Abstract

We continue our recent work of characterizing the plasma content of high-redshift damped and sub-damped Lyman-a 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 (6-8kms -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 3cr upper limits on the N V column density. The detection rate of N V in DLAs is therefore 13 -4 +5%. Two sub-DLA N V detections are found among a sample of 18, at a similar detection rate of 11 -7 +15%.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] = [NI/H I] > -2.3. The N V and CIV 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 7Y(N V) > 14 or log N(O VI) > 14.2), which present challenges to either galactic inflow or outflow models. © ESO 2009.