The nature of $\ion{N}{v}$ absorbers at high redshift

Abstract

Aims. We present a study of N v absorption systems at 1.5 z 2.5 in the spectra of 19 QSOs, based on data obtained with the VLT/UVES instrument. Our analysis includes both the absorbers arising from the intergalactic medium, as well as systems in the vicinity of the background quasar. Methods. We construct detailed photoionization models to study the physical conditions and abundances in the absorbers and to constrain the spectral hardness of the ionizing radiation. Results. The rate of incidence for intervening N v components is dN/dz = 3.38 ± 0.43, corresponding to dN/dX = 1.10 ± 0.14. The column density distribution function is fitted by the slope β = 1.89 ± 0.22, consistent with measurements of C iv and O vi .T he narrow line widths (bNv ∼ 6k m s −1 ) imply photoionization rather than collisions as the dominating ionization process. The column densities of C iv and N v are correlated but show different slopes for intervening and associated absorbers, which indicates different ionizing spectra. Associated systems are found to be more metal-rich, denser, and more compact than intervening absorbers. This conclusion is independent of the adopted ionizing radiation. For the intervening N v systems we find typical values of [C/H] ∼− 0.6 and nH ∼ 10 −3.6 cm −3 and sizes of a few kpc, while for associated N v absorbers we obtain [C/H] ∼ +0.7, nH ∼ 10 −2.8 cm −3 and sizes of several 10 pc. The abundance of nitrogen relative to carbon [N/C] and α-elements like oxygen and silicon [N/α ]i s correlated with [N/H], indicating the enrichment by secondary nitrogen. The larger scatter in [N/α] in intervening systems suggests an inhomogeneous enrichment of the IGM. There is an anti-correlation between [N/α ]a nd [α/C], which could be used to constrain the initial mass function of the carbon- and nitrogen-producing stellar population.