THE NITROGEN-TO-OXYGEN RATIO IN GALAXIES, AND ITS IMPLICATIONS FOR THE ORIGIN OF NITROGEN

We re-examine the available data on nitrogen and oxygen abundances deduced from the spectra of giant H II regions in external galaxies. If account is taken of the likely differences in temperature between the [O II] and [O III] ionization zones, the derived N/O ratios as a function of O/H can consistently be explained by models in which nitrogen has both a (delayed) primary and a secondary component relative to oxygen, with the secondary component dominating at high abundance. There is also some evidence that the secondary component is delayed (by perhaps 2 x 10(9) yr), and that there is some spread in the ages of the discs of late-type spirals.