[论文解读] High resolution observations of HCN and HCO+ J=3-2 in the disk and outflow of Mrk231 -Detection of vibrationally excited HCN in the warped nucleus

S. Aalto, S. García‐Burillo+7 more|DIGITAL.CSIC (Spanish National Research Council (CSIC))|Nov 10, 2014

Astrophysics and Star Formation Studies参考文献 56被引用 48

一句话总结

本研究利用IRAM Plateau de Bure干涉仪对Mrk 231中的HCN和HCO⁺ J=3–2辐射进行了高分辨率观测，首次在扭曲的核盘面中探测到振动激发态HCN（ν₂=1 f）。核心处紧凑且高度倾斜的发射——由14 µm中红外辐射激发——揭示了一个致密、多尘、扭曲的内盘结构，其HCN丰度显著升高（X(HCN) ≈ 10⁻⁶），气体密度较高（n ≈ 10⁴–5×10⁵ cm⁻³），表明红外泵浦影响了主盘面约50%的发射；而展宽的谱线轮廓则表明存在碰撞激发的喷流状气体，速度高达±750 km s⁻¹。

ABSTRACT

We obtained high resolution (0."25 to 0."90) observations of HCN and HCO+ J=3-2 of the ultraluminous QSO galaxy Mrk231 with the IRAM Plateau de Bure Interferometer. We find luminous HCN and HCO+ 3-2 emission in the main disk and we detect compact (r<90 pc) vibrationally excited HCN 3-2, v2=1f emission centred on the nucleus. The velocity field of the vibrationally excited HCN is strongly inclined (PA=155 deg.) compared to the east-west rotation of the main disk. The nuclear molecular mass is estimated to 8e8 Msun with an average N(H2)of 1.2e24 cm-2. Prominent, spatially extended (>350 pc) line wings are found for HCN 3-2 with velocities +-750 km/s. Line ratios indicate that the emission is emerging in dense gas n=1e4 - 5e5 cm-3 of elevated HCN abundance X(HCN)=1e-8 to 1e-6. High X(HCN) also allows for the emission to originate in gas of more moderate density. We tentatively detect nuclear emission from the reactive ion HOC+ with HCO+/HOC+=10-20. The HCN v2=1f line emission is consistent with the notion of a hot, dusty, warped inner disk of Mrk231 where the v2=1f line is excited by bright mid-IR 14 micron continuum. We estimate the vibrational temperature T_vib to 200-400 K. We propose that 50% of the main HCN emission may have its excitation affected by the radiation field through IR pumping of the vibrational ground state. The HCN emission in the line wings, however, is more extended and thus likely not strongly affected by IR pumping. Our results reveal that dense clouds survive (and/or are formed) in the AGN outflow on scales of at least several hundred pc before evaporating or collapsing. The elevated HCN abundance in the outflow is consistent with warm chemistry possibly related to shocks and/or X-ray irradiated gas. An upper limit to the mass and momentum flux is 4e8 Msun and 12L_AGN/c, respectively, and we discuss possible driving mechanisms for the dense outflow.

研究动机与目标

• 研究超亮QSO星系Mrk 231核区及大尺度喷流中的分子气体性质。
• 确定活动星系核驱动喷流中致密分子气体的物理条件。
• 检验活动星系核的中红外辐射是否为HCN振动激发态的激发源。
• 估算喷流与核区致密气体的质量、动量通量及激发机制。

提出的方法

• 利用IRAM Plateau de Bure干涉仪对HCN和HCO⁺ J=3–2跃迁进行高分辨率（0.″25至0.″90）干涉观测。
• 通过空间分辨的谱线映射分析速度场、谱线展宽及发射形态。
• 通过谱线强度比分析推断气体密度（n）、HCN丰度（X(HCN)）及激发条件。
• 假设HCN在基态与第一激发振动态共空间分布，建模14 µm中红外泵浦引起的振动激发。
• 利用积分线流量与 beam-averaged 假设估算分子质量与柱密度。
• 比较HCO⁺/HOC⁺谱线强度比，推断离子化学及核区可能存在的活性离子。

实验结果

研究问题

• RQ1 Mrk 231核盘面及喷流中HCN和HCO⁺ J=3–2发射的空间分布与运动学特征如何？
• RQ2 观测到的核区HCN发射是否由14 µm中红外辐射泵浦ν₂=1 f振动态引起？
• RQ3 活动星系核驱动喷流中致密分子气体的物理条件（密度、丰度、温度）为何？
• RQ4 红外泵浦对主盘面与展宽谱线翼部的HCN激发影响程度如何？
• RQ5 致密分子喷流的质量与动量通量是多少？其驱动机制可能为何？

主要发现

• 首次在Mrk 231中探测到HCN J=3–2 ν₂=1 f振动跃迁，发射紧凑（FWHM r ≈ 0.″17，150 pc），中心位于核区。
• 振动激发态HCN的速度场表现出强烈倾角（PA ≈ 155° ± 10°），表明存在扭曲的多尘内盘结构。
• 核区分子质量估算为8×10⁸ M⊙，平均H₂柱密度为1.2×10²⁴ cm⁻²。
• HCN ν₂=1 f线由14 µm中红外连续辐射激发，振动温度为200–400 K。
• HCN J=3–2的谱线展宽超过350 pc（0.″4），速度达±750 km s⁻¹，表明存在大质量、致密喷流。
• 喷流中HCN丰度升高（X(HCN) ≈ 10⁻⁶），与温暖、致密气体一致，可能由激波或X射线辐照驱动。