On the Detectability of Lyman-alpha Emission in Star-forming Galaxies: The Role of Dust

Abstract

[Context] Lyman-alpha (Lyα) is now widely used to investigate the galaxy formation and evolution in the high redshift universe. However, without a rigorous understanding of the processes which regulate the Lyα escape fraction, physical interpretations of high-z observations remain questionable.

[Aims] To 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 aim to understand the Lyα escape physics and infer the implications for high-redshift studies.

[Methods] We use HST/ACS 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 appears not to correlate 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 declining relationship between Lyα and E(B-V).

[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.