Lyman-alpha imaging of starburst galaxies in the local universe and beyond

Abstract: The last decade has seen huge advances in studies of astrophysical cosmology, primarily as a result of developments in telescopic facilities. One of the primary observational signatures of actively star forming galaxies in the distant universe is the Lyman-alpha emission line (Lyα). The line is used either to search for objects or as a spectral feature for definite redshift confirmation. In recent years, high-z Lyα surveys have been used to constrain cosmic star formation history, investigate large scale structure, and examine the neutral hydrogen fraction of the universe. This doctoral thesis is directly concerned with studies of the Lyα emission from star-forming galaxies and the validity of Lyα as a cosmological tool.The approach is to study a sample of local actively star forming galaxies using data obtained with the Hubble Space Telescope (HST). Imaging observations have been performed in the Lyα line, Hα, and various continuum bandpasses in the ultraviolet and optical wavelength domains. Sophisticated tools have been developed for the analysis of the images, resulting also in theoretical exploration of Lyα-related observables from galaxies at high-z. Model simulations are presented, along with a methodology by which to interpret high-z survey data. HST imaging results call into question the interpretations of many high-z Lyα surveys. More specifically, the first direct observational evidence is presented for the emission of Lyα photons after resonant scattering in neutral hydrogen and low surface-brightness Lyα halos are found as a result. Imaging reveals Lyα morphologies that systematically differ from morphologies probed by stellar light or non-resonant tracers of the nebular gas. Based upon Hα observations and recombination theory, the fraction of Lyα photons that escape is found never to exceed 20% in any of the observed targets, despite the violent star-formation known to be taking place. Even after internal dust corrections, a deficit from the predicted Lyα/Hα line ratio is always found. The interpretation is that scattering events systematically enhance the probability of absorption of Lyα by dust grains. If these galaxies are representative of those that fall into the data-sets of high-z Lyα surveys, some cosmological estimates may be in error by an order of magnitude.