Metal-enriched plasma in protogalactic halos - A survey of N?V absorption in high-z damped and sub-damped Lyman-$mathsf{lpha}$ systems

摘要

We continue our recent work of characterizing the plasma content of high-redshift damped and sub-damped Lyman-systems (DLAs/sub-DLAs), which represent multi-phase gaseous (proto)galactic disks and halos seen toward a background source. We survey N Vabsorption in a sample of 91DLAs and 18sub-DLAs in the redshift range 1.67< < 4.28 with unblended coverage of the N Vdoublet. Our dataset includes high-resolution (6-8kms-1 FWHM) quasar spectra obtained with VLT/UVES and Keck/HIRES, together with medium-resolution (40kms-1 FWHM) quasar spectra from Keck/ESI. In DLAs, we find eight secure N Vdetections, four marginal detections, and 79non-detections, for which we place 3upper limits on the N Vcolumn density. The detection rate of N V in DLAs is therefore 13+5-4%. Two sub-DLA N Vdetections are found among a sample of18, at a similar detection rate of 11+15-7%. We show that the N Vdetection rate is a strong function of neutral-phase nitrogen abundance, increasing by a factor of4 at[N/H]= [N I/H I]> -2.3. The N V and C IVcomponent b-value distributions in DLAs arestatistically similar, but the medianb(N V) of 18kms-1 is narrower than the medianb(O VI) of25kms-1. Some 20% of the N Vcomponents have b < 10kms-1 and thus arise in warm, photoionized plasma at log(T/K)< 4.92; local sources of ionizing radiation (asopposed to the extragalactic background) are required to keep the cloud sizes physically reasonable. The nature of the remaining 80% of (broad) N Vcomponents 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 (<0.01solar) 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.Key words: quasars: absorption lines - cosmology: observations - galaxies: high-redshift - galaxies: halos - galaxies: ISM