The N/Si Abundance Ratio in Fifteen Damped Lyman-alpha Galaxies: Implications for the Origin of Nitrogen

Abstract

Galactic chemical evolution model calculations 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 toward decreasing metallicity. 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 Lyman-alpha galaxies for gaining further insights into the origin of N and discussed abundances in three damped Lyman-alpha galaxies. Since abundance measurements for O are generally unavailable for damped Lyman-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 Lyman-alpha galaxies. We discuss observations of heavy element abundances in 15 high-redshift (z>2) damped Lyman-alpha 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 Lyman-alpha galaxies exhibit a very large scatter (about 1 dex) at [Si/H]~-2 and there is some indication that the scatter increases toward decreasing metallicity. Considerations of various sources of uncertainties suggest that they are not likely the main causes of the large scatter. These results thus provide strong support for the time-delay model of primary N production in intermediate mass stars if, indeed, O/Si=solar in damped Lyman-alpha galaxies.