The effect of dust geometry on the Lyman-alpha output of galaxies

TL;DR

This study investigates how dust geometry affects Lyman-alpha emission in galaxies, finding that a clumpy dust screen model best explains observed line ratios without requiring complex multi-phase media.

Contribution

It demonstrates that a clumpy dust screen geometry accurately reproduces observed Lyman-alpha and Balmer line ratios, challenging the traditional uniform dust screen assumption.

Findings

Clumpy dust geometry explains observed line ratios.

Uniform dust models cannot reproduce high Lya/Halpha ratios.

No need for multi-phase media to account for Lya escape.

Abstract

We present the optical spectroscopic follow-up of 31 z=0.3 Lyman-alpha (Lya) emitters, previously identified by Deharveng et al. (2008). We find that 17% of the Lya emitters have line ratios that require the hard ionizing continuum produced by an AGN. The uniform dust screen geometry traditionally used in studies similar to ours is not able to simultaneously reproduce the observed high Lya/Halpha and Halpha/Hbeta 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 (Lya/Halpha)_C values consistent with Case B recombination theory, whereas a uniform dust screen model would imply values (Lya/Halpha)_C higher than 8.7. Our analysis shows that there is no need to invoke "ad-hoc" multi phase media in which the Lya photons only scatter between the dusty clouds and eventually escape.