Lyα Escape Fraction Research Articles

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.

Lyα Line Properties at z = 3.78 and Their Environmental Dependence: A Case Study around a Massive Protocluster

Abstract Lyα-emitting galaxies (LAEs) are easily detectable in the high-redshift universe and are potentially efficient tracers of large-scale structure at early epochs, as long as their observed properties do not depend strongly on environment. We investigate the luminosity and equivalent width functions of LAEs in the overdense field of a protocluster at redshift z ≃ 3.78. Using a large sample of LAEs (many spectroscopically confirmed), we find that the Lyα luminosity distribution is well represented by a Schechter function with log ( L * / erg s − 1 ) = 43.26 − 0.22 + 0.20 and log ( ϕ * / Mpc − 3 ) = − 3.40 − 0.04 + 0.03 with α = −1.5. Fitting the equivalent width distribution as an exponential, we find a scale factor of ω = 79 − 15 + 15 Å. We also measured the Lyα luminosity and equivalent width functions using the subset of LAEs lying within the densest cores of the protocluster, finding similar values for L* and ω. Hence, despite having a mean overdensity more than 2× that of the general field, the shapes of the Lyα luminosity function and equivalent width distributions in the protocluster region are comparable to those measured in the field LAE population by other studies at similar redshift. While the observed Lyα luminosities and equivalent widths show correlations with the UV continuum luminosity in this LAE sample, we find that these are likely due to selection biases and are consistent with no intrinsic correlations within the sample. This protocluster sample supports the strong evolutionary trend observed in the Lyα escape fraction and suggests that LAEs at lower redshift can be on average significantly more dusty that their counterparts at higher redshift.

The Role of Dust, UV Luminosity and Large-scale Environment on the Escape of Lyα Photons: A Case Study of a Protocluster Field at z = 3.1

Abstract We present a detailed characterization of the Lyα properties for 93 Lyα emitters (LAEs) at z ∼ 3.1 selected from the D1 field of the Canada–France–Hawaii Telescope Legacy Survey, including 24 members of a massive protocluster. The median-stacked Lyα image shows an extended Lyα halo (LAH) surrounding the galaxy with the exponential scale length 4.9 ± 0.7 kpc, which accounts for roughly half of the total line flux. Accounting for the LAH contribution, the total Lyα escape fraction, f esc, is 40% ± 26%. Combining the data set with existing measurements, we find a dependence of f esc on the galaxy’s UV slope (β) and UV luminosity (L UV). The simultaneous use of both parameters allows prediction of f esc within 0.18 dex, a substantial improvement over 0.23 dex when only β is used. The correlation between f esc and E(B − V) suggests that Lyα photons undergo interstellar dust attenuation in a similar manner to continuum photons. Yet, Lyα transmission is typically higher than that expected for continuum photons at a similar wavelength by a factor, which depends on UV luminosity, up to 2 in the samples we studied. These results hint at complex geometries and physical conditions of the interstellar medium, which affect the Lyα transmission or production. Alternatively, the dust law may change with luminosity leading to an over- or underestimation of f esc. Finally, we report that protocluster LAEs tend to be bluer and more UV luminous than their field cousins, resulting in systematically higher f esc values. We speculate that it may be due to the widespread formation of young low-mass galaxies in dense gas-rich environments.

The HETDEX Survey: The Lyα Escape Fraction from 3D-HST Emission-Line Galaxies at z ∼ 2

Abstract In this work, we measure the Lyα escape fraction of 935 [O iii]-emitting galaxies between 1.9 < z < 2.35 by comparing stacked spectra from the Hubble Space Telescope/WFC3's near-IR grism to corresponding stacks from the Hobby–Eberly Telescope Dark Energy Experiment’s Internal Data Release 2. By measuring the stacks’ Hβ to Lyα ratios, we determine the Lyα escape fraction as a function of stellar mass, star-formation rate, internal reddening, size, and [O iii]/Hβ ratio. We show that the escape fraction of Lyα correlates with a number of parameters, such as galaxy size, star-formation rate, and nebular excitation. However, we also demonstrate that most of these relations are indirect, and that the primary variables controlling the escape of Lyα are likely to be stellar mass and internal extinction. Overall, the escape of Lyα declines from ≳16% in galaxies with to ≲1% for systems with , with the sample’s mean escape fraction being .

