The Lyman Alpha Reference Sample

XII. Morphology of extended Lyman alpha emission in star-forming galaxies

¹ Department of Astronomy, Oscar Klein Centre, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
e-mail: armin.rasekh@astro.su.se
² European Southern Observatory, Av. Alonso de Córdova 3107, 763 0355 Vitacura, Santiago, Chile
³ Institut d’Astrophysique de Paris (IAP), 98bis Boulevard Arago, 75014 Paris, France
⁴ Centro de Astrobiología (CSIC–INTA), Departamento de Astrofísica, 28692 Villanueva de la Cañada, Spain
⁵ Observatoire de Genéve, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
⁶ Department of Physics & Astronomy, Macalester College, 1600 Grand Avenue, Saint Paul, MN 55105, USA

Received: 5 March 2021
Accepted: 10 September 2021

Abstract

Aims. We use Hubble Space Telescope data of 45 nearby star-forming galaxies to investigate properties of Lyman-alpha (Lyα) halos, Lyα morphology, and the star-forming characteristics of galaxies. We study how the morphology of Lyα emission is related to other Lyα observables. Furthermore, we study the interdependencies of Lyα morphological quantities.

Methods. We studied the spatial extent of Lyα using surface brightness profiles in the following two ways (i) using circular apertures and (ii) within faint Lyα isophotes. We also 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 Lyα, ultraviolet continuum, and I band maps.

Results. We found that galaxies with more extended star-forming regions possess larger Lyα halos. Furthermore, galaxies with more elongated Lyα morphology are also more extended in Lyα. Our data suggest that Lyα bright galaxies appear rounder in their Lyα morphology, and there is less of a contribution from their Lyα halo to their overall luminosity. We compared our results with studies at high redshift and found that whilst the Lyα extent in the inner regions of the galaxies in our sample are similar to the high-z Lyα emitters (LAEs), Lyα halos are more extended in high-z LAEs.

Conclusions. Our analysis suggests that the Lyα morphology affects the measurement of other observable quantities concerning Lyα emission, and some of the conclusions drawn from high redshift LAEs might be biased towards galaxies with specific Lyα shapes. In particular, faint Lyα emitters have larger Lyα scale lengths and halo fractions. This implies that faint Lyα emitters are harder to detect at high redshift than previously believed.