Eliot Ayache and Kris Youakim
Friday 14 October
10:30 - 11:30
The Lyman-alpha (Lyα) emission is predicted to be the strongest spectral line emitted by galaxies. This emission line is regularly used to detect and observe 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, Lyα emitting 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 Lyα morphology of the largest sample of nearby star-forming galaxies dedicated to Lyα studies called 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 fraction (HF). In other words, in galaxies that are faint in Lyα, most of the Lyα luminosity originates from their Lyα halos. This result suggests that we can study faint Lyα objects by focusing on low Lyα surface brightness halos, which explains why observing faint Lyα emitters (LAEs) at high redshift is hard. Hence, the conclusions drawn from studying data that only sampled 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. Depending on the intensity of the outflow of gas in the centre, we discovered that galaxies show Lyα absorption or lower Lyα flux than expected, which ultimately influences the Lyα HF.
Finally, we studied a sample of six intense star-forming galaxies in the low-z Universe. Due to the strong mission of Hβ + [O III], nebular emission lines and the appearance 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 (LCE). We found that one of the primary LCE diagnostics — peak separation — correlates with HF. This result suggests that a new diagnostic for LyC escape is the Lyα halo luminosity fraction.