Lyα Escape Fraction Research Articles

Physical properties and evolutionary state of the Lyman alpha emitting starburst galaxy IRAS 08339+6517

Though Lyman alpha emission (Lya) is one of the most used tracers of massive star formation at high redshift, a correct understanding of radiation transfer effects by neutral gas is required to properly quantify the star formation rate along the history of the Universe. We are embarked in a program to study the properties of the Lya emission (spectral profile, spatial distribution, relation to Balmer lines intensity,...) in several local starburst galaxies. We present here the results obtained for IRAS 08339+6517. Using evolutionary population synthesis models, we have characterized the properties of the starburst (UV continuum, Halpha, total infrared and X-ray emissions, etc.), which transformed 1.4e+8 Mo of gas into stars around 5-6 Myr ago. In addition to the central compact emission blob, we have identified a diffuse Lya emission component smoothly distributed over the whole central area of IRAS 08339+6517. This diffuse emission is spatially decoupled from the UV continuum, the Halpha emission or the Halpha/Hbeta ratio. Both locally and globally, the Lya/Halpha ratio is lower than the Case B predictions, even after reddening correction, with an overall Lya escape fraction of only 4%. We conclude that in IRAS 08339+6517 the resonant scattering of Lya photons by an outflowing shell of neutral gas causes their highly-efficient destruction by dust, which explains the low Lya escape fraction measured. These results stress again the importance of a proper correction of scattering and transfer effects when using Lya to derive the star formation rate in high-redshift galaxies.

Z ~ 1 Lyα EMITTERS. I. THE LUMINOSITY FUNCTION.

We construct a flux-limited sample of 135 candidate z ~ 1 Lyα emitters (LAEs) from Galaxy Evolution Explorer (GALEX) grism data using a new data cube search method. These LAEs have luminosities comparable to those at high redshifts and lie within a 7 Gyr gap present in existing LAE samples. We use archival and newly obtained optical spectra to verify the UV redshifts of these LAEs. We use the combination of the GALEX UV spectra, optical spectra, and X-ray imaging data to estimate the active galactic nucleus (AGN) fraction and its dependence on Lyα luminosity. We remove the AGNs and compute the luminosity function (LF) from 60 z ~ 1 LAE galaxies. We find that the best-fit LF implies a luminosity density increase by a factor of ~1.5 from z ~ 0.3 to z ~ 1 and ~20 from z ~ 1 to z ~ 2. We find a z ~ 1 volumetric Lyα escape fraction of 0.7% ± 0.4%.

Influence of physical galaxy properties on Lyαescape in star-forming galaxies

[abridged] Among the different observational techniques used to select high-redshift galaxies, the hydrogen recombination line Lyman-alpha (Lya) is of particular interest as it gives access to the measurement of cosmological quantities such as the star formation rate of distant galaxy populations. However, the interpretation of this line and the calibration of such observables is still subject to serious uncertainties. Therefore, it important to understand under what conditions the Lya line can be used as a reliable star formation diagnostic tool. We use a sample of 24 Lya emitters at z ~ 0.3 with an optical spectroscopic follow-up to calculate the Lya escape fraction and its dependency upon different physical properties. We also examine the reliability of Lya as a star formation rate indicator. We combine these observations with a compilation of Lya emitters selected at z = 0 - 0.3 to assemble a larger sample. The Lya escape fraction depends clearly on the dust extinction following the relation fesc(Lya) = C(Lya) x 10^(-0.4 E(B-V) k(Lya)), but with a shallower slope than previously reported, with k(Lya) ~ 6.67 and C(Lya) = 0.22. However, the correlation does not follow the expected curve for a simple dust attenuation. We explore the various mechanisms than lead to fesc(Lya) values above the continuum extinction curve, i.e. to an enhancement of the Lya output. We also observe that the strength of Lya and the escape fraction appear unrelated to the galaxy metallicity. Regarding the reliability of Lya as a star formation rate (SFR) indicator, we show that the deviation of SFR(Lya) from the true SFR (as traced by the UV continuum) is a function of the observed SFR(UV), which can be seen as the decrease of fesc(Lya) with increasing UV luminosity. Moreover, we observe a redshift-dependence of this relationship revealing the underlying evolution of fesc(Lya) with redshift.

