The N/Si abundance ratio in 15 damped Lyα galaxies:: Implications for the origin of nitrogen

Calculations derived from Galactic chemical evolution models indicate that there should be considerable scatter in the observed N/O ratios at a fixed metallicity (O/H) for galaxies with very low metallicities, due to the delayed release of primary N from intermediate-mass stars relative to that of O from short-lived massive stars. Moreover, the scatter should increase progressively as metallicity decreases. Such effects have not been convincingly demonstrated by observations of H II regions in nearby metal-poor galaxies, raising doubts about the time-delay model of primary N production. Pettini et al. and Lipman et al. realized the utility of high-redshift damped Ly alpha galaxies for gaining further insights into the origin of N, and discussed abundances in three damped Ly alpha galaxies. Since abundance measurements for O are generally unavailable for damped Ly alpha galaxies, they used N/Si or N/S in place of N/O, under the reasonable assumption that the abundance ratios O/Si and O/S are the same as solar in damped Lya galaxies. We discuss observations of heavy-element abundances in 15 high-redshift (z > 2) damped Lya galaxies, many of which have metallicities comparable to or lower than the lowest metallicity galaxy known locally (I Zw 18). We find that the N/Si ratios in damped Lya galaxies exhibit a very large scatter (similar to 1 dex) at [Si/H] similar to -2 and that there is some indication that the scatter increases with decreasing metallicity. Consideration of various sources of uncertainty suggests that they are not likely the main causes of the large scatter. These results provide strong support for the time-delay model of primary N production in intermediate-mass stars if,indeed, O/Si similar or equal to solar in damped Ly alpha galaxies.