CNO Abundances in Dwarf and Spiral Galaxies

Abstract

CNO abundances in galaxies bear on issues of galactic evolution as well as stellar evolution and nucleosynthesis. Knowledge about them in dwarf and spiral galaxies depends mainly on emission lines from H II regions, with information from stars and supernova remnants available for galaxies in the Local Group. Oxygen abundances can be related to both local and global properties of the parent galaxies such as gas fraction, surface brightness, galaxy luminosity, rotation speed and morphological type, and in H II regions they range from 1/30th solar (using the new calibration) to slightly above solar. C/O is a more or less smoothly increasing function of O/H, flat at low metallicities and rising above 12+log(O/H) = 8.0 with a 45 degree slope. N/O behaves quite similarly in the traditional `primary followed by secondary' style, but with a substantial scatter at least around O-abundance 8.0. C/N ratios show no clear trend with metallicity.