The Source of Leaking Ionizing Photons from Haro11: Clues from HST/COS Spectroscopy of Knots A, B, and C*

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

The Origin and Evolution of Lyα Blobs in Cosmological Galaxy Formation Simulations

Abstract High-redshift Lyα blobs (LABs) are an enigmatic class of objects that have been the subject of numerous observational and theoretical investigations. It is of particular interest to determine the dominant power sources for their luminosity, as direct emission from H ii regions, cooling gas, and fluorescence due to the presence of active galactic nuclei (AGNs) can all contribute significantly. In this paper, we present the first theoretical model to consider all of these physical processes in an attempt to develop a model for the origin of LABs. This is achieved by combining a series of high-resolution cosmological zoom-in simulations with ionization and Lyα radiative transfer models. We find that massive galaxies display a range of Lyα luminosities and spatial extents (which strongly depend on the limiting surface brightness used) over the course of their lives, though regularly exhibit luminosities and sizes consistent with observed LABs. The model LABs are typically powered from a combination of recombination in star-forming galaxies, as well as cooling emission from gas associated with accretion. When AGNs are included in the model, the fluorescence caused by active galactic nucleus-driven ionization can be a significant contributor to the total Lyα luminosity as well. Within our modeled mass range, there are no obvious threshold physical properties that predict the appearance of LABs, and only weak correlations of the luminosity with the physical properties of the host galaxy. This is because the emergent Lyα luminosity from a system is a complex function of the gas temperature, ionization state, and Lyα escape fraction.

Can Galaxy Evolution Mimic Cosmic Reionization?

Abstract Lyα emitting galaxies are powerful tools to probe the late stages of cosmic reionization. The observed sudden drop in Lyα fraction at z > 6 is often interpreted as a sign of reionization, since the intergalactic medium (IGM) is more neutral and opaque to Lyα photons. Crucially, this interpretation of the observations is only valid under the assumption that galaxies themselves experience a minimal evolution at these epochs. By modeling Lyα radiative transfer effects in and around galaxies, we examine whether a change in the galactic properties can reproduce the observed drop in the Lyα fraction. We find that an increase in the galactic neutral hydrogen content or a reduction in the outflow velocity toward higher redshift both lead to a lower Lyα escape fraction, and can thus mimic an increasing neutral fraction of the IGM. We furthermore find that this change in galactic properties leads to systematically different Lyα spectra which can be used to differentiate the two competing effects. Using the CANDELSz7 survey measurements which indicate slightly broader lines at z ∼ 6, we find that the scenario of a mere increase in the galactic column density toward higher z is highly unlikely. We also show that a decrease in outflow velocity is not ruled out by existing data but leads to more prominent blue peaks at z > 6. Our results caution using Lyα observations to estimate the IGM neutral fraction without accounting for the potential change in the galactic properties, e.g., by mapping out the evolution of Lyα spectral characteristics.

The Effects of Biconical Outflows on Lyα Escape from Green Peas

Abstract We analyze the spectra of 10 Green Pea galaxies, previously studied by Henry et al., using a semi-analytical line transfer model to interpret emission and absorption features observed in UV galactic spectra. We focus our analysis on various ionization states of silicon, associated with the cool (∼104 K) and warm (∼105 K) gas. By analyzing low-ionization lines, we study the relationships between the distribution and kinematics of the outflowing H i gas and the observed Lyα escape fraction, , as well as the Lyα emission peak separation, Δpeak. We find that outflow geometries that leave a portion of the source uncovered along the line of sight create the best conditions for Lyα escape and have narrow peak separations, while geometries that block the observer’s view of the source create the worst conditions for Lyα escape and have large peak separations. To isolate the effects of outflow kinematics, we restrict our testing set to galaxies with spherical outflows and find that and the Lyα luminosity both increase with the extent of the galactic winds. A simple estimate suggests that the collisional excitation of neutral hydrogen by free electrons in the cool gas of the winds can account for the Lyα luminosity observed in these objects. Finally, we speculate on the relationship between outflows and the escape of ionizing radiation from the circumgalactic medium.

