H-alpha in the Outskirts of Galaxies: Constraining the Ionization Sources of Diffuse Extraplanar Gas and the Outer Disk Initial Mass Function

Most stars are born in the dense inner parts of disk galaxies, surrounded by clouds of hydrogen gas that they ionize with their energetic radiation. However, both ionized gas and young stars themselves are found in the diffuse outer disk, as well as above and below the disk. In this thesis I study star formation and ionization of gas in the outskirts of nine nearby disk galaxies. I analyze data obtained with state of the art telescopes and model gas ionization with simulations. In agreement with earlier studies, I find that in all but one of my sample galaxies the gas at the outskirts seems to be ionized by radiation escaping from young massive stars in the inner parts of the galaxies, in combination with shock ionization. Interestingly, this does not seem to be the case for the galaxy ESO 544-27, for which my analysis and simulations support a model where the gas outside the disk is ionised by the radiation of old evolved stars (white dwarfs). Additionally I find that the distribution of the masses of newborn stars in the outskirts of the galaxy M 83 lacks massive stars compared to what is commonly observed in the inner parts of galaxies. These results demonstrate how local conditions determine how stars are born and gas ionized in the outskirts of galaxies. Understanding energetic phenomena (such as star formation and gas ionization) in the diffuse outskirts of galaxies is important for refining cosmological models.