The role of dust
Institut d'Astrophysique de Paris (IAP), 98bis boulevard Arago, 75014 Paris, France e-mail: email@example.com
2 Stockholm Observatory, AlbaNova University Centre, 106 91 Stockholm, Sweden
3 Centro de Astrobiología (CSIC–INTA), 28850 Torrejón de Ardoz, Spain
Accepted: 20 May 2008
Context. Lyman-alpha (Lyα) 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 Lyα 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 Lyα. Thereby, we attempt to understand the Lyα escape physics and infer the implications for high-redshift studies.
Methods. We use HST/ACS imaging to produce continuum-subtracted Lyα maps, and ground-based observations (ESO/NTT and NOT) to map the Hα emission and the extinction in the gas phase derived from the Balmer decrement Hα/Hβ.
Results. When large outflows are present, the Lyα emission does not appear to be correlated 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 negative correlation between Lyα and . We found that the Lyα 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 Lyα line, star formation rate based on Lyα luminosity is underestimated with respect to that derived from UV luminosity. Simple reddening correction does not reconcile SFR(Lyα) with the total star formation rate.
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.
Key words: galaxies: starburst / galaxies: ISM / ultraviolet: galaxies / ISM: dust, extinction / galaxies: general
This work was supported by the Centre National d'Études Spatiales (CNES). It is based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated under NASA contract NAS 5-26555. These observations are associated with programs #GO 9470 and #GO 10575.
Based on observations made with ESO Telescopes at the La Silla Observatories under programme IDs 073.B-0785 and 70.B-0639.
© ESO, 2008