The Lyman Alpha Reference Sample

Context. Lyman-α (Lyα) is the brightest emission line in star-forming galaxies. However, its interpretation in terms of physical properties is hampered by the resonant nature of Lyα photons. In order to remedy this complicated situation, the Lyman Alpha Reference Sample (LARS) was defined, enabling the study of Lyα production and escape mechanisms in 14 local star-forming galaxies. Aims. With this paper, we complement our efforts and study the global dust and (molecular) gas content as well as the properties of gas associated with photon-dominated regions. We aim to characterize the interstellar medium of LARS galaxies, allowing us to relate these newly derived properties to quantities relevant for Lyα escape. Methods. We observed LARS galaxies with Herschel, SOFIA, the IRAM 30m telescope, and APEX, targeting far-infrared (FIR) continuum and emission lines of [C II]158 μm, [O I]63 μm, [O III]88 μm, and low-J CO lines. Using Bayesian methods we derived dust model parameters and estimated the total gas masses for all LARS galaxies, taking into account a metallicity-dependent gas-to-dust ratio. Star formation rates were estimated from FIR, [C II]158 μm, and [O I]63 μm luminosities. Results. LARS covers a wide dynamic range in the derived properties, with FIR-based star formation rates from ∼0.5−100 M⊙ yr−1, gas fractions between ∼15−80%, and gas depletion times ranging from a few hundred megayears up to more than ten gigayears. The distribution of LARS galaxies in the Σgas versus ΣSFR (Kennicutt–Schmidt plane) is thus quite heterogeneous. However, we find that LARS galaxies with the longest gas depletion times, that is, relatively high gas surface densities (Σgas) and low star formation rate densities (ΣSFR), have by far the highest Lyα escape fraction. A strong approximately linear relation is found between the Lyα escape fraction and the total gas (HI+H2) depletion time. We argue that the Lyα escape in those galaxies is driven by turbulence in the star-forming gas that shifts the Lyα photons out of resonance close to the places where they originate. We further report on an extreme [C II]158 μm excess in LARS 5, corresponding to ∼14 ± 3% of the FIR luminosity, which probably is the most extreme [C II]-to-FIR ratio observed in a galaxy (without active nucleus) to date.

SuperBoRG: Exploration of Point Sources at z ∼ 8 in HST Parallel Fields*

Abstract To extend the search for quasars in the epoch of reionization beyond the tip of the luminosity function, we explore point-source candidates at redshift z ∼ 8 in SuperBoRG, a compilation of ∼0.4 deg2 archival medium-deep (m F160W ∼ 26.5 ABmag, 5σ) parallel infrared (IR) images taken with the Hubble Space Telescope (HST). Initial candidates are selected by using the Lyman-break technique. We then carefully analyze source morphology, and robustly identify three point sources at z ∼ 8. Photometric redshift analysis reveals that they are preferentially fit by extragalactic templates, and we conclude that they are unlikely to be low-z interlopers, including brown dwarfs. A clear IRAC ch2 flux excess is seen in one of the point sources, which is expected if the source has strong Hβ+[O iii] emission with a rest-frame equivalent width of ∼3000 Å. Deep spectroscopic data taken with Keck/MOSFIRE, however, do not reveal Lyα emission from the object. In combination with the estimated Hβ+[O iii] equivalent width, we place an upper limit on its Lyα escape fraction of . We estimate the number density of these point sources as ∼1 × 10−6 Mpc−3 mag−1 at M UV ∼ −23 mag. The final interpretation of our results remains inconclusive: extrapolation from low-z studies of faint quasars suggests that ≳100× survey volume may be required to find one of this luminosity. The James Webb Space Telescope will be able to conclusively determine the nature of our luminous point-source candidates, while the Roman Space Telescope will probe ∼200 times the area of the sky with the same observing time considered in this HST study.

