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 the high-z universe. We focus mainly on the C IV/He II and C III]/C IV flux ratios that are sensitive to both the 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. In combining our new spectra with data from the literature, we examine the possible redshift evolution of the NLRmetallicity for 57HzRGs at .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 .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 5HzRGs 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.Key words: galaxies: active - galaxies: evolution - galaxies: nuclei - quasars: emission lines - quasars: general
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机译:我们提出了在VLT上使用FORS2获得的z> 2.7的9个高z射电星系(HzRGs)的新深光谱。这些静止帧紫外光谱用于推断窄线区域(NLR)的金属性,以研究高z宇宙中星系的化学演化。我们主要关注对气体金属性和电离参数均敏感的C IV / He II和C III] / C IV通量比。尽管N V发射已被广泛用于推断气体的金属性,但它通常太弱而无法精确测量NLR。通过将我们的新光谱与文献数据相结合,我们研究了在57HzRGs时NLR金属性可能发生的红移演变。基于观察到的发射线通量比与我们的光电离模型计算结果之间的比较,我们发现没有明显的金属性结果表明,大型星系几乎以更高的红移(z> 5)完成了化学演化。最后,尽管我们在z> 2.7的5HzRGs中检测到了很强的NV发射谱线,但我们指出高NV / He II比并不表示高金属性,而是与NLR中气云的高电离参数相对应。 -星系:进化-星系:原子核-类星体:发射线-类星体:一般
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