2022A&A...659A.183K

Context. The hydrogen Lyman α line is often the only measurable feature in optical spectra of high-redshift galaxies. Its shape and strength are influenced by radiative transfer processes and the properties of the underlying stellar population. High equivalent widths of several hundred Å are especially hard to explain by models and could point towards unusual stellar populations, for example with low metallicities, young stellar ages, and a top-heavy initial mass function. Other aspects influencing equivalent widths are the morphology of the galaxy and its gas properties. Aims. The aim of this study is to better understand the connection between the Lyman α rest-frame equivalent width (EW₀) and spectral properties as well as ultraviolet (UV) continuum morphology by obtaining reliable EW₀ histograms for a statistical sample of galaxies and by assessing the fraction of objects with large equivalent widths. Methods. We used integral field spectroscopy from the Multi Unit Spectroscopic Explorer (MUSE) combined with broad-band data from the Hubble Space Telescope (HST) to measure EW₀. We analysed the emission lines of 1920 Lyman α emitters (LAEs) detected in the full MUSE-Wide (one hour exposure time) and MUSE-Deep (ten hour exposure time) surveys and found UV continuum counterparts in archival HST data. We fitted the UV continuum photometric images using the Galfit software to gain morphological information on the rest-UV emission and fitted the spectra obtained from MUSE to determine the double peak fraction, asymmetry, full-width at half maximum, and flux of the Lyman α line. Results. The two surveys show different histograms of Lyman α EW₀. In MUSE-Wide, 20% of objects have EW₀ > 240 Å, while this fraction is only 11% in MUSE-Deep and ≃16% for the full sample. This includes objects without HST continuum counterparts (one-third of our sample), for which we give lower limits for EW₀. The object with the highest securely measured EW₀ has EW₀ = 589 ± 193 Å (the highest lower limit being EW₀ = 4464 Å). We investigate the connection between EW₀ and Lyman α spectral or UV continuum morphological properties. Conclusions. The survey depth has to be taken into account when studying EW₀ distributions. We find that in general, high EW₀ objects can have a wide range of spectral and UV morphological properties, which might reflect that the underlying causes for high EW₀ values are equally varied.