2002A&A...391...21P

We take a fresh look at the abundance of nitrogen in damped Lyman α systems (DLAs) with oxygen abundances between ∼1/10 and ∼1/100 of solar. This is a metallicity regime poorly sampled in the local universe and where QSO absorbers may hold clues to both the nucleosynthetic origin of N and the chemical evolution of high redshift galaxies. We combine new VLT UVES observations with others from the literature to form a sample of 10 DLAs in which the abundances of N and of one of the two α-capture elements O or S have been measured. The sample consists exclusively of high resolution, high signal-to-noise ratio data obtained with 8-10m class telescopes. We confirm earlier reports that the (N/O) ratio exhibits a larger range of values than other ratios of heavy elements in DLAs; however, all 10 DLAs fall in the region of the (N/O) vs. (O/H) plot delimited by the primary and secondary levels of nitrogen production. Our results provide empirical evidence in support of the proposal that intermediate mass stars (4≲M/M_☉≲7) are the main source of primary nitrogen, so that its release into the interstellar medium lags behind that of oxygen, which is produced by Type II supernovae. A high proportion (40%) of the DLAs in our sample have apparently not yet attained the full primary level of N enrichment; this finding may be an indication that the metallicity regime we are studying preferentially picks out galaxies which have only recently condensed out of the intergalactic medium and begun forming stars. Alternatively, the delay in the release of N following an episode of star formation may increase with decreasing metallicity, if stars of progressively lower masses than 4M_☉ can synthesize N in their hotter interiors. In this general picture, the uniform value of (N/O) measured locally in metal-poor star-forming galaxies implies that they are not young, and is consistent with the presence of older stellar populations revealed by imaging studies with the Hubble Space Telescope.