The ALPINE-ALMA [CII] survey

Context. The Lyman-α line in the ultraviolet (UV) and the [CII] line in the far-infrared (FIR) are widely used tools to identify galaxies in the early Universe and to obtain insights into interstellar medium (ISM) properties in high-redshift galaxies. By combining data obtained with ALMA in band 7 at ∼320 GHz as part of the ALMA Large Program to INvestigate [CII] at Early Times (ALPINE) with spectroscopic data from DEIMOS at the Keck Observatory, VIMOS and FORS2 at the Very Large Telescope, we assembled a unique sample of 53 main-sequence star-forming galaxies at 4.4 < z < 6 in which we detect both the Lyman-α line in the UV and the [CII] line in the FIR. Aims. The goal of this paper is to constrain the properties of the Lyα emission in these galaxies in relation to other properties of the ISM. Methods. We used [CII], observed with ALMA, as a tracer of the systemic velocity of the galaxies, and we exploited the available optical spectroscopy to obtain the Lyα-[CII] and ISM-[CII] velocity offsets. Results. We find that 90% of the selected objects have Lyα-[CII] velocity offsets in the range 0 < ΔvLyα − [CII] < 400 km s−1, in line with the few measurements available so far in the early Universe, and significantly smaller than those observed at lower redshifts. At the same time, we observe ISM-[CII] offsets in the range −500 < ΔvISM−[CII] < 0 km s−1, in line with values at all redshifts, which we interpret as evidence for outflows in these galaxies. We find significant anticorrelations between ΔvLyα−[CII] and the Lyα rest-frame equivalent width EW0(Lyα) (or equivalently, the Lyα escape fraction fesc(Lyα)): galaxies that show smaller ΔvLyα−[CII] have larger EW0(Lyα) and fesc(Lyα). Conclusions. We interpret these results in the framework of available models for the radiative transfer of Lyα photons. According to the models, the escape of Lyα photons would be favored in galaxies with high outflow velocities, producing large EW0(Lyα) and small ΔvLyα-[CII], in agreement with our observations. The uniform shell model would also predict that the Lyα escape in galaxies with slow outflows (0 < vout < 300 km s−1) is mainly determined by the neutral hydrogen column density (NHI) along the line of sight, while the alternative model by Steidel et al. (2010, ApJ, 717, 289) would more highly favor a combination of NHI at the systemic velocity and covering fraction as driver of the Lyα escape. We suggest that the increase in Lyα escape that is observed in the literature between z ∼ 2 and z ∼ 6 is not due to a higher incidence of fast outflows at high redshift, but rather to a decrease in average NHI along the line of sight, or alternatively, a decrease in HI covering fraction.

The origin of the escape of Lyman α and ionizing photons in Lyman continuum emitters

Context. Identifying the physical mechanisms driving the escape of Lyman continuum (LyC) photons is crucial for the search of Lyman continuum emitter (LCE) candidates. Aims. To understand the physical properties involved in the leakage of LyC photons, we investigate the connection between the H I covering fraction, H I velocity width, the Lyman α (Lyα) properties, and the escape of LyC photons in a sample of 22 star-forming galaxies, which includes 13 confirmed LCEs. Methods. We fit the stellar continuum, dust attenuation, and absorption lines between 920 Å and 1300 Å to extract the H I covering fractions and dust attenuation. Additionally, we measure the H I velocity widths of the optically thick Lyman series and derive the Lyα equivalent widths (EW), escape fractions (fesc), peak velocities, and fluxes at the minimum of the observed Lyα profiles. Results. Overall, we highlight strong observational correlations between the presence of low H I covering fractions and the observation of (1) low Lyα peak velocities; (2) more flux at the profile minimum; and (3) larger EW(Lyα), fesc(Lyα), and fescobs(LyC). Hence, low column density channels are crucial ISM ingredients for the leakage of Lyα and LyC photons. Additionally, galaxies with narrower H I absorption velocity widths have higher Lyα equivalent widths, larger Lyα escape fractions, and lower Lyα peak velocity separations. This may suggest that these galaxies have low H I column density. Finally, we find that dust also regulates the amount of Lyα and LyC radiation that actually escapes the ISM. Conclusions. The ISM porosity is one of the origins of strong Lyα emission, enabling the escape of ionizing photons in low-z leakers. However, this is not sufficient to explain the largest fescobs(LyC), which indicates that the most extreme LCEs are likely to be density-bounded along all lines of sight to the observer. Overall, the neutral gas porosity provides a constraint for a lower limit to the escape fraction of LyC and Lyα photons, which offers a key estimator for assessing the leakage of ionizing photons.

