On the detectability of Lyα emission in star forming galaxies -: The role of dust

Context. Lyman-alpha (Ly alpha) 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 Lya 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 Ly alpha. Thereby, we attempt to understand the Ly alpha escape physics and infer the implications for high-redshift studies. Methods. We use HST/ACS imaging to produce continuum-subtracted Ly alpha maps, and ground-based observations (ESO/NTT and NOT) to map the H alpha emission and the extinction E(B-V) in the gas phase derived from the Balmer decrement H alpha/H beta. Results. When large outflows are present, the Ly alpha emission does not appear to be correlated with the dust content, confirming the role of the Hi kinematics in the escape of Ly alpha photons. In the case of a dense, static Hi covering, we observe a damped absorption with a negative correlation between Ly alpha and E(B-V). We found that the Ly alpha 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 Ly alpha line, star formation rate based on Ly alpha luminosity is underestimated with respect to that derived from UV luminosity. Simple reddening correction does not reconcile SFR(Ly alpha) with the total star formation rate. Conclusions. The dust is not necessarily the main Ly alpha escape regulatory factor. ISM kinematics and geometry may play a more significant role. The failure of simple dust correction to recover the intrinsic Ly alpha/H alpha 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 Ly alpha alone. To this end, we propose a more realistic calibration for SFR(Ly alpha), which accounts for dust attenuation and resonant scattering effects via the Ly alpha escape fraction.