Chemical evolution of high-redshift radio galaxies

Abstract

We present new deep optical spectra of 9 high-z radio galaxies (HzRGs) at z > 2.7 obtained with FORS2 on VLT. These rest-frame ultraviolet spectra are used to infer the metallicity of the narrow-line regions (NLRs) in order to investigate the chemical evolution of galaxies in high-z universe. We focus mainly on the C iv/He ii and C iii]/C iv flux ratios that are sensitive to gas metallicity and ionization parameter. Although the N v emission has been widely used to infer the gas metallicity, it is often too weak to be measured accurately for NLRs. By combining our new spectra with data from the literature, we examine the possible redshift evolution of the NLR metallicity for 57 HzRGs at 1 < ∼ z < ∼ 4. Based on the comparison between the observed emission-line flux ratios and the results of our photoionization model calculations, we find no significant metallicity evolution in NLRs of HzRGs, up to z ∼ 4. Our results imply that massive galaxies had almost completed their chemical evolution at much higher redshift (z > 5). Finally, although we detect strong N v emission lines in 5 HzRGs at z > 2.7, we point out that high N v/He ii ratios are not indicative of high metallicities but correspond to high ionization parameters of gas clouds in NLRs.

Strong [CII] emission at high redshift

“…However, the BLR is a very tiny region (<1 pc) in quasar nuclei, which is probably not representative of the ISM in the host galaxy, and may undergo quick enrichment with just a few supernova explosions (see detailed discussion in Juarez et al 2009). A few studies have investigated the metallicity on the larger scales (∼100 pc-10 kpc) of the narrow line region (NLR) in high-z AGNs (Nagao et al 2006b;Matsuoka et al 2009;Vernet et al 2001;De Breuck et al 2000;Humphrey et al 2008). These studies are currently limited 3 BR 1202-0725 was included in Maiolino et al (2005) as an upper limit (from Benford et al, in prep.…”

Weak13CO in the Cloverleaf quasar: evidence for a young, early generation starburst

“…1, solar or super-solar metallicities are common in quasars up to high redshifts. This does not only refer to so-called "α-elements" being rapidly synthesized in short-lived massive stars but also to iron (e.g., Iwamuro et al 2004;Kurk et al 2007;Sameshima et al 2009), carbon (e.g., Jiang et al 2007;Juarez et al 2009), and, even more importantly, nitrogen (Hamann & Ferland 1999;De Breuck et al 2000;Vernet et al 2001;Hamann et al 2002;Nagao et al 2006;Matsuoka et al 2009), with 14 N being mainly a secondary nucleus produced by CNO burning just like 13 C. A possible explanation for the contradictory results obtained at or near optical wavelengths and the microwave data presented here may be different locations. It is well possible that mainly secondary nuclei like 13 C and 14 N are enriched close to the quasar, in the Broad and Narrow Line Regions and in outflows originating from the active galactic nucleus (AGN).…”

“…There is evidence for solar or super-solar metallicities in the circumnuclear environments of quasars out to redshifts z > 4 (e.g., Hamann & Ferland 1999;Kurk et al 2007;Jiang et al 2007;Juarez et al 2009;Matsuoka et al 2009). This evidence, mainly from optical lines, is supported by millimeter detections of CO and dust in high-redshift sources, indicating rapid metal enrichment due to starbursts in the circumnuclear regions of at least some galaxies in the early universe (e.g., Solomon & Vanden Bout 2005).…”

Enhanced Far-IR Line Emission at High-Z: Implications for SPICA

“…However, the BLR is a very tiny region (<1pc) in quasar nuclei, which is probably not representative of the ISM in the host galaxy, and may undergo quick enrichment with just a few supernova explosions (see detailed discussion in Juarez et al, 2009). A few studies have investigated the metallicity on the larger scales (∼100pc-10kpc) of the narrow line region (NLR) in high-z AGNs (Nagao et al, 2006b;Matsuoka et al, 2009;Vernet et al, 2001;De Breuck et al, 2000;Humphrey et al, 2008). Although these studies are currently limited to z<4, the inferred NLR metallicities are much lower than in the BLR, about solar or sub-solar, which is indeed less than the metallicity observed in local massive galaxies (Maiolino et al, 2008).…”