The spatial distribution of Lyman alpha from star-forming galaxies in the low redshift Universe

Sammanfattning: The Lyman alpha (Lyα) emission line from the recombination of hydrogen atoms is predicted to be the strongest spectral feature of galaxies. This emission line is regularly used to detect and characterise high redshift galaxies. However, studying this spectral line is challenging. Lyα is a resonant line, meaning it interacts strongly with neutral hydrogen. Consequently, the interpretation of Lyα observations of galaxies is very complicated. Nonetheless, this complexity provides a wealth of information. One way to learn how to extract this information is to focus on low-z star-forming galaxies. The characteristics of these galaxies, such as their gas kinematics and dust, affect Lyα escape from galaxies. This is the primary motivation for the projects included in this thesis, where we focus on Lyα observations in the low-z Universe.We studied the morphology of Lyα emission in galaxies from the largest galaxy sample with available Lyα imaging, the Lyman Alpha Reference Sample (LARS). We investigated the Lyα light distribution and how it affects the Lyα global observables, such as Lyα luminosity, Lyα escape fraction and Lyα equivalent width. We found that the Lyα luminosity anti-correlate with the Lyα halo luminosity fraction. In other words, in galaxies that are faint in Lyα, most of the Lyα luminosity originates from their Lyα halos. Because of surface brightness dimming, this could introduce a detection bias against low luminosity Lyα emitters at high redshift, which explains why observing faint Lyα emitters at high redshift is hard. Hence, the conclusions drawn from studying data that only sample bright LAEs may have been biased.We also investigated the origin of the extended Lyα halo emission using line-of-sight kinematic information in the LARS sample. We found that the gas kinematics in the centres of galaxies plays a vital role. We discovered that Lyα flux in the central regions of galaxies varies with the intensity of the outflow of gas in the centre. If the outflow is weak, galaxies show Lyα absorption or lower Lyα flux than expected (based on the observed Hα flux), which ultimately affects the measured Lyα halo luminosity fraction.Finally, we studied a sample of six intensely star-forming galaxies in the low-z Universe. Due to the strong emission of Hβ + [O III] nebular emission lines and the compact nature of these galaxies, they are called Green Pea galaxies. We looked for any relationship between the Lyα properties and well-established diagnostics for determining whether a galaxy is a Lyman continuum emitter. We found that one of the primary Lyman continuum emitter diagnostics -- Lyα spectral peak separation -- correlates with the Lyα halo luminosity fraction. This result suggests that the Lyα halo luminosity fraction could be used as a diagnostic of Lyman continuum escape.