2009A&A...503..731F

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 NV 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 NV doublet. Our dataset includes high-resolution (6-8km/s FWHM) quasar spectra obtained with VLT/UVES and Keck/HIRES, together with medium-resolution (≃40km/s 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 18km/s is narrower than the median b(OVI) of ≃25km/s. Some ≃20% of the NV components have b<10km/s 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.