EDP Sciences
Free access
Volume 503, Number 3, September I 2009
Page(s) 731 - 746
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361/200911717
Published online 19 May 2009
A&A 503, 731-746 (2009)
DOI: 10.1051/0004-6361/200911717

Metal-enriched plasma in protogalactic halos

A survey of N V absorption in high-z damped and sub-damped Lyman- $\mathsf{\alpha}$ systems
A. J. Fox1, J. X. Prochaska2, C. Ledoux1, P. Petitjean3, A. M. Wolfe4, and R. Srianand5

1  European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago, Chile
    e-mail: afox@eso.org
2  University of California/Lick Observatory, UC Santa Cruz, 1156 High Street, CA 95064, USA
3  Institut d'Astrophysique de Paris, UMR7095 CNRS, UPMC, 98bis Blvd Arago, 75014 Paris, France
4  Center for Astrophysics and Space Sciences, UC San Diego, 9500 Gilman Drive, CA 92093, USA
5  Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganesh Khind, Pune 411 007, India

Received 21 January 2009 / Accepted 7 May 2009

We continue our recent work of characterizing the plasma content of high-redshift damped and sub-damped Lyman-$\alpha$ systems (DLAs/sub-DLAs), which represent multi-phase gaseous (proto)galactic disks and halos seen toward a background source. We survey $\ion{N}{v}$ absorption in a sample of 91 DLAs and 18 sub-DLAs in the redshift range 1.67 < $z_{\rm abs}$ < 4.28 with unblended coverage of the $\ion{N}{v}$ doublet. Our dataset includes high-resolution (6-8 km s-1 FWHM) quasar spectra obtained with VLT/UVES and Keck/HIRES, together with medium-resolution ($\approx$40 km s-1 FWHM) quasar spectra from Keck/ESI. In DLAs, we find eight secure $\ion{N}{v}$ detections, four marginal detections, and 79 non-detections, for which we place 3$\sigma$ upper limits on the $\ion{N}{v}$ column density. The detection rate of $\ion{N}{v}$ in DLAs is therefore 13+5-4%. Two sub-DLA $\ion{N}{v}$ detections are found among a sample of 18, at a similar detection rate of 11+15-7%. We show that the $\ion{N}{v}$ detection rate is a strong function of neutral-phase nitrogen abundance, increasing by a factor of $\approx$4 at [N/H] = [ $\ion{N}{i}$/ $\ion{H}{i}$] > -2.3. The $\ion{N}{v}$ and $\ion{C}{iv}$ component b-value distributions in DLAs are statistically similar, but the median b( $\ion{N}{v}$) of 18 km s-1 is narrower than the median b( $\ion{O}{vi}$) of $\approx$25 km s-1. Some $\approx$20% of the $\ion{N}{v}$ components have b < 10 km s-1 and thus arise in warm, photoionized plasma at log (T/K) < 4.92; local sources of ionizing radiation (as opposed to the extragalactic background) are required to keep the cloud sizes physically reasonable. The nature of the remaining $\approx$80% of (broad) $\ion{N}{v}$ components is unclear; models of radiatively-cooling collisionally-ionized plasma at log (T/K) = 5.2-5.4 are fairly successful in reproducing the observed integrated high-ion column density ratios and the component line widths, but we cannot rule out photoionization by local sources. Finally, we identify several unusual DLAs with extremely low metallicity (<0.01 solar) but strong high-ion absorption (log N( $\ion{N}{v}$) > 14 or log N( $\ion{O}{vi}$) > 14.2), which present challenges to either galactic inflow or outflow models.

Key words: quasars: absorption lines -- cosmology: observations -- galaxies: high-redshift -- galaxies: halos -- galaxies: ISM

© ESO 2009