| 1. |
- Atek, H., et al.
(author)
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Empirical estimate of Lyα escape fraction in a statistical sample of Lyα emitters
- 2009
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 506:2, s. L1-L4
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Journal article (peer-reviewed)abstract
- Context: The Lyman-alpha (Lyα) recombination line is a fundamental tool for galaxy evolution studies and modern observational cosmology. However, subsequent interpretations are still prone to a number of uncertainties. Besides numerical efforts, empirical data are urgently needed for a better understanding of the Lyα escape process. Aims: We empirically estimate the Lyα escape fraction in a statistically significant sample of galaxies in a redshift range z ~ 0 - 0.3. This estimate will constrain interpretations of current high-redshift Lyα observations. Methods: An optical spectroscopic follow-up of a sub-sample of 24 Lyα emitters detected by GALEX at z ~ 0.2 - 0.3, combined with a UV-optical sample of local starbursts, both with matched apertures, allow us to quantify the dust extinction through Balmer lines, and to estimate the Lyα escape fraction from the Hα flux corrected for extinction in the framework of the recombination theory. Results: The global escape fraction of Lyα radiation spans a wide range of values and fesc(Lyα) clearly decreases with increasing nebular dust extinction E(B-V). Several objects show fesc(Lyα) greater than fesc(continuum), which may be taken as observational evidence for a clumpy ISM geometry or for an aspherical ISM. Selection biases and aperture size effects may still prevail between z ~ 0.2 - 0.3 Lyman-alpha emitters (LAEs) and local starbursts and may explain the difference observed for fesc(Lyα).
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| 2. |
- Atek, Hakim, et al.
(author)
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On the detectability of Ly-alpha emission in star forming galaxies. The role of dust
- 2008
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 488:491
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Journal article (pop. science, debate, etc.)abstract
- 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.
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| 3. |
- Mas-Hesse, J. M., et al.
(author)
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Local Lyman alpha emitters and their relevance to high-redshift star-forming galaxies
- 2009
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In: Astrophysics and Space Science. - : Springer Science and Business Media LLC. - 0004-640X .- 1572-946X. ; 320:1-3, s. 35-38
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Journal article (peer-reviewed)abstract
- The Ly alpha line is an important diagnostic of star formation and activity in galaxies. The analysis of Ly alpha is complicated due to the resonant nature of the line and radiative transfer effects. High spectral resolution studies of local starburst galaxies with the unprecedented UV capabilities of the HST have shown that this line is either seen in absorption or in emission and in the latter case with a P Cygni profile indicative of a large scale outflow of neutral gas. Moreover, HST imaging obtained with HST-ACS of a sample of 6 star-forming galaxies has revealed that a substantial fraction of the Ly alpha photons are diffused far away from the emissive knots. Since the importance of Ly alpha for tracing large scale structure, correlation functions, and galaxy formation is recognized, Ly alpha will remain a very important probe of the distant universe for the foreseeable future, and it is therefore imperative to acquire a better understanding of what mechanisms regulate our ability to detect this line.
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