dust. The H i absorption EW is uniquely crucial for predicting f esc for the strongest LyC emitters, which show properties similar to weaker LyC emitters and whose high f esc may therefore result from favorable orientation. In the absence of H i information, star formation rate surface density (ΣSFR) and [O iii]/[O ii] ratio are the most predictive variables and highlight the connection between feedback and f esc. We generate a model suitable for z > 6, which uses only the UV slope, ΣSFR, and [O iii]/[O ii]. We find that ΣSFR is more important in predicting f esc at higher stellar masses, whereas [O iii]/[O ii] plays a greater role at lower masses. We also analyze predictions for other parameters, such as the ionizing-to-nonionizing flux ratio and Lyα escape fraction. These multivariate models represent a promising tool for predicting f esc at high redshift.

Peering into cosmic reionization: Lyα visibility evolution from galaxies at z = 4.5–8.5 with JWST

The resonant scattering interaction between Lyα photons and neutral hydrogen implies that a partially neutral intergalactic medium has the ability to significantly impact the detectability of Lyα emission in galaxies. Thus, the redshift evolution of the Lyα equivalent width distribution of galaxies offers a key observational probe of the degree of ionization during the Epoch of Reionization (EoR). Previous in-depth investigations at z ≥ 7 were limited by ground-based instrument capabilities. We present an extensive study of the evolution of Lyα emission from galaxies at 4.5 &lt; z &lt; 8.5, observed as part of the CEERS and JADES surveys in the JWST NIRSpec/PRISM configuration. The sample consists of 235 galaxies in the redshift range of 4.1 &lt; z &lt; 9.9. We identified 65 of them as Lyα emitters. We first measured the Lyα escape fractions from Lyα to Balmer line flux ratios and explored the correlations with the inferred galaxies’ physical properties, which are similar to those found at lower redshift. We also investigated the possible connection between the escape of Lyα photons and the inferred escape fractions of LyC photons obtained from indirect indicators, finding no secure correlation. We then analyzed the redshift evolution of the Lyα emitter fraction, finding lower average values at z = 5 and 6 compared to previous ground-based observations. At z = 7, the GOODS-S results are aligned with previous findings, whereas the visibility in the EGS field appears to be enhanced. This discrepancy in Lyα visibility between the two fields could potentially be attributed to the presence of early reionized regions in the EGS. Such a broad variance is also expected in the Cosmic Dawn II radiation-hydrodynamical simulation. The average Lyα emitter fraction obtained from the CEERS+JADES data continues to increase from z = 5 to 7, ultimately declining at z = 8. This suggests a scenario in which the ending phase of the EoR is characterized by ∼1 pMpc ionized bubbles around a high fraction of moderately bright galaxies. Finally, we characterize such two ionized regions found in the EGS at z = 7.18 and z = 7.49 by estimating the radius of the ionized bubble that each of the spectroscopically-confirmed members could have created.

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.

Nebular dominated galaxies: insights into the stellar initial mass function at high redshift

ABSTRACT We identify a low-metallicity ($12+\log ({\rm O}/{\rm H})=7.59$) Ly $\alpha$-emitting galaxy at $z=5.943$ with evidence of a strong Balmer jump, arising from nebular continuum. While Balmer jumps are sometimes observed in low-redshift star-forming galaxies, this galaxy also exhibits a steep turnover in the UV continuum. Such turnovers are typically attributed to absorption by a damped Ly $\alpha$ system (DLA); however, the shape of the turnover and the high observed Ly $\alpha$ escape fraction ($f_{\rm esc,Ly\alpha }~\sim 27~{{\ \rm per\ cent}}$) is also consistent with strong nebular two-photon continuum emission. Modelling the UV turnover with a DLA requires extreme column densities ($N_{\rm HI}\,\,\gt\,\, 10^{23}$ cm$^{-2}$), and simultaneously explaining the high $f_{\rm esc,Ly\alpha }$ requires a fine-tuned geometry. In contrast, modelling the spectrum as primarily nebular provides a good fit to both the continuum and emission lines, motivating scenarios in which (a) we are observing only nebular emission or (b) the ionizing source is powering extreme nebular emission that outshines the stellar emission. The nebular-only scenario could arise if the ionizing source has ‘turned off’ more recently than the recombination time-scale ($\sim$1000 yr), hence we may be catching the object at a very specific time. Alternatively, hot stars with $T_{\rm eff}\gtrsim 10^5$ K (e.g. Wolf–Rayet or low-metallicity massive stars) produce enough ionizing photons such that the two-photon emission becomes visible. While several stellar SEDs from the literature fit the observed spectrum well, the hot-star scenario requires that the number of $\gtrsim 50~{\rm M}_\odot$ stars relative to $\sim 5\!-\!50~{\rm M}_\odot$ stars is significantly higher than predicted by typical stellar initial mass functions (IMFs). The identification of more galaxies with similar spectra may provide evidence for a top-heavy IMF at high redshift.

