Relation between ionized gas kinematics and Lyman-alpha observables in galaxies

Abstract

In the early universe, galaxies can be detected via their Lyman-alpha emission, which is redshifted in the optical wavelength range (1215:7 Å, 2p to 1s transition of hydrogen). However, not all galaxies in the early universe show Ly-alpha emission. The reasons for this are only partly understood. Neutral hydrogen has a high absorption cross-section for Ly-alpha photons. In galaxies, this leads to a spatial and spectral random walk of the Ly-alpha photons in the interstellar-medium (ISM), which increases the absorption probability by interstellar dust of the Ly-alpha photons and is a main reason that not all galaxies show Ly-alpha emission. The random walk of the photons leads to a diffusion spatially and spectrally, which makes it harder to detect the Ly-alpha radiation. The study of spatially resolved observations with samples of galaxies with and without Ly-alpha emission can give important insights in the prevailing ISM conditions for promoting Ly-alpha escape. Such studies can be performed well on a sample of nearby galaxies, which resemble galaxies from the early universe. For nearby galaxies multi-wavelength observations can be obtained, which allow a detailed study of the ISM conditions influencing the Ly-alpha escape. This thesis uses integral field spectroscopic data obtained from the Potsdam Multi Aperture Spectrophotometer at the Calar Alto 3.5 m telescope to investigate the kinematics of ionized gas in 42 nearby galaxies with young stellar populations and active star formation. We use the Balmer-alpha line (6562:8 Å, 3 to 2 transition) as a tracer for the intrinsic Ly-alpha radiation field in the galaxies. Additionally, we use photometric observations from the Hubble Space Telescope for the Lyman-alpha Reference Sample (LARS) and Extended Lyman-alpha Reference Sample (eLARS) galaxies to obtain the Ly-alpha observables. Turbulent kinematics may shift emitting and absorbing material out of resonance, increasing the likelihood of Ly-alpha escaping from galaxies. To test this hypothesis, we perform a global analysis of the kinematic properties of the LARS and eLARS sample, along with their Ly-alpha observables. We derive velocity fields and velocity dispersion maps from the H-alpha observations, and then we focus on the relation between integrated kinematic quantities and the Ly-alpha observables (luminosity, equivalent width and escape fraction). Prior to the analysis, we apply a newly introduced gradient method to correct our data for point spread function smearing. Our results from Kendall tau statistic tests between ionized gas kinematics and Ly-alpha observables support the hypothesis that galaxies dominated by turbulent kinematics, rather than ordered motions, favor the escape of Ly-alpha. Furthermore, we apply a multivariate linear regression method on the Ly-alpha observables luminosity, equivalent width and escape fraction to asses the importance of the integrated kinematic parameters. Again, we find that intrinsic velocity dispersion is an important parameter in affecting the emergence of Ly-alpha emission. We therefore suggest that dispersion dominated ionized gas kinematics may be a necessary, but not a sufficient, condition for facilitating Ly-alpha escape.