The Structure and Environment of H II Galaxies

Abstract

H II galaxies are dwarf star forming galaxies found in objective prism surveys through their strong emission lines. They have been targets of extensive studies of their spectroscopic properties such as chemical abundances, physical conditions and kinematics. Little surface photometry work has been done on H II galaxies up to now. This thesis is primarily a CCD surface photometry study of the structural properties, optical colors and a statistical analysis of the environment of H II galaxies. In the chapter "The Morphology of H II Galaxies" I have used a sample of 39 H II galaxies with CCD images for a study of their overall morphology and luminosity profiles. The results show that H II galaxies can be classified in two main types: type I H II galaxies are luminous and have disturbed and irregular outer shapes while type II H II galaxies are less luminous and have regular shapes. The outer parts of profiles of all HIi galaxies are well represented by an exponential as in other types of known dwarf galaxies. In "The Dynamics of H II Galaxies" I have investigated the relation between the linear size of the starburst, luminosity and velocity dispersion. It is found that H II galaxies have similar relations as virialized systems which supports the gravitational origin of their overall kinematics. The possible bi-parametric behavior ("fundamental plane") of H II galaxies is also investigated. However, a conclusive answer to this question still requires more and better data. In "The Colors of H II Galaxies" I have presented a high spatial resolution CCD surface photometry study. The colors of the underlying stellar continuum within the starburst are obtained by removing the flux contribution of the emission lines in the broad band filters. Comparison with recent evolutionary population synthesis models show that the observational errors and the uncertainties in the models are still too large to put strict constraints on their past star formation history. However, the distribution of colors of the underlying galaxy in H II galaxies is similar to the colors of some dwarf galaxies which suggests a kinship of these with the quiescent phases of H II galaxies. In "The Environment of H II Galaxies" I have studied the environment of 51 H II galaxies for which I have CCD images, H-beta luminosity, velocity dispersion, H-beta equivalent width and oxygen abundance. The aim of the study is to check the tidal origin of the starburst in H II galaxies and the effect of the environment on their intrinsic properties. It is found that only 12 H II galaxies have a bright neighbour within a projected distance of 1 Mpc and 250 km s-1 in velocity difference. Surprisingly, isolated H II galaxies tend to be of high luminosity and disturbed morphology while H II galaxies with neighbours tend to be low luminosity regular H II galaxies. Furthermore, the metal abundance and the equivalent width of the emission lines in H II galaxies do not depend on the presence of a companion. These results are opposed to what is expected if interaction with a luminous companion is the main mechanism triggering the starbursts. It is also found a loose group of four H II galaxies with no luminous companion. For this, there is the additional difficulty of understanding how these starbursts are synchronized on time scales of less than 107 yrs in systems separated by ~1-2 Mpc.

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