On the origin of nitrogen in low-metallicity galaxies: Blue compact galaxies versus damped Lya absorbers

Abstract

A remarkably small scatter in the N/O ratios for the HII regions in low-metallicity blue compact galaxies (BCG) has been found recently. It lead to the conclusion that N and O are both produced by massive stars. Conversely, the N/Si ratios in damped Lya absorbers (DLA) show a large scatter. This result provides support for the time-delay model of nitrogen production in intermediate-mass stars, nitrogen production in massive stars being not required. The goal of this study is to test whether these observational data are compatible with each other and with the existing ideas on the chemical evolution of galaxies. We find that it is possible to reconcile the constancy of N/O ratios in low-metallicity BCGs and the scatter of N/Si ratios in DLAs under the following three assumptions: 1) a significant part of nitrogen is produced by intermediate-mass stars, 2) star formation in BCGs and DLAs occurs in bursts separated by quiescent periods, 3) the previous star formation events are responsible for measured heavy element abundances of HII regions in BCGs. Since the reality of low N/Si ratios in DLAs is not beyond question, the possibility that the nitrogen in low-metallicity BCGs has been produced by massive stars cannot be rejected without additional observations.