The Lyman Alpha Reference Sample: V. The impact of neutral ISM kinematics and geometry on Lyman Alpha escape

TL;DR

This study investigates how the kinematics and geometry of the neutral interstellar medium influence the escape of Lyman Alpha photons in low-redshift star-forming galaxies, revealing complex interactions and multiple contributing factors.

Contribution

It provides high-resolution UV spectra of 14 galaxies, analyzing neutral ISM properties and their effects on Lyman Alpha escape, and distinguishes between different ISM covering models.

Findings

Lyα escape correlates with high outflow velocities and low covering fractions.

No single factor solely governs Lyα radiative transfer and escape.

A strong anticorrelation exists between Hα equivalent width and covering factor.

Abstract

We present high-resolution far-UV spectroscopy of the 14 galaxies of the Lyman Alpha Reference Sample; a sample of strongly star-forming galaxies at low redshifts ($0.028 < z < 0.18$). We compare the derived properties to global properties derived from multi band imaging and 21 cm HI interferometry and single dish observations, as well as archival optical SDSS spectra. Besides the Lyman $\alpha$ line, the spectra contain a number of metal absorption features allowing us to probe the kinematics of the neutral ISM and evaluate the optical depth and and covering fraction of the neutral medium as a function of line-of-sight velocity. Furthermore, we show how this, in combination with precise determination of systemic velocity and good Ly$\alpha$ spectra, can be used to distinguish a model in which separate clumps together fully cover the background source, from the "picket fence" model named by Heckman et al. (2011). We find that no one single effect dominates in governing Ly$\alpha$ radiative transfer and escape. Ly$\alpha$ escape in our sample coincides with a maximum velocity-binned covering fraction of $\lesssim 0.9$ and bulk outflow velocities of $\gtrsim 50$ km s$^{-1}$, although a number of galaxies show these characteristics and yet little or no Ly$\alpha$ escape. We find that Ly$\alpha$ peak velocities, where available, are not consistent with a strong backscattered component, but rather with a simpler model of an intrinsic emission line overlaid by a blueshifted absorption profile from the outflowing wind. Finally, we find a strong anticorrelation between H$\alpha$ equivalent width and maximum velocity-binned covering factor, and propose a heuristic explanatory model.