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Issue A&A
Volume 496, Number 1, March II 2009
Page(s) 31 - 44
Section Cosmology (including clusters of galaxies)
DOI http://dx.doi.org/10.1051/0004-6361/200810421
Published online 30 January 2009

A&A 496, 31-44 (2009)
DOI: 10.1051/0004-6361/200810421

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

C. Fechner and P. Richter

Institut für Physik und Astronomie, Universität Potsdam, Haus 28, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
    e-mail: [cfech;prichter]@astro.physik.uni-potsdam.de

Received 19 June 2008 / Accepted 14 January 2009

Abstract
Aims. We present a study of $\ion{N}{v}$ absorption systems at $1.5 \lesssim z \lesssim 2.5$ 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 $\ion{N}{v}$ components is d $\mathcal{N}/{\rm d}z = 3.38 \pm 0.43$, corresponding to d $\mathcal{N}/{\rm d}X = 1.10 \pm 0.14$. The column density distribution function is fitted by the slope $\beta = 1.89 \pm 0.22$, consistent with measurements of $\ion{C}{iv}$ and $\ion{O}{vi}$. The narrow line widths ( $b_\ion{N}{v} \sim 6\,\mathrm{km\,s}^{-1}$) imply photoionization rather than collisions as the dominating ionization process. The column densities of $\ion{C}{iv}$ and $\ion{N}{v}$ 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 $\ion{N}{v}$ systems we find typical values of $\mathrm{[C/H]} \sim -0.6$ and $n_{\element{H}} \sim 10^{-3.6}\,\mathrm{cm}^{-3}$ and sizes of a few kpc, while for associated $\ion{N}{v}$ absorbers we obtain $\mathrm{[C/H]} \sim +0.7$, $n_{\element{H}} \sim 10^{-2.8}\,\mathrm{cm}^{-3}$ 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.


Key words: galaxies: intergalactic medium -- galaxies: quasars: absorption lines -- cosmology: observations



© ESO 2009

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