Quantifying the Escape of Lyα at z ≈ 5–6: A Census of Lyα Escape Fraction with Hα-emitting Galaxies Spectroscopically Confirmed by JWST and VLT/MUSE

The James Webb Space Telescope provides an unprecedented opportunity for unbiased surveys of Hα-emitting galaxies at z > 4 with the NIRCam's wide-field slitless spectroscopy (WFSS). In this work, we present a census of Lyα escape fraction (f esc,Lyα ) of 165 star-forming galaxies at z = 4.9–6.3, utilizing their Hα emission directly measured from FRESCO NIRCam/WFSS data. We search for Lyα emission of each Hα-emitting galaxy in the Very Large Telescope/MUSE data. The overall f esc,Lyα measured by stacking is 0.090 ± 0.006. We find that f esc,Lyα displays a strong dependence on the observed UV slope (β obs) and E(B − V), such that the bluest galaxies (β obs ∼ −2.5) have the largest escape fractions (f esc,Lyα ≈ 0.6), indicative of the crucial role of dust and gas in modulating the escape of Lyα photons. f esc,Lyα is less well related to other parameters, including the UV luminosity and stellar mass, and the variation in f esc,Lyα with them can be explained by their underlying coupling with E(B − V) or β obs. Our results suggest a tentative decline in f esc,Lyα at z ≳ 5, implying increasing intergalactic medium attenuation toward higher redshift. Furthermore, the dependence of f esc,Lyα on β obs is proportional to that of the ionizing photon escape fraction (f esc,LyC), indicating that the escape of Lyα and ionizing photon may be regulated by similar physical processes. With f esc,Lyα as a proxy to f esc,LyC, we infer that UV-faint (M UV > −16) galaxies contribute >70% of the total ionizing emissivity at z = 5–6. If these relations hold during the epoch of reionization, UV-faint galaxies can contribute the majority of UV photon budget to reionize the Universe.

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).

Haro 11: The Spatially Resolved Lyman Continuum Sources

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 × 107 M ⊙ 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.

JWST spectroscopy of z ∼ 5–8 UV-selected galaxies: new constraints on the evolution of the Ly α escape fraction in the reionization era

ABSTRACT We describe JWST/NIRSpec prism measurements of Ly α emission in z ≳ 5 galaxies. We identify Ly α detections in 10 out of 69 galaxies with robust rest-optical emission-line redshift measurements at 5 ≤ z &amp;lt; 7 in the Cosmic Evolution Early Release Science (CEERS) and DDT-2750 observations of the Extended Groth Strip field. Galaxies at z ≃ 6 with faint continuum (F150W=27–29 mag) are found with extremely large rest-frame Ly α equivalent widths (EWs; ranging up to 286 Å). Likely Ly α detections are also seen in two new z &amp;gt; 7 galaxies (z = 7.49 and 7.17) from the second epoch of CEERS observations, both showing large Ly α EWs that likely indicate significant transmission through the intergalactic medium (IGM). We measure high Ly α escape fractions in the 12 Ly α emitters in our sample (median 0.28), two of which show $f_{\rm esc}^{ {\rm Ly}\alpha }$ near unity (&amp;gt;0.80). We find that $50_{-11}^{+11}$ per cent of z ≃ 6 galaxies with [O iii] + H β EW&amp;gt;1000 Å have $f_{\rm esc}^{ {\rm Ly}\alpha }$ &amp;gt;0.2, consistent with the fractions found in lower redshift samples with matched [O iii] + H β EWs. While uncertainties are still significant, we find that only $10_{-5}^{+9}$ per cent of z &amp;gt; 7 galaxies with similarly strong rest optical emission lines show such large $f_{\rm esc}^{ {\rm Ly}\alpha }$, as may be expected if IGM attenuation of Ly α increases towards higher redshifts. We identify photometric galaxy overdensities near the z ≳ 7 Ly α emitters, potentially providing the ionizing flux necessary to create large ionized sightlines that facilitate Ly α transmission. Finally, we investigate the absence of Ly α emission in a comparable (and spectroscopically confirmed) galaxy overdensity at z = 7.88 in the Abell 2744 field, discussing new prism spectra of the field obtained with the UNCOVER program.

