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Metal-enriched plasma in protogalactic halos. A survey of N V absorption in high-z damped and sub-damped Lyman-α systems

Abstract : We continue our recent work of characterizing the plasma content of high-redshift damped and sub-damped Lyman-α systems (DLAs/sub-DLAs), which represent multi-phase gaseous (proto)galactic disks and halos seen toward a background source. We survey N V absorption in a sample of 91 DLAs and 18 sub-DLAs in the redshift range 1.67 < z_abs < 4.28 with unblended coverage of the 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 (≈40 km s-1 FWHM) quasar spectra from Keck/ESI. In DLAs, we find eight secure N V detections, four marginal detections, and 79 non-detections, for which we place 3σ upper limits on the N V column density. The detection rate of N V in DLAs is therefore 13+5-4%. Two sub-DLA N V detections are found among a sample of 18, at a similar detection rate of 11+15-7%. We show that the N V detection rate is a strong function of neutral-phase nitrogen abundance, increasing by a factor of ≈4 at [N/H] = [{N I}/{H I}] > -2.3. The N V and C IV component b-value distributions in DLAs are statistically similar, but the median b(N V) of 18 km s-1 is narrower than the median b(O VI) of ≈25 km s-1. Some ≈20% of the 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 ≈80% of (broad) 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(N V) > 14 or log N(O VI) > 14.2), which present challenges to either galactic inflow or outflow models.

Based on observations taken with the Ultraviolet and Visual Echelle Spectrograph (UVES) on the Very Large Telescope (VLT) Unit 2 (Kueyen) at Paranal, Chile, operated by the European Southern Observatory (ESO), and with the High Resolution Echelle Spectrograph (HIRES) and Echelle Spectrograph and Imager (ESI) instruments located at the W. M. Keck Observatory on Mauna Kea, Hawaii.

Appendix A is only available in electronic form at

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A. J. Fox, J. X. Prochaska, C. Ledoux, P. Petitjean, A. M. Wolfe, et al.. Metal-enriched plasma in protogalactic halos. A survey of N V absorption in high-z damped and sub-damped Lyman-α systems. Astronomy and Astrophysics - A&A, EDP Sciences, 2009, 503, pp.731-746. ⟨10.1051/0004-6361/200911717⟩. ⟨hal-03646268⟩



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