On the influence of physical galaxy properties on Lyman-alpha escape 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.