A Survey of Lyα Emission around Damped Lyα Absorbers at z ≈ 2 with the Keck Cosmic Web Imager

We present Keck Cosmic Web Imager Lyα integral field spectroscopy of the fields surrounding 14 damped Lyα absorbers (DLAs) at z ≈ 2. Of these 14 DLAs, nine have high metallicities ([M/H] > − 0.3), and four of those nine feature a CO-emitting galaxy at an impact parameter ≲30 kpc. Our search reaches median Lyα line flux sensitivities of ∼2 × 10−17 erg s−1 cm−2 over apertures of ∼6 kpc and out to impact parameters of ∼50 kpc. We recover the Lyα flux of three known Lyα-emitting H i-selected galaxies in our sample. In addition, we find two Lyα emitters at impact parameters of ≈50–70 kpc from the high-metallicity DLA at z ≈ 1.96 toward QSO B0551-366. This field also contains a massive CO-emitting galaxy at an impact parameter of ≈15 kpc. Apart from the field with QSO B0551-366, we do not detect significant Lyα emission in any of the remaining eight high-metallicity DLA fields. Considering the depth of our observations and our ability to recover previously known Lyα emitters, we conclude that H i-selected galaxies associated with high-metallicity DLAs at z ≈ 2 are dusty and therefore might feature low Lyα escape fractions. Our results indicate that complementary approaches—using Lyα, CO, Hα, and [C ii] 158 μm emission—are necessary to identify the wide range of galaxy types associated with z ≈ 2 DLAs.

JADES: Discovery of extremely high equivalent width Lyman-α emission from a faint galaxy within an ionized bubble at z = 7.3

We report the discovery of a remarkable Lyα emitting galaxy at z = 7.2782, JADES-GS+53.16746−27.7720 (shortened to JADES-GS-z7-LA), with rest-frame equivalent width, EW0(Lyα) = 388.0 ± 88.8 Å and UV magnitude −17.0. The spectroscopic redshift is confirmed via rest-frame optical lines [O II], Hβ and [O III] in its JWST/NIRSpec Micro-Shutter Assembly (MSA) spectrum. The Lyα line is detected in both lower resolution (R ∼ 100) PRISM as well as medium resolution (R ∼ 1000) G140M grating spectra. The line spread function-deconvolved Lyα full width at half maximum in the grating is 383.9 ± 56.2 km s−1 and the Lyα velocity offset compared to the systemic redshift is 113.3 ± 80.0 km s−1, indicative of very little neutral gas or dust within the galaxy. We estimate the Lyα escape fraction to be &gt; 70%. JADES-GS-z7-LA has a [O III]/[O II] ratio (O32) of 11.1 ± 2.2 and a ([O III] + [O II])/Hβ ratio (R23) of 11.2 ± 2.6, consistent with low metallicity and high ionization parameters. Deep NIRCam imaging also revealed a close companion source (separated by 0.23″), which exhibits similar photometry to that of JADES-GS-z7-LA, with a photometric excess in the F410M NIRCam image consistent with [O III] + Hβ emission at the same redshift. The spectral energy distribution of JADES-GS-z7-LA indicates a “bursty” star formation history, with a low stellar mass of ≈107 M⊙. Assuming that the Lyα transmission through the intergalactic medium is the same as its measured escape fraction, an ionized region of size &gt; 1.5 pMpc is needed to explain the high Lyα EW and low velocity offset compared to systemic seen in JADES-GS-z7-LA. Owing to its UV-faintness, we show that it is incapable of single-handedly ionizing a region large enough to explain its Lyα emission. Therefore, we suggest that JADES-GS-z7-LA (and possibly the companion source) may be a part of a larger overdensity, presenting direct evidence of overlapping ionized bubbles at z &gt; 7.

