[论文解读] Molecular Gas Chemistry in AGN I. The IRAM Survey of NGC1068

A. Usero, S. García‐Burillo+3 more|ArXiv.org|Feb 24, 2004

Astrophysics and Star Formation Studies被引用 92

一句话总结

本研究利用IRAM 30米望远镜观测，调查了赛佛特2型星系NGC 1068的核区盘（CND）中的分子气体化学。研究提出，X射线辐照通过X射线主导区（XDR）化学过程驱动了异常的分子丰度，特别是高SiO和HOC+丰度，其中HCO+/HOC+比值为30–80，是迄今在分子气体中测得的最低值。

ABSTRACT

There is observational evidence that nuclear winds and X-rays can heavily influence the physical conditions and chemical abundances of molecular gas in the circumnuclear disks (CND) of Active Galactic Nuclei (AGN). In this paper we probe the chemical status of molecular gas in the CND of NGC1068, a prototypical Seyfert 2 galaxy. Precedent claims that the chemistry of molecular gas in the nucleus of NGC1068 is abnormal by galactic standards were based on the high HCN/CO luminosity ratio measured in the CND. Results from new observations obtained in this survey have served to derive abundances of molecular species such as SiO, CN, HCO+, HOC+, H13CO+ and HCO. These estimates are complemented by a re-evaluation of molecular abundances for HCN, CS and CO, based on previously published single-dish and interferometer observations of NGC1068. We report on the first detection of SiO emission in the CND of NGC1068. The estimated large abundance of SiO in the CND, X(SiO)~5-10x10^{-9}, cannot be attributed to shocks related to star formation, as there is little evidence of a recent starburst in the nucleus of NGC1068. Alternatively, we propose that silicon chemistry is driven by intense X-ray processing of molecular gas. We also report on the first extragalactic detection of the reactive ion HOC+. Most remarkably, the estimated HCO+/HOC+ abundance ratio in the nucleus of NGC1068, ~30-80, is the smallest ever measured in molecular gas. The abundances derived for all molecules that have been the subject of this survey are compared with the predictions of models invoking either oxygen-depletion or X-ray chemistry in molecular gas. Our conclusions favour an overall scenario where the CND of NGC1068 has become a giant X-ray Dominated Region (XDR).

研究动机与目标

• 确定原型赛佛特2型星系NGC 1068的核区盘（CND）中分子气体的化学状态。
• 解决观测到的高HCN/CO比值与氧耗竭模型及X射线驱动化学模型之间的矛盾。
• 检验X射线辐照是否能解释CND中SiO和HOC+等活性物种丰度增强的现象。
• 将观测到的分子丰度与XDR模型和氧耗竭模型的预测进行比较，以识别主导的化学机制。

提出的方法

• 利用IRAM 30米望远镜在毫米波段对NGC 1068进行分子线谱线调查。
• 测量了SiO(3–2)、SiO(2–1)、HOC+、CN、HCO+、H13CO+、HCO、HCN、CS和CO的发射线。
• 将新的单孔径数据与先前发表的干涉仪和单孔径观测数据结合，用于丰度计算。
• 在所有物种中采用一致的激发和光学厚度假设，重新评估分子丰度。
• 将观测到的丰度比与XDR模型（X(e⁻) ~ 10⁻⁶–10⁻⁴）和氧耗竭模型的预测进行比较。
• 利用空间分辨数据比较东、西CND结块之间的化学差异，并与钱德拉X射线图像相关联。

实验结果

研究问题

• RQ1 NGC 1068的CND中HCN/CO光度比异常偏高，其原因是什么？是X射线处理还是氧耗竭所致？
• RQ2 为何CND中SiO丰度显著高于星形成区域？星形成引发的激波能否解释这一现象？
• RQ3 为何HCO+/HOC+丰度比达到30–80的极端值，这是迄今在分子气体中测得的最低值？
• RQ4 东、西CND结块间分子丰度的空间变化是否与X射线照射差异相关？
• RQ5 XDR模型能否同时再现NGC 1068的CND中HCN、SiO、HOC+和CN的观测丰度？

主要发现

• 首次在NGC 1068的CND中探测到SiO(3–2)和SiO(2–1)发射，其全局丰度为X(SiO) ~ (5–10) × 10⁻⁹。
• CND中SiO丰度比周围星暴环高10倍，排除了星形成驱动激波作为其起源的可能性。
• 首次在河外星系中探测到活性离子HOC+，其HCO+/HOC+丰度比约为30–80，为迄今最低测量值。
• HOC+谱线轮廓呈不对称性，峰值位于红移速度，表明CND中X射线处理或形成效率不均。
• XDR模型成功解释了观测到的HCN/HCO+和CN/HCN丰度比，而氧耗竭模型则无法解释。
• 空间分辨数据显示西CND结块的X射线照射更强，与钱德拉图像显示该区域Fe Kα发射更强的结果一致。