Gas metallicity in the narrow-line regions of high-redshift active galactic nuclei

Abstract

We analyze optical (UV rest-frame) spectra of X-ray selected narrow-line QSOs at redshift 1.5 < ∼ z < ∼ 3.7 found in the Chandra Deep Field South and of narrow-line radio galaxies at redshift 1.2 < ∼ z < ∼ 3.8 to investigate the gas metallicity of the narrow-line regions and their evolution in this redshift range. Such spectra are also compared with UV spectra of local Seyfert 2 galaxies. The observational data are inconsistent with the predictions of shock models, suggesting that the narrow-line regions are mainly photoionized. The photoionization models with dust grains predict line flux ratios which are also in disagreement with most of the observed values, suggesting that the high-ionization part of the narrowline regions (which is sampled by the available spectra) is dust-free. The photoionization dust-free models provide two possible scenarios which are consistent with the observed data: low-density gas clouds (n H < ∼ 10 3 cm −3 ) with a sub-solar metallicity (0.2 < ∼ Z gas /Z < ∼ 1.0), or high-density gas clouds (n H ∼ 10 5 cm −3 ) with a wide range of gas metallicity (0.2 < ∼ Z gas /Z < ∼ 5.0). Regardless of the specific interpretation, the observational data do not show any evidence for a significant evolution of the gas metallicity in the narrow-line regions within the redshift range 1.2 < ∼ z < ∼ 3.8. Instead, we find a trend for more luminous active galactic nuclei to have more metal-rich gas clouds (luminosity-metallicity relation), which is in agreement with the same finding in the studies of the broad-line regions. The lack of evolution for the gas metallicity of the narrow-line regions implies that the major epoch of star formation in the host galaxies of these active galactic nuclei is at z > ∼ 4.

Nuclear activity versus star formation: emission-line diagnostics at ultraviolet and optical wavelengths

“…According to Figs 5 and 6, and modulo the above remark about potential contamination by circumnuclear star formation, the models in best agreement with AGN data are those with nH 10 3 cm −3 (see also Nagao et al 2006). At fixed other parameters, increasing nH means lowering the volume-filling factor, (equation 4).…”

“…We note that other authors sometimes use the ionization parameter at the inner edge rin of the narrow-line region, noted U0, as an adjustable parameter (e.g. Groves et al 2004b;Nagao et al 2006). According to equation (2), U0 will be larger than US for rin RS and more similar to US for rin RS.…”

“…Yet, N v λ1240-based diagnostics are not optimal because of their strong dependence on the details of secondary nitrogen enhancement. Also, as indicated by the large dynamic range spanned by the abscissae in Figs 7 and 8, N v λ1240 emission is very weak and hence difficult to detect at low metallicity (Nagao et al 2006). …”

“…Ultraviolet emission-line ratios involving the N v λ1240 multiplet, such as N v λ1240/C iv λ1550 and N v λ1240/He ii λ1640, have been exploited to interpret observations of active galaxies using AGN photoionization models, in particular as metallicity diagnostics (Groves et al 2004b;Nagao et al 2006, and references therein). Here, we investigate the extent to which N v λ1240-based emission-line ratios can be used as diagnostics of the nature of the photoionization source in a galaxy.…”

Abundance anomalies in tidal disruption events

“…These are low enough compared to the estimated TDE rate for evolved stars in Kochanek (2015) that there seems no difficulty in creating the nitrogen rich quasars using the very long lived TDE's associated with the larger evolved stars. A more general puzzle about abundances inferred from quasar BLRs is that they appear to require very high metallicities, Z > ∼ 5Z⊙ (e.g., Dietrich et al 2003, Nagao et al 2006. It is somewhat challenging to reach these metallicities with global star formation models (e.g., Hamann & Ferland 1993, Friaca & Terlevich 1998, Romano et al 2002, leading to an alternative picture of star formation associated with the outer, self-gravitating parts of the accretion flow (e.g., Collin & Zahn 1999, Wang et al 2011.…”