SUBMILLIMETER-HCN DIAGRAM FOR ENERGY DIAGNOSTICS IN THE CENTERS OF GALAXIES
SUBMILLIMETER-HCN DIAGRAM FOR ENERGY DIAGNOSTICS IN THE CENTERS OF GALAXIES
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DOI：
10.3847/0004-637x/818/1/42
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发表时间：
2015-12
期刊：
影响因子：
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通讯作者：
T. Izumi;K. Kohno;S. Aalto;D. Espada;K. Fathi;N. Harada;B. Hatsukade;P. Hsieh;M. Imanishi
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文献类型：
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作者：
T. Izumi;K. Kohno;S. Aalto;D. Espada;K. Fathi;N. Harada;B. Hatsukade;P. Hsieh;M. Imanishi
从文献和阿塔卡马大型毫米/亚毫米阵列档案汇编数据，我们显示增强HCN（4-3）/HCO+（4-3）和/或HCN（4-3）/CS（7-6）的积分强度比周围的活动星系核（AGN）相比，在星爆（SB）星系（亚毫米HCN增强）的环核分子气体。样本星系的数量显着增加，从我们以前的工作。我们期望这一特征有可能成为星系核区的一个无反作用能量诊断工具。对上述分子发射谱线进行了非LTE辐射传输模拟，考虑了碰撞激发和辐射激发以及光子俘获效应，研究了活动星系核中高谱线比的原因.因此，我们发现，增强的丰度比HCN的HCO+和HCN CS的活动星系相比，SB星系的几个甚至10倍的一个合理的解释亚毫米HCN增强。然而，一个相反的论点，即活动星系核中的气体密度比SB星系中的气体密度高，也是一个合理的方案。虽然我们不能完全区分这两种情况下，在这个时候，由于数量不足的多跃迁，多物种的数据，前一种情况是指示异常的化学成分在活动星系核。关于实现的组合物的实际机制，我们认为，这是很难与传统的气相X射线为主的区域电离模型再现所观察到的高线比。我们可能必须考虑其他机制，例如在高温环境和/或机械加热区域中有效激活的中性-中性反应，以进一步了解AGN中的高线比。
Compiling data from literature and the Atacama Large Millimeter/submillimeter Array archive, we show enhanced HCN(4–3)/HCO+(4–3) and/or HCN(4–3)/CS(7–6) integrated intensity ratios in circumnuclear molecular gas around active galactic nuclei (AGNs) compared to those in starburst (SB) galaxies (submillimeter HCN enhancement). The number of sample galaxies is significantly increased from our previous work. We expect that this feature could potentially be an extinction-free energy diagnostic tool of nuclear regions of galaxies. Non-LTE radiative transfer modelings of the above molecular emission lines involving both collisional and radiative excitation, as well as a photon trapping effect, were conducted to investigate the cause of the high line ratios in AGNs. As a result, we found that enhanced abundance ratios of HCN to HCO+ and HCN to CS in AGNs as compared to SB galaxies by a factor of a few to even ≳10 are a plausible explanation for the submillimeter HCN enhancement. However, a counterargument of a systematically higher gas density in AGNs than in SB galaxies can also be a plausible scenario. Although we cannot fully distinguish these two scenarios at this moment owing to an insufficient amount of multi-transition, multi-species data, the former scenario is indicative of abnormal chemical composition in AGNs. Regarding the actual mechanism to realize the composition, we suggest that it is difficult with conventional gas-phase X-ray-dominated region ionization models to reproduce the observed high line ratios. We might have to take into account other mechanisms such as neutral–neutral reactions that are efficiently activated in high-temperature environments and/or mechanically heated regions to further understand the high line ratios in AGNs.