Forbidden collisionally excited optical atomic transitions from high-ionization-potential (IP ≥ 54.8 eV) ions, such as Ca⁴⁺, Ne⁴⁺, Fe⁶⁺, Fe¹⁰⁺, Fe¹³⁺, Ar⁹⁺, and S¹¹⁺, are known as optical coronal lines (CLs). The spectral energy distributions (SEDs) of active galactic nuclei (AGNs) typically extend to hundreds of electron volts and above, which should be able to produce such highly ionized gas. However, optical CLs are often not detected in AGNs. Here we use photoionization calculations with the cloudy spectral synthesis code to determine possible reasons for the rarity of these optical CLs. We calculate CL luminosities and equivalent widths from radiation-pressure-confined photoionized gas slabs exposed to an AGN continuum. We consider the role of dust, metallicity, and ionizing SED in the formation of optical CLs. We find that (i) dust reduces the strength of most CLs by ∼3 orders of magnitude, primarily as a result of depletion of metals onto the dust grains; (ii) in contrast to the CLs, the more widely observed lower-IP optical lines such as [O iii] 5007 Å are less affected by depletion, and some are actually enhanced in dusty gas; and (iii) many optical CLs become detectable in dustless gas, and are particularly strong for a hard ionizing SED. This implies that prominent CL emission likely originates in dustless gas. Our calculations also suggest optical CL emission is enhanced in galaxies with low-mass black holes characterized by a harder radiation field and a low dust-to-metals ratio. The fact that optical CLs are not widely observed in the early Universe with JWST may point to rapid dust formation at high redshift.

中文翻译：

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为什么光学日冕线在活跃的星系核中很暗？


来自高电离电位 （IP ≥ 54.8 eV） 离子（如 Ca⁴⁺、Ne⁴⁺、Fe⁶⁺、Fe¹⁰⁺、Fe¹³⁺、Ar⁹⁺ 和 S¹¹⁺）的禁止碰撞激发光学原子跃迁称为光学日冕线 （CL）。活跃星系核 （AGN） 的光谱能量分布 （SED） 通常延伸到数百电子伏特及以上，这应该能够产生如此高度电离的气体。然而，在 AGN 中通常未检测到光学 CL。在这里，我们使用带有混浊光谱合成代码的光电离计算来确定这些光学 CL 稀有性的可能原因。我们从暴露于 AGN 连续体的辐射-压力限制的光电离气体板中计算 CL 光度和等效宽度。我们考虑了灰尘、金属性和电离 SED 在光学 CL 形成中的作用。我们发现 （i） 尘埃将大多数 CL 的强度降低了 ∼ 3 个数量级，主要是由于金属耗尽到尘埃颗粒上;（ii） 与 CL 相比，更广泛观察到的低 IP 光线（如 [O iii] 5007 Å）受损耗的影响较小，有些实际上在含尘气体中得到增强;（iii） 许多光学 CL 可以在无尘气体中检测到，并且对于硬电离 SED 来说特别强。这意味着显著的 CL 发射可能来自无尘气体。我们的计算还表明，在具有低质量黑洞的星系中，光学阴极发光发射增强，其特点是辐射场更硬，尘埃与金属比低。使用 JWST 在早期宇宙中没有广泛观察到光学 CL 的事实可能表明在高红移时尘埃会迅速形成。