The Importance of Star Formation Intensity in Lyα Escape from Green Pea Galaxies and Lyman Break Galaxy Analogs

Abstract We have studied ultraviolet images of 40 Green Pea galaxies and 15 local Lyman Break Galaxy Analogs to understand the relation between Lyα photon escape and central UV photometric properties. We measured star-formation intensity (SFI; star formation rate per unit area) from the central 250 pc region ( ) using Cosmic Origins Spectrograph near-ultraviolet images from the Hubble Space Telescope. The measured of our sample Green Peas ranges from 2.3–46 M ☉ yr−1 kpc−2, with a geometric mean of 15 M ☉ yr−1 kpc−2 and a standard deviation of 0.266 dex, forming a relatively narrow distribution. The Lyman Break Galaxy Analogs show a similarly narrow distribution of (0.271 dex), though with a larger mean of 28 M ☉ yr−1 kpc−2. We show that while the Lyα equivalent width (EW(Lyα)) and the Lyα escape fraction ( ) are not significantly correlated with the central SFI ( ), both are positively correlated with the ratio of surface brightness to galaxy stellar mass (S 250 pc/M star), with correlation coefficients (p-values) of 0.702 (1 × 10−8) and 0.529 (5 × 10−4) with EW(Lyα) and , respectively. These correlations suggest a scenario where intense central star formation can drive a galactic wind in galaxies with relatively shallow gravitational potential wells, thus clearing channels for the escape of Lyα photons.

Rest-frame UV properties of luminous strong gravitationally lensed Lyα emitters from the BELLS GALLERY Survey

ABSTRACT We present deep rest-frame UV spectroscopic observations using the Gran Telescopio Canarias of six gravitationally lensed Lyα emitters (LAEs) at 2.36 < z < 2.82 selected from the BELLS GALLERY survey. By taking the magnifications into account, we show that LAEs can be as luminous as LLyα ≃ 30 × 1042 erg s−1 and MUV ≃ −23 (AB) without invoking an AGN component, in contrast with previous findings. We measure Lyα rest-frame equivalent widths, $EW_{0}\,\rm (Ly\alpha)$, ranging from 16 to 50 Å and Lyα escape fractions, $f_{\rm esc}\, \rm (Ly\alpha)$, from 10 per cent to 40 per cent. Large $EW_{0}\, \rm (Ly\alpha)$ and $f_{\rm esc}\, \rm (Ly\alpha)$ are found predominantly in LAEs showing weak low-ionization ISM absorption (EW0 ≲ 1 Å) and narrow Lyα profiles (≲300 km s−1 FWHM) with their peak close (≲80 km s−1) to their systemic redshifts, suggestive of less scatter from low H i column densities that favours the escape of Lyα photons. We infer stellar metallicities of Z/Z⊙ ≃ 0.2 in almost all LAEs by comparing the P-Cygni profiles of the wind lines N v1240 Å and C iv1549 Å with those from stellar synthesis models. We also find a trend between MUV and the velocity offset of ISM absorption lines, such as the most luminous LAEs experience stronger outflows. The most luminous LAEs show star formation rates up to ≃180 M⊙ yr−1, yet they appear relatively blue (βUV ≃ −1.8 to −2.0) showing evidence of little dust attenuation [E(B − V) = 0.10–0.14]. These luminous LAEs may be particular cases of young starburst galaxies that have had no time to form large amounts of dust. If so, they are ideal laboratories to study the early phase of massive star formation, stellar and dust mass growth, and chemical enrichment histories of starburst galaxies at high-z.

New Insights on Lyα and Lyman Continuum Radiative Transfer in the Greenest Peas*

Abstract As some of the only Lyman continuum (LyC) emitters at z ∼ 0, Green Pea (GP) galaxies are possible analogs of the sources that reionized the universe. We present HST COS spectra of 13 of the most highly ionized GPs, with [O iii]/[O ii] = 6–35, and investigate correlations between Lyα, galaxy properties, and low-ionization UV lines. Galaxies with high [O iii]/[O ii] have higher Hα equivalent widths (EWs), and high intrinsic Lyα production may explain the prevalence of high Lyα EWs among GPs. While the Lyα escape fraction is closely linked to low gas covering fractions, implying a clumpy gas geometry, narrow Lyα velocity peak separation ( ) correlates with the ionization state, suggesting a density-bounded geometry. We therefore suggest that may trace the residual transparency of low-column-density pathways. Metallicity is associated with both [O iii]/[O ii] and . This trend may result from catastrophic cooling around low-metallicity star clusters, which generates a compact geometry of dense clouds within a low-density inter-clump medium. We find that the relative strength of low-ionization UV emission to absorption correlates with Lyα emission strength and is related to Lyα profile shape. However, as expected for optically thin objects, the GPs with the lowest show both weak low-ionization emission and weak absorption. The strengths of the low-ionization absorption and emission lines in a stacked spectrum do not correspond to any individual spectrum. Galaxies with high [O iii]/[O ii] contain a high fraction of LyC emitter candidates, but [O iii]/[O ii] alone is an insufficient diagnostic of LyC escape.

