用稠密气体示踪剂的高J线观测探测NGC 1068核中的X射线辐照
Probing X-ray irradiation in the nucleus of NGC 1068 with observations of high-J lines of dense gas tracers
摘要：
结合高J分子谱线，我们的目标是限制在塞弗特2星系NGC 1068的中心区域的致密气体的物理条件，并确定AGN或星爆贡献的签名。我们用麦克斯韦望远镜观测了NGC 1068中HCN、HNC和HCO+的J=4-3跃迁，以及CN的N_J=2_{5/2}-1_3/2}和N_J=3_{5/2}-2_5/2}跃迁。根据谱线强度比和辐射转移模型，我们估计了HCN、HNC和CN的激发条件。我们发现，HCN、HNC、CN和高J HCO+的体发射来自n（H_2）≥ 10^5 cm^-3的稠密气体。然而，低J的HCO+谱线（主要是HCO+柱密度）追踪的是密度较低（n（H_2）<10^5 cm^-3）和温度较低（T_K<=20 K）的气体，而高J的HCO+谱线则来自温度较高（>30 K）的气体。HCO+ J=4-3谱线强度与较低的跃迁谱线和HCN J=4-3谱线相比，支持局部XDR环境的影响。估算的N（CN）/N（HCN）~（1-4）柱密度比表明XDR/AGN环境可能有X射线或冲击引起的颗粒表面化学贡献。
英文摘要：
With the incorporation of high-J molecular lines, we aim to constrain the physical conditions of the dense gas in the central region of the Seyfert 2 galaxy NGC 1068 and to determine signatures of the AGN or the starburst contribution. We used the James Clerk Maxwell Telescope to observe the J=4-3 transition of HCN, HNC, and HCO+, as well as the CN N_J=2_{5/2}-1_{3/2} and N_J=3_{5/2}-2_{5/2}, in NGC 1068. We estimate the excitation conditions of HCN, HNC, and CN, based on the line intensity ratios and radiative transfer models. We find that the bulk emission of HCN, HNC, CN, and the high-J HCO+ emerge from dense gas n(H_2)>=10^5 cm^-3). However, the low-J HCO+ lines (dominating the HCO+ column density) trace less dense (n(H_2)<10^5 cm^-3) and colder (T_K<=20 K) gas, whereas the high-J HCO+ emerges from warmer (>30 K) gas than the other molecules. The HCO+ J=4-3 line intensity, compared with the lower transition lines and with the HCN J=4-3 line, support the influence of a local XDR environment. The estimated N(CN)/N(HCN)~1-4 column density ratios are indicative of an XDR/AGN environment with a possible contribution of grain-surface chemistry induced by X-rays or shocks.
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