Influence of physical galaxy properties on Lyα escape in star-forming galaxies

Abstract

Context. Among the different observational techniques used to select high-redshift galaxies, the hydrogen recombination line Lyman-alpha (Lyα) is of particular interest because it gives access to the measurement of cosmological quantities such as the star formation rate (SFR) of distant galaxy populations. However, interpreting this line and calibrating such observables are still subject to serious uncertainties. Aims. In this context, it important to understand the mechanisms responsible for the attenuation of Lyα emission, and under what conditions the Lyα emission line can be used as a reliable star formation diagnostic tool. Methods. We used a sample of 24 Lyα emitters at z ~ 0.3 with an optical spectroscopic follow-up to calculate the Lyα escape fraction and its dependence upon different physical properties. We also examined the reliability of Lyα as a SFR indicator. We combined these observations with a compilation of Lyα emitters selected at z = 0−0.3 from the literature to assemble a larger sample. Results. We confirm that the Lyα escape fraction clearly depends on the dust extinction following the relation f_(esc)(Lyα) = C_(Lyα) × 10^(−0.4 E(B−V)kLyα) where k_(Lyα) ~ 6.67 and C_(Lyα) = 0.22. However, the correlation does not follow the expected curve for a simple dust attenuation. A higher attenuation can be attributed to a scattering process, while f_(esc)(Lyα) values that are clearly above the continuum extinction curve can be the result of various mechanisms that can lead to an enhancement of the Lyα output. We also observe that the strength of Lyα and the escape fraction appear unrelated to the galaxy metallicity. Regarding the reliability of Lyα as a SFR indicator, we show that the deviation of SFR(Lyα) 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 in f_(esc)(Lyα) with increasing UV luminosity. Moreover, we observe redshift dependence of this relationship, revealing the underlying evolution of f_(esc)(Lyα) with redshift.

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