Emission from the circumgalactic medium: from cosmological zoom-in simulations to multiwavelength observables

ABSTRACT We simulate the flux emitted from galaxy haloes in order to quantify the brightness of the circumgalactic medium (CGM). We use dedicated zoom-in cosmological simulations with the hydrodynamical adaptive mesh refinement code ramses, which are evolved down to z = 0 and reach a maximum spatial resolution of 380 h−1 pc and a gas mass resolution up to $1.8\times 10^{5} \, h^{-1}\, \rm {M}_{\odot }$ in the densest regions. We compute the expected emission from the gas in the CGM using cloudy emissivity models for different lines (e.g. Lyα, C iv, O vi, C vi, O viii) considering UV background fluorescence, gravitational cooling and continuum emission. In the case of Lyα, we additionally consider the scattering of continuum photons. We compare our predictions to current observations and find them to be in good agreement at any redshift after adjusting the Lyα escape fraction. We combine our mock observations with instrument models for Faint Intergalactic Redshifted Emission Balloon-2 (FIREBall-2; UV balloon spectrograph) and HARMONI (visible and NIR IFU on the ELT) to predict CGM observations with either instrument and optimize target selections and observing strategies. Our results show that Lyα emission from the CGM at a redshift of 0.7 will be observable with FIREBall-2 for bright galaxies (NUV∼18 mag), while metal lines like O vi and C iv will remain challenging to detect. HARMONI is found to be well suited to study the CGM at different redshifts with various tracers.

The dominant origin of diffuse Lyα halos around Lyα emitters explored by spectral energy distribution fitting and clustering analysis

AbstractThe physical origin of diffuse Lyα halos (LAHs) around star-forming galaxies is still a matter of debate. We present the dependence of LAH luminosity [L(Lyα)H] on the stellar mass (M⋆), star formation rate, color excess [E(B − V)⋆], and dark matter halo mass (Mh) of the parent galaxy for ∼900 Lyα emitters (LAEs) at z ∼ 2 divided into ten subsamples. We calculate L(Lyα)H using the stacked observational relation between L(Lyα)H and central Lyα luminosity of Momose et al. (2016, MNRAS, 457, 2318), which we find agrees with the average trend of VLT/MUSE-detected individual LAEs. We find that our LAEs have relatively high L(Lyα)H despite low M⋆ and Mh, and that L(Lyα)H remains almost unchanged with M⋆ and perhaps with Mh. These results are incompatible with the cold stream (cooling radiation) scenario and the satellite-galaxy star-formation scenario, because the former predicts fainter L(Lyα)H and both predict steeper L(Lyα)H vs. M⋆ slopes. We argue that LAHs are mainly caused by Lyα photons escaping from the main body and then scattering in the circum-galactic medium. This argument is supported by LAH observations of Hα emitters (HAEs). When LAHs are taken into account, the Lyα escape fractions of our LAEs are about ten times higher than those of HAEs with similar M⋆ or E(B − V)⋆, which may partly arise from lower H i gas masses implied from lower Mh at fixed M⋆, or from another Lyα source in the central part.

Neutral Gas Properties and Lyα Escape in Extreme Green Pea Galaxies∗

Abstract Mechanisms regulating the escape of Lyα photons and ionizing radiation remain poorly understood. To study these processes, we analyze Very Large Array 21 cm observations of one Green Pea (GP), J160810+352809 (hereafter J1608), and Hubble Space Telescope Cosmic Origins Spectrograph (COS) spectra of 17 GP galaxies at . All are highly ionized: J1608 has the highest [O iii] λ5007/[O ii] λ3727 for star-forming galaxies in Sloan Digital Sky Survey, and the 17 GPs have [O iii]/[O ii] ≥ 6.6. We set an upper limit on J1608's H i mass of , near or below average compared to similar-mass dwarf galaxies. In the COS sample, eight GPs show Lyα absorption components, six of which also have Lyα emission. The H i column densities derived from Lyα absorption are high, cm−2 = 19–21, well above the LyC optically thick limit. Using low-ionization absorption lines, we measure covering fractions ( ) of 0.1–1 and find that strongly anticorrelates with Lyα escape fraction. Low covering fractions may facilitate Lyα and LyC escape through dense neutral regions. GPs with all have low neutral gas velocities, while GPs with lower have a larger range of velocities. Conventional mechanical feedback may help establish low in some cases, whereas other processes may be important for GPs with low velocities. Finally, we compare with proposed indicators of LyC escape. Ionizing photon escape likely depends on a combination of neutral gas geometry and kinematics, complicating the use of emission-line diagnostics for identifying LyC emitters.

