Metal-Enriched Plasma in Protogalactic Halos: A Survey of N V Absorption in High-z Damped & Sub-Damped Lyman-alpha Systems

Abstract

We continue our recent work to characterize 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 Nv absorption in a sample of 91 DLAs and 18 sub-DLAs in the redshift range 1.67<zabs<4.28 with unblended coverage of the Nv doublet. Our dataset includes high-resolution (6–8 kms −1 FWHM) quasar spectra obtained with VLT/UVES and Keck/HIRES, together with medium-resolution (≈40 kms −1 FWHM) quasar spectra from Keck/ESI. In DLAs, we find eight secure Nv detections, four marginal detections, and 79 non-detections, for which we place 3σ upper limits on the Nv column density. The detection rate of Nv in DLAs is therefore 13+5 −4%. Two sub-DLA Nv detections are found among a sample of 18, at a similar detection rate of 11+15 −7 %. We show that the Nv detection rate is a strong function of neutral-phase nitrogen abundance, increasing by a factor of ≈4 at [N/H]=[Ni/Hi]>−2.3. The Nv and Civ component b-value distributions in DLAs are statistically similar, but the median b(Nv) of 18 kms −1 is narrower than the median b(Ovi) of ≈25 kms −1 . Some ≈20% of the Nv components have b<10 kms −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) Nv 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 [logN(Nv)>14 or logN(Ovi)>14.2], which present challenges to either galactic inflow or outflow models.