The effect of dust geometry on the Lyα output of galaxies

Abstract

We present the optical spectroscopic follow-up of 31 z = 0.3 Lyα emitters, previously identified by Deharveng et al. We find that 17% of the Lyα emitters have line ratios that require the hard ionizing continuum produced by an active galactic nucleus. The uniform dust screen geometry traditionally used in studies similar to ours is not able to simultaneously reproduce the observed high Lyα/Hα and Hα/Hβ line ratios. We consider different possibilities for the geometry of the dust around the emitting sources. We find that also a uniform mixture of sources and dust does not reproduce the observed line ratios. Instead, these are well reproduced by a clumpy dust screen. This more realistic treatment of the geometry results in extinction corrected (Lyα/Hα)C values consistent with case B recombination theory, whereas a uniform dust screen model would imply values (Lyα/Hα)C higher than 8.7. Our analysis shows that there is no need to invoke ad hoc multiphase media in which the Lyα photons only scatter between the dusty clouds and eventually escape. © 2009 The American Astronomical Society. All rights reserved.