Predicting Lyα escape fractions with a simple observable

Lyman-α (Lyα) is intrinsically the brightest line emitted from active galaxies. While it originates from many physical processes, for star-forming galaxies the intrinsic Lyα luminosity is a direct tracer of the Lyman-continuum (LyC) radiation produced by the most massive O- and early-type B-stars (M⋆ ≳ 10 M⊙) with lifetimes of a few Myrs. As such, Lyα luminosity should be an excellent instantaneous star formation rate (SFR) indicator. However, its resonant nature and susceptibility to dust as a rest-frame UV photon makes Lyα very hard to interpret due to the uncertain Lyα escape fraction, fesc, Lyα. Here we explore results from the CAlibrating LYMan-α with Hα (CALYMHA) survey at z = 2.2, follow-up of Lyα emitters (LAEs) at z = 2.2 − 2.6 and a z ∼ 0−0.3 compilation of LAEs to directly measure fesc, Lyα with Hα. We derive a simple empirical relation that robustly retrieves fesc, Lyα as a function of Lyα rest-frame EW (EW0): fesc,Lyα = 0.0048 EW0[Å] ± 0.05 and we show that it constrains a well-defined anti-correlation between ionisation efficiency (ξion) and dust extinction in LAEs. Observed Lyα luminosities and EW0 are easy measurable quantities at high redshift, thus making our relation a practical tool to estimate intrinsic Lyα and LyC luminosities under well controlled and simple assumptions. Our results allow observed Lyα luminosities to be used to compute SFRs for LAEs at z ∼ 0−2.6 within ±0.2 dex of the Hα dust corrected SFRs. We apply our empirical SFR(Lyα,EW0) calibration to several sources at z ≥ 2.6 to find that star-forming LAEs have SFRs typically ranging from 0.1 to 20 M⊙ yr−1 and that our calibration might be even applicable for the most luminous LAEs within the epoch of re-ionisation. Our results imply high ionisation efficiencies (log10[ξion/Hz erg−1] = 25.4−25.6) and low dust content in LAEs across cosmic time, and will be easily tested with future observations with JWST which can obtain Hα and Hβ measurements for high-redshift LAEs.

Accurately predicting the escape fraction of ionizing photons using rest-frame ultraviolet absorption lines

The fraction of ionizing photons that escape high-redshift galaxies sensitively determines whether galaxies reionized the early Universe. However, this escape fraction cannot be measured from high-redshift galaxies because the opacity of the intergalactic medium is large at high redshifts. Without methods to measure the escape fraction of high-redshift galaxies indirectly, it is unlikely that we will know what reionized the Universe. Here, we analyze the far-ultraviolet (UV) H I (Lyman series) and low-ionization metal absorption lines of nine low-redshift, confirmed Lyman continuum emitting galaxies. We use the H I covering fractions, column densities, and dust attenuations measured in a companion paper to predict the escape fraction of ionizing photons. We find good agreement between the predicted and observed Lyman continuum escape fractions (within 1.4σ) using both the H I and ISM absorption lines. The ionizing photons escape through holes in the H I, but we show that dust attenuation reduces the fraction of photons that escape galaxies. This means that the average high-redshift galaxy likely emits more ionizing photons than low-redshift galaxies. Two other indirect methods accurately predict the escape fractions: the Lyα escape fraction and the optical [O III]/[O II] flux ratio. We use these indirect methods to predict the escape fraction of a sample of 21 galaxies with rest-frame UV spectra but without Lyman continuum observations. Many of these galaxies have low escape fractions (fesc ≤ 1%), but 11 have escape fractions >1%. Future studies will use these methods to measure the escape fractions of high-redshift galaxies, enabling upcoming telescopes to determine whether star-forming galaxies reionized the early Universe.