Small Region, Big Impact: Highly Anisotropic Lyman-continuum Escape from a Compact Starburst Region with Extreme Physical Properties

Extreme, young stellar populations are considered to be the primary contributor to cosmic reionization. How the Lyman continuum (LyC) escapes these galaxies remains highly elusive, and it is challenging to observe this process in actual LyC emitters without resolving the relevant physical scales. We investigate the Sunburst Arc, a strongly lensed LyC emitter at z = 2.37 that reveals an exceptionally small-scale (tens of parsecs) region of high LyC escape. The small (<100 pc) LyC-leaking region has extreme properties: a very blue UV slope (β = −2.9 ± 0.1), a high ionization state ([O iii] λ5007/[O ii] λ3727 = 11 ± 3 and [O iii] λ5007/Hβ = 6.8 ± 0.4), strong oxygen emission (EW([O iii]) = 1095 ± 40 Å), and a high Lyα escape fraction (0.3 ± 0.03), none of which are found in nonleaking regions of the galaxy. The leaking region’s UV slope is consistent with approximately “pure” stellar light that is minimally contaminated by the surrounding nebular continuum emission or extinguished by dust. These results suggest a highly anisotropic LyC escape process such that LyC is produced and escapes from a small, extreme starburst region where the stellar feedback from an ionizing star cluster creates one or more “pencil-beam” channels in the surrounding gas through which LyC can directly escape. Such anisotropic escape processes imply that random sight-line effects drive the significant scatters between measurements of galaxy properties and LyC escape fraction, and that strong lensing is a critical tool for resolving the processes that regulate the ionizing budget of galaxies for reionization.

Astraeus – VIII. A new framework for Lyman-α emitters applied to different reionization scenarios

ABSTRACT We use the astraeus framework to investigate how the visibility and spatial distribution of Lyman-α (Lyα) emitters (LAEs) during reionization is sensitive to a halo mass-dependent fraction of ionizing radiation escaping from the galactic environment (fesc) and the ionization topology. To this end, we consider the two physically plausible bracketing scenarios of fesc increasing and decreasing with rising halo mass. We derive the corresponding observed Lyα luminosities of galaxies for three different analytic Lyα line profiles and associated Lyα escape fraction ($f_\mathrm{esc}^\mathrm{Ly\alpha }$) models: importantly, we introduce two novel analytic Lyα line profile models that describe the surrounding interstellar medium (ISM) as dusty gas clumps. They are based on parameterizing results from radiative transfer simulations, with one of them relating $f_\mathrm{esc}^\mathrm{Ly\alpha }$ to fesc by assuming the ISM of being interspersed with low-density tunnels. Our key findings are: (i) for dusty gas clumps, the Lyα line profile develops from a central to double peak dominated profile as a galaxy’s halo mass increases; (ii) LAEs are galaxies with $M_h\gtrsim 10^{10}\, {\rm \rm M_\odot }$ located in overdense and highly ionized regions; (iii) for this reason, the spatial distribution of LAEs is primarily sensitive to the global ionization fraction and only weakly in second-order to the ionization topology or a halo mass-dependent fesc; (iv) furthermore, as the observed Lyα luminosity functions reflect the Lyα emission from more massive galaxies, there is a degeneracy between the fesc-dependent intrinsic Lyα luminosity and the Lyα attenuation by dust in the ISM if fesc does not exceed $\sim 50~{{\ \rm per \, cent}}$.

Early Results from GLASS-JWST. XXII. Rest-frame UV–Optical Spectral Properties of Lyα Emitting Galaxies at 3 < z < 6

Lyα emission is possibly the best indirect diagnostic of Lyman continuum (LyC) escape since the conditions that favor the escape of Lyα photons are often the same that allow for the escape of LyC photons. In this work, we present the rest-frame UV–optical spectral characteristics of 11 Lyα emitting galaxies at 3 < z < 6—the redshift range that optimizes between intergalactic medium attenuation effects and temporal proximity to the epoch of reionization. From a combined analysis of JWST/NIRSpec and MUSE data, we present the Lyα escape fraction and study its correlation with other physical properties of galaxies that might facilitate Lyα escape. We find that our galaxies have low masses (80% of the sample with ), compact sizes (median R e ∼ 0.7 kpc), low dust content, moderate [O iii]/[O ii] flux ratios (mean ∼ 6.8 ± 1.2), and moderate Lyα escape fractions (mean 0.11). Our sample shows characteristics that are broadly consistent with low-redshift galaxies with Lyα emission, which are termed as “analogs” of the high-redshift population. We predict the LyC escape fraction in our sample to be low (0.03–0.07), although larger samples in the postreionization epoch are needed to confirm these trends.

The Lyα Reference Sample. XIV. Lyα Imaging of 45 Low-redshift Star-forming Galaxies and Inferences on Global Emission

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.

