AGN周围的稠密分子气体：NGC3227中的HCN/CO
Dense Molecular Gas around AGN: HCN/CO in NGC3227
摘要：
现在有令人信服的证据表明，HCN分子线发射的强度在活动星系核周围增强。本文研究了Seyfert星系NGC 3227的特殊情况，它的HCN（1-0）88 GHz和CO（2-1）230 GHz转动谱线的亚角秒分辨率数据使我们能够在空间上分离出半径为140 pc的环核区和半径为40 pc的内核区。HCN（1-0）/CO（2-1）通量比在这两个区域之间相差超过一个数量级。我们进行大速度梯度（LVG）的计算，以确定的参数范围（气体温度和密度，HCN/CO丰度比，柱密度和速度梯度），产生物理上合理的解决方案，在中央100 pc的观测通量比。在原子核中观测到的HCN/CO强度比与在非常致密（>=10^5cm^{-3}）的气体中非常光学厚的热化发射是一致的，在这种情况下，那里的HCN/CO丰度比是不受约束的。或者，HCN/CO强度比可能是由于光学较薄的发射，但具有非常高的（~10^{-2}）HCN/CO丰度比。这种可能性与塞弗特星系NGC 1068和NGC 6951的核中的CO和HCN排放更加一致。这将意味着速度梯度很大，云可能是重力不受约束的。我们估计，在NGC 3227中心半径小于20秒差距处的X射线电离率超过10^{-13}s^{-1}。X射线电离和加热可能会导致高HCN/CO的比例在温暖的气体在一个高电离的分子相附近的活动星系核。
英文摘要：
There is now convincing evidence that the intensity of HCN molecular line emission is enhanced around active galactic nuclei. In this paper we examine the specific case of the Seyfert galaxy NGC3227, for which there are subarcsecond resolution data for the HCN (1-0) 88 GHz and CO (2-1) 230 GHz rotational lines, enabling us to spatially separate a circumnuclear ring at a radius of 140pc and an inner nuclear region within 40pc of the AGN. The HCN(1-0)/CO(2-1) flux ratio differs by more than an order of magnitude between these two regions. We carry out large velocity gradient (LVG) computations to determine the range of parameters (gas temperature and density, HCN/CO abundance ratio, column densities and velocity gradients) that yield physically plausible solutions for the observed flux ratio in the central 100pc. The observed HCN/CO intensity ratio in the nucleus is consistent with very optically thick thermalized emission in very dense (>=10^5cm^{-3}) gas, in which case the HCN/CO abundance ratio there is unconstrained. Alternatively, the HCN/CO intensity ratio could be due to optically thinner emissions but with very high (~10^{-2}) HCN/CO abundance ratios. This possibility is more consistent with the CO and HCN emissions seen in the nuclei of the Seyfert galaxies NGC1068 and NGC6951. It would imply the velocity gradients are large and the clouds may be gravitationally unbound. We estimate that the X-ray ionisation rate at radii less than 20pc in the centre of NGC3227 exceeds 10^{-13}s^{-1}. X-ray ionisation and heating may lead to high HCN/CO ratios in warm gas in a high-ionisation molecular phase near the AGN.
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