The UCSD HIRES/Keck I Damped Lyα Abundance Database. IV. Probing Galactic Enrichment Histories with Nitrogen

We present 14 N^0 measurements from our HIRES/Keck database of damped Lyaabundances. These data are combined with measurements from the recent and pastliterature to build an homogeneous, uniform set of observations. We examinephotoionization diagnostics like Fe^++ and Ar^0 in the majority of the completesample and assess the impact of ionization corrections on N/alpha and alpha/Hvalues derived from observed ionic column densities of N^0, Si^+, H^0, and S^+.Our final sample of 19 N/alpha, alpha/H pairs appears bimodal; the majority ofsystems show N/alpha values consistent with metal-poor emission regions in thelocal universe but a small sub-sample exhibit significantly lower N/alpharatios. Contrary to previous studies of N/alpha in the damped systems, oursample shows little scatter within each sub-sample. We consider variousscenarios to explain the presence of the low N/alpha sightlines and account forthe apparent bimodality. We favor a model where at least some galaxies undergoan initial burst of star formation with suppressed formation ofintermediate-mass stars. We found a power-law IMF with slope 0.10 or a mass cutof ~5-8 Msolar would successfully reproduce the observed LN-DLA values. If thebimodal distribution is confirmed by a larger sample of measurements, this maypresent the first observational evidence for a top heavy initial mass functionin some early stellar populations.