On the evolution of the size of Lyman alpha haloes across cosmic time: no change in the circumgalactic gas distribution when probed by line emission

ABSTRACT Lyman α (Lyα) is now routinely used as a tool for studying high-redshift galaxies, and its resonant nature means it can trace neutral hydrogen around star-forming galaxies. Integral field spectrograph measurements of high-redshift Lyα emitters indicate that significant extended Lyα halo emission is ubiquitous around such objects. We present a sample of redshift 0.23 to 0.31 galaxies observed with the Hubble Space Telescope selected to match the star formation properties of high-z samples while optimizing the observations for detection of low surface brightness Lyα emission. The Lyα escape fractions range between 0.7 and 37 per cent, and we detect extended Lyα emission around six out of seven targets. We find Lyα halo to UV scale length ratios around 6:1, which is marginally lower than high-redshift observations, and halo flux fractions between 60 and 85 per cent – consistent with high-redshift observations – when using comparable methods. However, our targets show additional extended stellar UV emission: we parametrize this with a new double exponential model. We find that this parametrization does not strongly affect the observed Lyα halo fractions. We find that deeper Hα data would be required to firmly determine the origin of Lyα halo emission; however, there are indications that Hα is more extended than the central FUV profile, potentially indicating conditions favourable for the escape of ionizing radiation. We discuss our results in the context of high-redshift galaxies, cosmological simulations, evolutionary studies of the circumgalactic medium in emission, and the emission of ionizing radiation.

Clustering dependence on Lyα luminosity from MUSE surveys at 3 &lt; z &lt; 6

We investigate the dependence of Lyα emitter (LAE) clustering on Lyα luminosity and connect the clustering properties of ≈L⋆ LAEs with those of much fainter ones, namely, ≈0.04L⋆. We use 1030 LAEs from the MUSE-Wide survey, 679 LAEs from MUSE-Deep, and 367 LAEs from the to-date deepest ever spectroscopic survey, the MUSE Extremely Deep Field. All objects have spectroscopic redshifts of 3 &lt; z &lt; 6 and cover a large dynamic range of Lyα luminosities: 40.15 &lt; log(LLyα/erg s−1) &lt; 43.35. We apply the Adelberger et al. K-estimator as the clustering statistic and fit the measurements with state-of-the-art halo occupation distribution (HOD) models. We find that the large-scale bias factor increases weakly with an increasing line luminosity. For the low-luminosity (log⟨LLyα/[erg s−1]⟩ = 41.22) and intermediate-luminosity (log⟨LLyα/[erg s−1]⟩ = 41.64) LAEs, we compute consistent bias factors blow = 2.43−0.15+0.15 and binterm. = 2.42−0.09+0.10, whereas for the high-luminosity (log⟨LLyα/[erg s−1]⟩ = 42.34) LAEs we calculated bhigh = 2.65−0.11+0.13. Consequently, high-luminosity LAEs occupy dark matter halos (DMHs) with typical masses of log(Mh/[h−1 M⊙]) = 11.09−0.09+0.10, while low-luminosity LAEs reside in halos of log(Mh/[h−1 M⊙]) = 10.77−0.15+0.13. The minimum masses to host one central LAE, Mmin, and (on average) one satellite LAE, M1, also vary with Lyα luminosity, growing from log(Mmin/[h−1M⊙]) = 10.3−0.3+0.2 and log(M1/[h−1 M⊙]) = 11.7−0.2+0.3 to log(Mmin/[h−1 M⊙]) = 10.7−0.3+0.2 and log(M1/[h−1 M⊙]) = 12.4−0.6+0.4 from low- to high-luminosity samples, respectively. The satellite fractions are ≲10% (≲20%) at 1σ (3σ) confidence level, supporting a scenario in which DMHs typically host one single LAE. We next bisected the three main samples into disjoint subsets to thoroughly explore the dependence of the clustering properties on LLyα. We report a strong (8σ) clustering dependence on Lyα luminosity, not accounting for cosmic variance effects, where the highest luminosity LAE subsample (log(LLyα/erg s−1) ≈ 42.53) clusters more strongly (bhighest = 3.13−0.15+0.08) and resides in more massive DMHs (log(Mh/[h−1M⊙] )= 11.43−0.10+0.04) than the lowest luminosity one (log(LLyα/erg s−1) ≈ 40.97), which presents a bias of blowest = 1.79−0.06+0.08 and occupies log(Mh/[h−1M⊙]) = 10.00−0.09+0.12 halos. We discuss the implications of these results for evolving Lyα luminosity functions, halo mass dependent Lyα escape fractions, and incomplete reionization signatures.

