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 luminosi ty. We find that both mechanisms contribute about equally to the range in o xygen abundance between low (logL(B) similar to 8) and high (logL(B) simila r to 10.5) luminosity galaxies. We also consider the transition from spiral s to irregulars, finding that both the oxygen abundance deficiency (indicat ing the degree of mass exchange between a galaxy and its environment) and t he gas fraction vary smoothly along the sequence. Thus we find no 'irregula r versus spiral dichotomy'.