THE N SI ABUNDANCE RATIO IN 15 DAMPED LY-ALPHA 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 a t a fixed metallicity (O/H) for galaxies with very low metallicities, due to the delayed release of primary N from intermediate-mass stars r elative to that of O from short-lived massive stars. Moreover, the sca tter should increase progressively as metallicity decreases. Such effe cts have not been convincingly demonstrated by observations of H II re gions in nearby metal-poor galaxies, raising doubts about the time-del ay model of primary N production. Pettini et al. and Lipman et al. rea lized the utility of high-redshift damped Ly alpha galaxies for gainin g further insights into the origin of N, and discussed abundances in t hree 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 abundanc e 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 comparab le 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 ver y large scatter (similar to 1 dex) at [Si/H] similar to -2 and that th ere is some indication that the scatter increases with decreasing meta llicity. Consideration of various sources of uncertainty suggests that they are not likely the main causes of the large scatter. These resul ts provide strong support for the time-delay model of primary N produc tion in intermediate-mass stars if,indeed, O/Si similar or equal to so lar in damped Ly alpha galaxies.