ART2: coupling Lyα line and multi-wavelength continuum radiative transfer

Narrow-band Lyα line and broad-band continuum have played important roles in the discovery of high-redshift galaxies in recent years. Hence, it is crucial to study the radiative transfer of both Lyα and continuum photons in the context of galaxy formation and evolution in order to understand the nature of distant galaxies. Here, we present a three-dimensional Monte Carlo radiative transfer code, All-wavelength Radiative Transfer with Adaptive Refinement Tree (ART2), which couples Lyα line and multi-wavelength continuum, for the study of panchromatic properties of galaxies and interstellar medium. This code is based on the original version of Li et al., and features three essential modules: continuum emission from X-ray to radio, Lyα emission from both recombination and collisional excitation, and ionization of neutral hydrogen. The coupling of these three modules, together with an adaptive refinement grid, enables a self-consistent and accurate calculation of the Lyα properties, which depend strongly on the UV continuum, ionization structure and dust content of the object. Moreover, it efficiently produces multi-wavelength properties, such as the spectral energy distribution and images, for direct comparison with multi-band observations. As an example, we apply ART2 to a cosmological simulation that includes both star formation and black hole growth, and study in detail a sample of massive galaxies at redshifts z = 3.1–10.2. We find that these galaxies are Lyα emitters (LAEs), whose Lyα emission traces the dense gas region, and that their Lyα lines show a shape characteristic of gas inflow. Furthermore, the Lyα properties, including photon escape fraction, emergent luminosity and equivalent width, change with time and environment. Our results suggest that LAEs evolve with redshift, and that early LAEs such as the most distant one detected at z ∼ 8.6 may be dwarf galaxies with a high star formation rate fuelled by infall of cold gas, and a low Lyα escape fraction.

ON THE REDSHIFT EVOLUTION OF THE Lyα ESCAPE FRACTION AND THE DUST CONTENT OF GALAXIES

The Lyα emission line has been proven to be a powerful tool for studying evolving galaxies at the highest redshift. However, in order to use Lyα as a physical probe of galaxies, it becomes vital to know the Lyα escape fraction (fLyαesc). Unfortunately, due to the resonant nature of Lyα, fLyαesc may vary unpredictably and requires empirical measurement. Here, we compile Lyα luminosity functions (LFs) between redshifts z = 0 and 8 and, combined with Hα and ultraviolet data, assess how fLyαesc evolves with redshift. We find a strong upward evolution in fLyαesc over the range z = 0.3–6, which is well fit by the power law fLyαesc∝(1 + z)ξwith ξ = (2.57+0.19−0.12). This predicts that fLyαesc should reach unity at z = 11.1. By comparing fLyαesc and EB−V in individual galaxies we derive an empirical relationship between fLyαesc and EB−V, which includes resonance scattering and can explain the redshift evolution of fLyαesc between z = 0 and 6 purely as a function of the evolution in the dust content of galaxies. Beyond z ≈ 6.5, fLyαesc drops more substantially, an effect attributed to either ionizing photon leakage, or an increase in the neutral gas fraction of the intergalactic medium. While distinguishing between these two scenarios may be extremely challenging, by framing the problem this way we remove the uncertainty of the halo mass from Lyα-based tests of reionization. We finally derive a new method by which to estimate the dust content of galaxies, based purely upon the observed Lyα and UV LFs. These data are characterized by an exponential with an e-folding scale of zEBV ≈ 3.4.

SERENDIPITOUS DISCOVERY OF AN OVERDENSITY OF Lyα EMITTERS ATz∼ 4.8 IN THE CL1604 SUPERCLUSTER FIELD

We present results of a spectroscopic search for Lyα emitters (LAEs) in the Cl1604 supercluster field using the extensive spectroscopic Keck/DEep Imaging Multi-Object Spectrograph database taken as part of the Observations of Redshift Evolution in Large-Scale Environments survey. A total of 12 slitmasks were observed and inspected in the Cl1604 field, spanning a survey volume of 1.365 × 10^4 comoving Mpc^3. We find a total of 17 high-redshift (4.39 ≤ z ≤ 5.67) LAE candidates down to a limiting flux of 1.9 × 10^(–18) erg s^(–1) cm^(–2) (L(Lyα) = 4.6 × 10^(41) erg s^(–1) or ~0.1 L _* at z ~ 5), 13 of which we classify as high quality. The resulting LAE number density is nearly double that of LAEs found in the Subaru deep field at z ~ 4.9 and nearly an order of magnitude higher than in other surveys of LAEs at similar redshifts, an excess that is essentially independent of LAE luminosity. We also report on the discovery of two possible LAE group structures at z ~ 4.4 and z ~ 4.8 and investigate the effects of cosmic variance of LAEs on our results. Fitting a simple truncated single Gaussian model to a composite spectrum of the 13 high-quality LAE candidates, we find a best-fit stellar velocity dispersion of 136 km s^(–1). Additionally, we see modest evidence of a second peak in the composite spectrum, possibly caused by galactic outflows, offset from the main velocity centroid of the LAE population by ~440 km s^(–1) as well as evidence for a nontrivial Lyα escape fraction. We find an average star formation rate density (SFRD) of ~5 × 10^(–3) M_☉ yr^(–1) Mpc^(–3) with moderate evidence for negative evolution in the SFRD from z ~ 4.6 to z ~ 5.7. By simulating the statistical flux loss due to our observational setup, we measure a best-fit luminosity function characterized by Φ_* L_* = 2.2^(+3.9)_(–1.3) × 10^(39) erg s^(–1) Mpc^(–3) for α = –1.6, generally consistent with measurements from other surveys at similar epochs. Finally, we investigate any possible effects from weak or strong gravitational lensing induced by the foreground supercluster, finding that our LAE candidates are minimally affected by lensing processes.

