标题： 星系中心的能量诊断亚毫米HCN图示 显示英文标题

标题： Submillimeter-HCN Diagram for an Energy Diagnostics in the Centers of Galaxies

摘要： 利用文献数据和ALMA档案，我们展示了活动星系核（AGN）周围的核区分子气体中增强的HCN(4-3)/HCO$^+$(4-3)和/或HCN(4-3)/CS(7-6)积分强度比，与星暴（SB）星系中的相比（亚毫米波段HCN增强）。样本星系的数量相较于我们的先前工作显著增加。我们预期这一特征可能成为星系核区域的一种无消光能量诊断工具。 针对上述分子发射线进行了非局部热平衡（Non-LTE）辐射转移建模，包括碰撞激发和辐射激发以及光捕获效应，以研究AGN中高线比的原因。结果表明，AGN中HCN相对于HCO$^+$和CS的丰度比相较于SB星系增加了几倍甚至$>$10，这是亚毫米波段HCN增强的一个合理解释。然而，另一种可能性——AGN中气体密度系统性高于SB星系——也是一个合理的场景。 由于缺乏多跃迁、多物种的数据，目前我们无法完全区分这两种情景。前者等效于主张AGN中存在异常化学成分。就实现这种成分的实际机制而言，我们认为用传统的气体相X射线主导区（XDR）电离模型难以重现观测到的高线比。为了进一步理解AGN中的高线比，我们需要考虑其他机制，例如在高温环境和/或机械加热区域中高效激活的中性-中性反应等。 显示英文摘要

摘要： Compiling data from literature and the ALMA 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 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 is a plausible explanation for the submillimeter HCN-enhancement. However, a counter argument of a systematically higher gas density in AGNs than in SB galaxies can also be a plausible scenario. Although we could not fully discriminate these two scenarios at this moment due to insufficient amount of multi-transition, multi-species data, the former scenario equivalently claims for abnormal chemical composition in AGNs. Regarding the actual mechanism to realize the composition, we suggest it is difficult with conventional gas phase X-ray dominated region (XDR) 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 at high temperature environments and/or mechanically heated regions to further understand the high line ratios in AGNs.