An Hα Impression of Lyα Galaxies at z ≃ 6 with Deep JWST/NIRCam Imaging

We present a study of seven spectroscopically confirmed (Lyα-emitting) galaxies at redshift z ≃ 6 using the JWST/NIRCam imaging data. These galaxies, with a wide range of Lyα luminosities, were recently observed in a series of NIRCam broad and medium bands. We constrain the rest-frame UV/optical continua and measure the Hα line emission of the galaxies using the combination of JWST/NIRCam and archival HST/WFC3 infrared photometry. We further estimate the escape fractions of their Lyα photons ( fescLyα ) and the production efficiency of ionizing photons (ξ ion). Among the sample, six out of seven galaxies have Lyα escape fractions of ≲10%, which might be the status for most of the star-forming galaxies at z ≃ 6. One UV-faint Lyα galaxy with an extremely blue UV slope has a large value of fescLyα reaching ≃50%. These galaxies have a broad range of ξ ion over log10 ξ ion,0 (Hz erg−1) ∼ 25.0–26.5. We find that UV-fainter galaxies with bluer UV-continuum slopes likely have higher escape fractions of Lyα photons. We also find that galaxies with higher Lyα line emission tend to produce ionizing photons more efficiently. The most Lyα-luminous galaxy in the sample has a very high ξ ion,0 of log10 ξ ion,0 (Hz erg−1) > 26. Our results support the scenario that Lyα galaxies may have served as an important contributor to cosmic reionization. Blue and bright Lyα galaxies are excellent targets for JWST follow-up spectroscopic observations.

Lyα Escape from Low-mass, Compact, High-redshift Galaxies

We investigate the effects of stellar populations and sizes on Lyα escape in 27 spectroscopically confirmed and 35 photometric Lyα emitters (LAEs) at z ≈ 2.65 in seven fields of the Boötes region of the NOAO Deep Wide-Field Survey. We use deep HST/WFC3 imaging to supplement ground-based observations and infer key galaxy properties. Compared to typical star-forming galaxies (SFGs) at similar redshifts, the LAEs are less massive (M ⋆ ≈ 107–109 M ⊙), younger (ages ≲1 Gyr), smaller (r e < 1 kpc), and less dust-attenuated (E(B−V) ≤ 0.26 mag) but have comparable star formation rates (SFRs ≈ 1–100 M ⊙ yr−1). Some of the LAEs in the sample may be very young galaxies having low nebular metallicities (Z neb ≲ 0.2 Z ⊙) and/or high ionization parameters (). Motivated by previous studies, we examine the effects of the concentration of star formation and gravitational potential on Lyα escape by computing SFR surface density, ΣSFR, and specific SFR surface density, ΣsSFR. For a given ΣSFR, the Lyα escape fraction is higher for LAEs with lower stellar masses. The LAEs have a higher ΣsSFR, on average, compared to SFGs. Our results suggest that compact star formation in a low gravitational potential yields conditions amenable to the escape of Lyα photons. These results have important implications for the physics of Lyα radiative transfer and for the type of galaxies that may contribute significantly to cosmic reionization.

Galactic Winds across the Gas-rich Merger Sequence. II. Lyα Emission and Highly Ionized O vi and N v Outflows in Ultraluminous Infrared Galaxies

This paper is the second in a series aimed at examining the gaseous environments of z ≤ 0.3 quasars and ultraluminous infrared galaxies (ULIRGs) as a function of AGN/host galaxy properties across the merger sequence. This second paper focuses on the Lyα emission and O vi 1032, 1038 and N v 1238, 1243 absorption features, tracers of highly ionized gas outflows, in ULIRGs observed with HST/COS. Lyα emission is detected in 15 out of 19 ULIRGs, and 12 of the 14 clear Lyα detections show emission with blueshifted velocity centroids and/or wings. The equivalent widths of the Lyα emission increase with increasing AGN luminosities and AGN bolometric fractions. The blueshifts of the Lyα emission correlate positively with those of the [O iii] λ5007 emission, where the latter traces the ionized gas outflows. The Lyα escape fractions tend to be slightly larger in objects with stronger AGN and larger outflow velocities, but they do not correlate with nebular line reddening. Among the 12 ULIRGs with good continuum signal-to-noise ratios, O vi and/or N v absorption features are robustly detected in six of them, all of which are blueshifted, indicative of outflows. In the combined ULIRG + quasar sample, the outflows are more frequently detected in the X-ray weak or absorbed sources. The absorption equivalent widths, velocities, and velocity dispersions of the outflows are also higher in the X-ray weak sources. No other strong correlations are visible between the properties of the outflows and those of the AGN or host galaxies.