The Lyman Alpha Reference Sample XII: Morphology of extended Lyman alpha emission in star-forming galaxies

Abstract

We use Hubble space telescope data of 45 nearby star-forming galaxies to investigate properties of Lyman-alpha (Ly$\hat{I}\pm$) halos, Ly$\hat{I}\pm$ morphology, and the star-forming characteristics of galaxies. We study how the morphology of Ly$\hat{I}\pm$ emission is related to other Ly$\hat{I}\pm$ observables. Furthermore, we study the interdependencies of Ly$\hat{I}\pm$ morphological quantities. We studied the spatial extent of Ly$\hat{I}\pm$ using surface brightness profiles in i) using circular apertures and ii) within faint Ly$\hat{I}\pm$ isophotes. We measured the average intensity and the size of the regions with a high star formation rate density. The morphology of the galaxies was quantified by computing centroid position, axis ratio, and position angle in the Ly$\hat{I}\pm$, ultraviolet continuum, and I band maps. We found that galaxies with more extended star-forming regions possess larger Ly$\hat{I}\pm$ halos. Furthermore, galaxies with more elongated Ly$\hat{I}\pm$ morphology are also more extended in Ly$\hat{I}\pm$. Our data suggest that Ly$\hat{I}\pm$ bright galaxies appear rounder in their Ly$\hat{I}\pm$ morphology, and there is less of a contribution from their Ly$\hat{I}\pm$ halo to their overall luminosity. We compared our results with studies at high redshift and found that whilst the Ly$\hat{I}\pm$ extent in the inner regions of the galaxies in our sample are similar to the high$-z$ Ly$\hat{I}\pm$ emitters (LAEs), Ly$\hat{I}\pm$ halos are more extended in high$-z$ LAEs. Our analysis suggests that the Ly$\hat{I}\pm$ morphology affects the measurement of other observable quantities concerning Ly$\hat{I}\pm$ emission, and some of the conclusions drawn from high redshift LAEs might be biased towards galaxies with specific Ly$\hat{I}\pm$ shapes. In particular, faint Ly$\hat{I}\pm$ emitters have larger Ly$\hat{I}\pm$ scale lengths and halo fractions. This implies that faint Ly$\hat{I}\pm$ emitters are harder to detect at high redshift than previously believed.