3C 84 环核盘内的空间分辨分子气体条件
Spatially resolved molecular gas conditions in the circumnuclear disc of 3C 84
AI总结：
本研究通过ALMA观测与贝叶斯框架建模，揭示3C 84环核盘的分子气体物理化学条件，发现HCN线比受光学深度饱和抑制，环核盘由纤维环境吸积塑造。
AI中文摘要：
英仙座星系团中心的最亮星系 NGC 1275 拥有射电噪活动星系核（AGN）3C 84 以及冷分子气体组成的环核盘（CND），大尺度 CO 纤维正吸积到该盘上，但 CND 内气体的物理与化学条件仍未得到充分约束。我们对阿尔玛（ALMA）的 CO(2-1)、HCN(3-2) 和 HCO⁺(3-2) 观测数据开展空间分辨分析，空间分辨率达 72 秒差距（pc）。数据被划分为与波束尺寸相当的六边形区域，并采用贝叶斯框架进行建模，该框架耦合了随时间演化的化学模型 UCLCHEM 的神经网络模拟器与非局部热动平衡（non-LTE）辐射传输模型 SpectralRadex，以推断每个区域的气体密度、动力学温度和宇宙射线电离率。三条谱线的线比均在内部盘区达到峰值，并随半径增大而降低。推断结果显示密度存在径向梯度（log₁₀ n(H₂) 约为 6.3 至 5）、动力学温度（约 200 开尔文（K）至 160 K）以及宇宙射线电离率（log₁₀(ζ/ζ₀) 约为 4.9 至 3）。尽管存在强大的射电 AGN，观测到的 HCN(3-2)/HCO⁺(3-2) 线比在整个盘区仍保持不超过 1（≤1）。我们的建模将此归因于 HCN(3-2) 的光学深度饱和（τ 约为 1 至 3），即使 HCN 的丰度超过 HCO⁺ 三倍或更多，也会抑制该线比。因此，若不考虑光学深度，HCN/HCO⁺ 线比无法用作丰度诊断，低线比并不一定意味着 AGN 对化学过程的影响较弱。方位角分辨剖面显示，盘区西部边界存在局部的 HCO⁺/CO 增强，与纤维-盘吸积界面重合，符合下落纤维与旋转盘之间的速度剪切产生的激波加工特征。这些结果表明，CND 的形成由其纤维状环境的吸积过程塑造。
英文摘要：
The brightest cluster galaxy NGC 1275, at the centre of the Perseus cluster, hosts the radio-loud AGN 3C 84 and a circumnuclear disc (CND) of cold molecular gas onto which large-scale CO filaments accrete, yet the physical and chemical conditions of the gas in the CND remain poorly constrained. We present a spatially resolved analysis of ALMA CO(2-1), HCN(3-2), and HCO$^+$(3-2) observations at 72 pc. The data are partitioned into beam-sized hexagonal regions and modelled with a Bayesian framework that couples a neural network emulator of time-dependent chemistry (UCLCHEM) with non-LTE radiative transfer (SpectralRadex) to infer the gas density, kinetic temperature, and cosmic ray ionisation rate in each region. All three line ratios peak in the inner disc and decline with radius. The inference shows radial gradients in density ($\log_{10} n({\rm H_2}) \approx 6.3$ to $\sim 5$), kinetic temperature (~200 K to ~160 K), and cosmic ray ionisation rate ($\log_{10}(ζ/ζ_0) \approx 4.9$ to $\sim 3$). Despite the powerful radio AGN, the observed HCN(3-2)/HCO$^+$(3-2) ratio remains $\lesssim 1$ across the disc. Our modelling attributes this to optical depth saturation of HCN(3-2) ($τ\sim 1$-3), which suppresses the intensity ratio even when the HCN abundance exceeds that of HCO$^+$ by a factor of three or more. The HCN/HCO$^+$ intensity ratio therefore cannot be used as an abundance diagnostic without accounting for optical depth, and a low ratio does not necessarily imply weak AGN influence on the chemistry. Azimuthally resolved profiles suggest a localised HCO$^+$/CO enhancement at the western disc boundary, coinciding with the filament-disc accretion interface and consistent with shock processing by velocity shear between the infalling filaments and the rotating disc. These results indicate that the CND is shaped by accretion from its filamentary environment.
