The nature of NV absorbers at high redshift

TL;DR

This study investigates NV absorption systems at high redshift, analyzing their physical conditions, metallicity, and ionization sources, revealing differences between intervening and associated absorbers and insights into cosmic chemical enrichment.

Contribution

The paper provides detailed photoionization models of NV absorbers at high redshift, distinguishing properties of intervening and associated systems, and explores their implications for cosmic chemical enrichment and stellar populations.

Findings

Intervening NV systems have typical [C/H] ~ -0.6 and sizes of a few kpc.

Associated NV systems are more metal-rich with [C/H] ~ +0.7 and sizes of several 10 pc.

Column density distribution slope beta = 1.89 ± 0.22, consistent with CIV and OVI.

Abstract

We present a study of NV absorption systems at 1.5 < z < 2.5 in the optical spectra of 19 QSOs. Our analysis includes both absorbers arising from the intergalactic medium as well as systems in the vicinity of the background quasar. 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. The rate of incidence for intervening NV 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 beta = 1.89 +/- 0.22, consistent with measurements for CIV and OVI. The narrow line widths (b_NV ~ 6 km/s) imply photoionization rather than collisions as dominating ionization process. The column densities of CIV and NV 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 NV systems we find typical values of [C/H] ~ -0.6 and n_H ~ 10^-3.6 cm^-3, and sizes of a few kpc, while for associated NV absorbers we obtain [C/H] ~ +0.7, n_H ~ 10^-2.8 cm^-3, and sizes of several 10 pc. The abundance of nitrogen relative to carbon [N/C] and alpha-elements like oxygen and silicon [N/alpha] is correlated with [N/H], indicating the enrichment by secondary nitrogen. The larger scatter in [N/alpha] in intervening systems suggests an inhomogeneous enrichment of the IGM. There is an anti-correlation between [N/alpha] and [alpha/C], which could be used to constrain the initial mass function of the carbon- and nitrogen-producing stellar population.