On the (non-)enhancement of the Lyman \alpha\ equivalent width by a multiphase interstellar medium

TL;DR

This paper investigates whether a multiphase interstellar medium can enhance the Lyman alpha equivalent width through radiative transfer effects, concluding that such enhancement is unlikely under realistic conditions and suggesting alternative explanations.

Contribution

The study systematically examines physical parameters affecting Lyman alpha escape, demonstrating that multiphase media do not significantly boost equivalent widths in realistic scenarios.

Findings

Clumpiness facilitates Lya escape but not preferentially over continuum

Realistic interstellar media unlikely to produce high Lya EWs via radiative transfer

High EWs more plausibly caused by cooling radiation or anisotropic escape

Abstract

It has been suggested that radiative transfer effects may explain the unusually high equivalent widths (EWs) of the Lya line, observed occasionally from starburst galaxies, especially at high redshifts. If the dust is locked up inside high-density clouds dispersed in an empty intercloud medium, the Lya photons could scatter off of the surfaces of the clouds, effectively having their journey confined to the dustless medium. The continuum radiation, on the other hand, does not scatter, and would thus be subject to absorption inside the clouds. This scenario is routinely invoked when Lya EWs higher than what is expected theoretically are observed, although the ideal conditions under which the results are derived usually are not considered. Here we systematically examine the relevant physical parameters in this idealized framework, testing whether any astrophysically realistic scenarios may lead to such an effect. It is found that although clumpiness indeed facilitates the escape of Lya, it is highly unlikely that any real interstellar media should result in a preferential escape of Lya over continuum radiation. Other possible causes are discussed, and it is concluded that the observed high EWs are more likely to be caused by cooling radiation from cold accretion and/or anisotropic escape of the Lya radiation.