Empirical estimate of Lyα escape fraction in a statistical sample of Lyα emitters

Context. The Lyman-alpha (Ly alpha) 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 Ly alpha escape process. Aims. We empirically estimate the Ly alpha escape fraction in a statistically significant sample of galaxies in a redshift range z similar to 0-0.3. This estimate will constrain interpretations of current high-redshift Ly alpha observations. Methods. An optical spectroscopic follow-up of a sub-sample of 24 Ly alpha emitters detected by GALEX at z similar to 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 Ly alpha escape fraction from the H alpha flux corrected for extinction in the framework of the recombination theory. Results. The global escape fraction of Ly alpha radiation spans a wide range of values and f(esc)(Ly alpha) clearly decreases with increasing nebular dust extinction E(B-V). Several objects show f(esc)(Ly alpha) greater than f(esc)(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 similar to 0.2-0.3 Lyman-alpha emitters (LAEs) and local starbursts and may explain the difference observed for f(esc)(Ly alpha).