What can Lyman α observations of nearby galaxies tell us about the high redshift Universe?

This discussion session started off with a quick review of some key results regarding Lyα emitters (LAEs) at low redshift: some illustrative images were shown, some intriguing results highlighted, and some comparison with high redshift LAEs made. Then followed an open discussion for about 1 h which touched upon questions such as: how similar are low- and high- z objects, Lyα escape fractions, what observations are needed to go from interesting to understandable results on low- z objects, how do we understand the complexity of Lyα in local galaxies and what does it imply for high- z objects?

On the detectability of Lyα emission in star forming galaxies

Context: Lyman-alpha (Lyα) radiation is now widely used to investigate the galaxy formation and evolution in the high redshift universe. However, without a rigorous understanding of the processes regulating the Lyα escape fraction, physical interpretations of high-z observations remain questionable. Aims: We examine six nearby star forming galaxies to disentangle the role of the dust from other parameters such as gas kinematics, geometry, and ISM morphology in the obscuration of Lyα. Thereby, we attempt to understand the Lyα escape physics and infer the implications for high-redshift studies. Methods: We use HST/ACS imaging to produce continuum-subtracted Lyα maps, and ground-based observations (ESO/NTT and NOT) to map the Hα emission and the extinction E(B-V) in the gas phase derived from the Balmer decrement Hα/Hβ. Results: When large outflows are present, the Lyα emission does not appear to be correlated with the dust content, confirming the role of the Hi kinematics in the escape of Lyα photons. In the case of a dense, static Hi covering, we observe a damped absorption with a negative correlation between Lyα and E(B-V). We found that the Lyα escape fraction does not exceed 10% in all our galaxies and is mostly about 3% or below. Finally, because of the radiative transfer complexity of the Lyα line, star formation rate based on Lyα luminosity is underestimated with respect to that derived from UV luminosity. Simple reddening correction does not reconcile SFR(Lyα) with the total star formation rate. Conclusions: The dust is not necessarily the main Lyα escape regulatory factor. ISM kinematics and geometry may play a more significant role. The failure of simple dust correction to recover the intrinsic Lyα/Hα ratio or the total star formation rate should prompt us to be more cautious when interpreting high-z observations and related properties, such as SFRs based on Lyα alone. To this end, we propose a more realistic calibration for SFR(Lyα), which accounts for dust attenuation and resonant scattering effects via the Lyα escape fraction.

Lyα‐Emitting Galaxies at 0.2 < z < 0.35 fromGALEXSpectroscopy

The GALEX (Galaxy Evolution Explorer) spectroscopic survey mode, with a resolution of about 8 A in the FUV (1350 - 1750 A) and about 20 A in the NUV (1950 - 2750 A) is used for a systematic search of Ly-a emitting galaxies at low redshift. This aims at filling a gap between high-redshift surveys and a small set of objects studied in detail in the nearby universe. A blind search of 7018 spectra extracted in 5 deep exposures (5.65 sq.deg) has resulted in 96 Ly-a emitting galaxy candidates in the FUV domain, after accounting for broad-line AGNs. The Ly-a EWs (equivalent width) are consistent with stellar population model predictions and show no trends as a function of UV color or UV luminosity, except a possible decrease in the most luminous that may be due to small-number statistics. Their distribution in EW is similar to that at z about 3 but their fraction among star-forming galaxies is smaller. Avoiding uncertain candidates, a sub-sample of 66 objects in the range 0.2 < z < 0.35 has been used to build a Ly-a LF (luminosity function). The incompleteness due to objects with significant Ly-a emission but a UV continuum too low for spectral extraction has been evaluated. A comparison with H-a LF in the same redshift domain is consistent with an average Ly-a/H-a of about 1 in about 15 % of the star-forming galaxies. A comparison with high-redshift Ly-a LFs implies an increase of the Ly-a luminosity density by a factor of about 16 from z about 0.3 to z about 3. By comparison with the factor 5 increase of the UV luminosity density in the same redshift range, this suggests an increase of the average Ly-a escape fraction with redshift.