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 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 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_IVcomponent b-value distributions in DLAs are statistically similar, but the median b(N_V) of 18 kms^-1 is narrower than the median b (O_VI) of ≈ 25 kms^-1. Some ≈ 20% of the 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) 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 (logN(N_V) > 14 or logN(O_VI) > 14.2), which present challenges to either galactic inflow or outflow models.