[论文解读] From the Circumnuclear Disk in the Galactic Center to thick, obscuring tori of AGNs: Modeling the molecular emission of a parsec-scale torus as found in NGC 1068

B. Vollmer, R. Davies+7 more|arXiv (Cornell University)|Jan 1, 2022

Astrophysics and Star Formation Studies参考文献 122被引用 5

一句话总结

该论文通过模拟气体云团向预存环状结构的动态碰撞，结合湍流加热、辐射转移和化学丰度，对NGC 1068中一个秒差距尺度、厚实且致密的尘埃环状结构的分子辐射进行了建模。模型成功在两倍以内重现了观测到的CO、HCN、HCO+和CS的谱线流量，表明湍流机械能输入（由径向吸积和云团碰撞驱动）是主要加热机制，而X射线吸收和红外泵浦仅起次要作用。

ABSTRACT

The high accretion rates needed to fuel the central black hole in a galaxy can be achieved via viscous torques in thick disks and rings, which can be resolved by millimeter interferometry within the inner ∼20 pc of the active galaxy NGC 1068 at comparable scales and sensitivity to single dish observations of the Circumnuclear Disk (CND) in the Galactic Center. To interpret observations of these regions and determine the physical properties of their gas distribution, we present a modeling effort that includes the following: (i) simple dynamical simulations involving partially inelastic collisions between disk gas clouds; (ii) an analytical model of a turbulent clumpy gas disk calibrated by the dynamical model and observations; (iii) local turbulent and cosmic ray gas heating and cooling via H<SUB>2</SUB>O, H<SUB>2</SUB>, and CO emission; and (iv) determination of the molecular abundances. We also consider photodissociation regions (PDRs) where gas is directly illuminated by the central engine. We compare the resulting model datacubes of the CO, HCN, HCO<SUP>+</SUP>, and CS brightness temperatures to available observations. In both cases the kinematics can be explained by one or two clouds colliding with a preexisting ring, in a prograde sense for the CND and retrograde for NGC 1068. And, with only dense disk clouds, the line fluxes can be reproduced to within a factor of about two. To avoid self-absorption of the intercloud medium, turbulent heating at the largest scales, comparable to the disk height, has to be decreased by a factor of 50-200. Our models indicate that turbulent mechanical energy input is the dominant gas-heating mechanism within the thick gas disks. Turbulence is maintained by the gain of potential energy via radial gas accretion, which is itself enhanced by the collision of the infalling cloud. In NGC 1068, we cannot exclude that intercloud gas significantly contributes to the molecular line emission. In this object, while the bulk of the X-ray radiation of the active galactic nucleus is absorbed in a layer of Compton-thick gas inside the dust sublimation radius, the optical and UV radiation may enhance the molecular line emission from photodissociation regions by ∼50% at the inner edge of the gas ring. Infrared pumping may also increase the HCN(3−2) line flux throughout the gas ring by about a factor of two. Our models support the scenario of infalling gas clouds onto preexisting gas rings in galactic centers, and it is viable and consistent with available observations of the CND in the Galactic Center and the dense gas distribution within the inner 20 pc of NGC 1068.

研究动机与目标

• 通过模拟气体云团向预存环状结构的下落过程，建立一个物理解释模型，以说明NGC 1068内区20 pc内致密分子气体的分布与运动学特征。
• 确定在厚实且致密的气体盘中，主导加热机制是湍流机械能输入，还是宇宙射线或X射线加热。
• 评估云间气体及光致离解区（PDRs）对分子谱线发射的贡献，特别是在活动星系核遮蔽和红外泵浦的背景下。
• 将CO、HCN、HCO+和CS的观测谱线流量与气体质量、吸积率和湍流等物理参数相协调。
• 探究被解释为外流的运动学特征是否实际上源于反向旋转系统中气体-气体碰撞的扰动。

提出的方法

• 利用部分非弹性碰撞模拟厚实湍流盘中团块的气体动力学，以建模云团下落与环状结构的相互作用。
• 基于动力学模拟结果和NGC 1068及银河系中心CND的观测约束，校准解析湍流致密盘模型。
• 通过H2O、H2和CO的转动辐射实现局部加热与冷却，包括宇宙射线和X射线电离效应。
• 在单个云团内计算辐射转移，并考虑光学厚度效应，假设光学厚云团可遮蔽后方发射。
• 采用修正的Toomre Q参数控制盘面稳定性和湍流，通过调节耗散率以匹配观测亮度温度。
• 评估红外泵浦对HCN(3-2)线以及内环边界处PDR效应的影响，尤其针对紫外/软X射线照射条件。

实验结果

研究问题

• RQ1 NGC 1068内区20 pc中观测到的CO、HCN、HCO+和CS的运动学特征与谱线流量，能否通过气体云团撞击预存环状结构的模型成功再现？
• RQ2 在厚实致密的尘埃环状结构中，主导加热机制是湍流机械能输入，还是宇宙射线或X射线/电离辐射？
• RQ3 在内区几秒差距范围内，云间气体对观测到的分子谱线发射贡献有多大？
• RQ4 红外泵浦和光致离解区（PDRs）如何影响HCN(3-2)线发射及内盘整体激发状态？
• RQ5 若中心X射线辐射为康普顿光学厚且主要在尘埃亚稳化半径内被吸收，是否仍能解释观测到的高HCN丰度？

主要发现

• 该模型在NGC 1068内区20 pc范围内，成功在约两倍以内重现了CO(2-1)、CO(3-2)、CO(6-5)、HCN(3-2)、HCN(4-3)、HCO+(3-2)、HCO+(4-3)和CS(7-6)的谱线流量。
• 湍流机械能输入是厚实气体盘中主导的加热机制，其能量来源于径向吸积带来的势能增加，并由云团碰撞进一步增强。
• 为避免云间介质中发生自吸收，最大尺度（与盘面高度相当）的湍流加热必须比标准估算值降低50至200倍。
• 由于红外泵浦作用，HCN(3-2)线的流量在整个气体环中增强最多达两倍；在半径1–2 pc处，PDR效应可使发射增强最多达两倍。
• 若电离率高于数×10−12 s−1，则无法再现观测到的高HCN丰度（xHCN > 10−8），表明中心X射线辐射为康普顿光学厚，且主要在尘埃亚稳化半径内被吸收。
• NGC 1068中被解释为外流的运动学特征，可能实为逆行气体-气体碰撞引起的扰动所致，且在R <∼3 pc区域内可能存在外流特征的叠加效应。