Chemical Composition and Evolution of Irregular and Blue Compact Galaxies

The paper presents spectrophotometric observations of H II regions in the irregular and blue compact galaxies. These observations are used to derive abundances of He, N, O, Ne, S, and Ar relative to H, and the results are consistent with the solution that nitrogen is in part a product of primary nucleosynthesis, and that the ratio of primary production of nitrogen to oxygen is N/O approximately equals 0.02. The relationship between the heavy element abundance and the total mass of galaxies is determined and it is found that the heavy element yield is constant. A model of galactic chemical evolution based on a new initial mass function and on stellar evolutionary models with mass loss agrees with the observations; it is concluded that irregular and blue compact galaxies seem to differ only in the present rate of star formation, but it was not determined whether they are old systems with a present burst, or systems where star formation started only recently.