The UCSD HIRES/Keck I damped Lyα abundance database.: IV.: Probing galactic enrichment histories with nitrogen

We present 14 N-0 measurements from our High Resolution Echelle Spectrometer/Keck database of damped Lyalpha abundances. These data are combined with measurements from the recent and past literature to build a homogeneous, uniform set of observations. We examine photoionization diagnostics such as Fe++ and Ar-0 in the majority of the complete sample and assess the impact of ionization corrections on N/alpha and alpha/H values 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 of systems show N/alpha values consistent with metal-poor emission regions in the local universe, but a small subsample exhibits significantly lower N/alpha ratios. Contrary to previous studies of N/alpha in the damped systems, our sample shows little scatter within each subsample. We consider various scenarios to explain the presence of the low N/alpha sight lines and account for the apparent bimodality. We favor a model where at least some galaxies undergo an initial burst of star formation with suppressed formation of intermediate-mass stars. We found that a power-law initial mass function with slope 0.10 or a mass cut of approximate to5-8 M-circle dot would successfully reproduce the observed low-nitrogen damped Lyalpha values. If the bimodal distribution is confirmed by a larger sample of measurements, this may present the first observational evidence for a top-heavy initial mass function in some early stellar populations.