High-ionization emission-line ratios from quasar broad-line regions: metallicity or density?
High-ionization emission-line ratios from quasar broad-line regions: metallicity or density?
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DOI：
10.1093/mnras/stab1610
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发表时间：
2021-06
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影响因子：
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通讯作者：
M. Temple;G. Ferland;A. Rankine;M. Chatzikos;P. Hewett
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作者：
M. Temple;G. Ferland;A. Rankine;M. Chatzikos;P. Hewett
高电离谱线的通量比通常被假定为指示发光类星体宽发射谱线区域的金属丰度。当考虑到它们的运动曲线的变化时，我们表明Nv/C iv， (Si iv+O iv])/C iv和Nv/Lyα线比率不随类星体连续体亮度，黑洞质量或吸积率而变化。利用CLOUDY的光电离模型，我们进一步表明，如果允许发射气体的物理条件在很大的密度和电离通量范围内变化，则观测到的这些线比的变化可以用具有太阳丰度的气体的发射来解释。类星体光谱中宽频线发射的多样性可以用一个模型来解释，该模型的发射来自两个运动上不同的区域，其中的线比表明这些区域要么具有非常不同的金属丰度，要么具有非常不同的密度。简单的星系演化模型和当前的星系演化模型都表明，在高密度云中形成部分光谱的接近太阳的丰度更有可能。在这个范例中，具有更强流出特征的物体显示出更强的气体发射，这些气体密度更大，位于离电离源更近的地方，其半径与线驱动圆盘风的模拟一致。使用宽线比值来推断化学富集历史的研究在估计金属丰度之前应该考虑密度和电离通量的变化。
The flux ratios of high-ionization lines are commonly assumed to indicate the metallicity of the broad emission line region in luminous quasars. When accounting for the variation in their kinematic profiles, we show that the Nv/C iv, (Si iv+O iv])/C iv and Nv/Lyα line ratios do not vary as a function of the quasar continuum luminosity, black hole mass, or accretion rate. Using photoionizationmodels from CLOUDY, we further show that the observed changes in these line ratios can be explained by emission from gas with solar abundances, if the physical conditions of the emitting gas are allowed to vary over a broad range of densities and ionizing fluxes. The diversity of broad line emission in quasar spectra can be explained by a model with emission from two kinematically distinct regions, where the line ratios suggest that these regions have either very different metallicity or density. Both simplicity and current galaxy evolution models suggest that near-solar abundances, with parts of the spectrum forming in high-density clouds, are more likely. Within this paradigm, objects with stronger outflow signatures show stronger emission from gas which is denser and located closer to the ionizing source, at radii consistent with simulations of line-driven disc-winds. Studies using broad-line ratios to infer chemical enrichment histories should consider changes in density and ionizing flux before estimating metallicities.