On the origin of the luminosity-metallicity relation for late-type galaxies - Spirals to irregulars transition

We consider the roles of two widely invoked mechanisms for the metallicity- luminosity correlation among late-type galaxies: higher astration level and decreasing efficiency of heavy-element loss with increasing luminosity. We find that both mechanisms contribute about equally to the range in oxygen abundance between low (logL(B) similar to 8) and high (logL(B) similar to 1 0.5) luminosity galaxies. We also consider the transition from spirals to irregulars, finding that bo th the oxygen abundance deficiency (indicating the degree of mass exchange between a galaxy and its environment) and the gas fraction vary smoothly al ong the sequence. Thus we And no "irregular versus spiral dichotomy".