The broad emission lines (BELs) of quasars and active galactic nuclei (AGNs) are important diagnostics of the relative abundances and overall metallicity in the gas. Here we present new theoretical predictions for several UV BELs. We focus specifically on the relative nitrogen abundance as a metallicity indicator, based on the expected secondary enrichment of nitrogen at metallicities Z > 0.2 Z_o. Among the lines we consider, N III] 1750/O III] 1664, N V 1240/(C IV 1549 + O VI 1034) and N V/He II 1640 are the most robust diagnostics. We argue, in particular, that the average N V BEL is not dominated by scattered Ly-alpha photons from a broad absorption line wind. We then compare our calculated line ratios with observations from the literature. The results support earlier claims that the gas-phase metallicities near quasars are typically near or several times above the solar value. We conclude that quasar activity is preceded by, or coeval with, an episode of rapid and extensive star formation in the surrounding galactic (or proto-galactic) nuclei. Chemical evolution models of these environments suggest that, to reach Z > Z_o in well-mixed interstellar gas, the star formation must have begun > 10^8 yr before the observed quasar activity.
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机译:类星体的宽发射谱线(BEL)和活跃的银河核(AGN)是气体中相对丰度和整体金属性的重要诊断。在这里,我们介绍了几种紫外线BEL的新理论预测。我们基于金属含量Z> 0.2 Z_o时预期的氮二次富集,特别关注相对氮丰度作为金属含量指标。在我们考虑的系列中,N III] 1750 / O III] 1664,N V 1240 /(C IV 1549 + O VI 1034)和N V / He II 1640是最可靠的诊断方法。我们特别指出,平均N V BEL不受来自宽吸收线风的散射Ly-α光子的支配。然后,我们将计算出的线比率与文献中的观察结果进行比较。这一结果支持了先前的说法,即类星体附近的气相金属度通常接近或高于太阳值几倍。我们得出的结论是,类星体活动是在周围银河(或原银河)核中快速且广泛的恒星形成之前或之后。这些环境的化学演化模型表明,要在充分混合的星际气体中达到Z> Z_o,恒星形成必须在观测到的类星体活动之前开始> 10 ^ 8 yr。
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