Luminosity-dependent line ratios in disks of spiral galaxies

The emission-line intensity ratio of N II forbidden line double-lambda 6548 + 6583 relative to S II forbidden line double-lambda 6717 + 6731 averaged over a spiral disk (excluding the nucleus) increases with increasing galaxy luminosity, M(B) in the range from -14 to -23. S II forbidden line is strong in disks of low-luminosity spirals; N II forbidden line is strong in disks of high-luminosity spirals. Because the N(+)/S(+) ratio is a good approximation to the N to S abundance ratio, it is inferred that the enhancement of N due to secondary nucleosynthetic processing in successive generations of stars is greater in disks of high-mass spirals than in disks of low-mass spirals.