The Lyman Alpha Reference Sample XIII: High-Angular Resolution 21cm HI observations of Ly$\alpha$ emitting galaxies

Abstract

The Ly$\alpha$ emission line is one of the main observables of galaxies at high redshift, but its output depends strongly on the neutral gas distribution and kinematics around the star-forming regions where UV photons are produced. We present observations of Ly$\alpha$ and 21-cm HI emission at comparable scales with the goal to qualitatively investigate how the neutral interstellar medium (ISM) properties impact Ly$\alpha$ transfer in galaxies. We have observed 21-cm HI at the highest angular resolution possible (~ 3" beam) with the VLA in two local galaxies from the Lyman Alpha Reference Sample. We contrast this data with HST Ly$\alpha$ imaging and spectroscopy, and MUSE and PMAS ionized gas observations. In LARS08, high intensity Ly$\alpha$ emission is co-spatial with high column density HI where dust content is the lowest. The Ly$\alpha$ line is strongly redshifted, consistent with velocity redistribution which allows Ly$\alpha$ escape from high column density neutral medium with low dust content. In eLARS01, high intensity Ly$\alpha$ emission is located in regions of low column density HI, below the HI data sensitivity limit ($<2\times10^{20}\,$cm$^{-2}$). The perturbed ISM distribution with low column density gas in front of the Ly$\alpha$ emission region plays an important role in the escape. In both galaxies, the faint Ly$\alpha$ emission ($\sim 1\times10^{-16}$erg.s$^{-1}$cm$^{-2}$arcsec$^{-2}$) traces intermediate H$\alpha$ emission regions where HI is found, regardless of the dust content. Dust seems to modulate, but not prevent, the formation of a faint Ly$\alpha$ halo. This study suggests the existence of scaling relations between dust, H$\alpha$, HI, and Ly$\alpha$ emission in galaxies.