Element Abundances at High Redshifts: The N/O Ratio in a Primeval Galaxy

The damped Lyman-α systems seen in the spectra of high-redshift QSOs offer the means to determine element abundances in galaxies observed while still at an early stage of evolution. Such measurements, which have only recently come within reach, complement and extend the data provided by studies of different stellar populations in our Galaxy and of extragalactic H ii regions which have up to now formed the basis of galactic chemical evolution models. In this paper we demonstrate the potential of this new approach with high- resolution echelle observations of several elements in the zabs = 2.27936 absorption system in the bright zem = 2.940 QSO 2348-147. The absorbing galaxy appears to be chemically unevolved, with heavy element abundances only ∼1/100 of solar; if it is the progenitor of a spiral galaxy like our own, it is unlikely to have collapsed to form a thin disk by z ≃ 2.3 (corresponding to a look-back time of ∼13 Gyr for H0 = 50 km s-1 Mpc-1 and q0 = 0.01). Our data allow us to measure the nitrogen-to-oxygen ratio at a metallicity lower than those of the most metal-poor dwarf galaxies known. We find that, relative to the solar scale, N is more underabundant than 0 by at least a factor of ∼15. This result is broadly in line with current ideas on the relative importance of primary and secondary production of N; future measurements in several damped Lyman-α systems will permit more stringent tests of models for the evolution of the N/O ratio with time. Oxygen and other α-elements are overabundant relative to Fe by no more than the factor ∼3 typical of metal-poor stars in the disk and halo of the Milky Way.