[论文解读] Inflowing gas onto a compact obscured nucleus in Arp 299A: Herschel spectroscopic studies of H2O and OH

N. Falstad, E. González-Alfonso+2 more|arXiv (Cornell University)|Nov 3, 2016

Galaxies: Formation, Evolution, Phenomena参考文献 88被引用 11

一句话总结

利用赫歇尔空间望远镜的远红外与亚毫米波谱线观测，本研究调查了Arp 299A星系致密遮蔽核区的流入分子气体，探测到多条H2O和OH谱线的吸收与发射。分析显示，存在一个康普顿光学厚、高表面亮度的核区成分，具有极端的分子丰度；红移的基态OH谱线表明存在低激发态的流入分子气体，支持该星系核区由分子流入供能的复合型AGN/星暴活动源，且16O/18O比值极高（>400）。

ABSTRACT

Aims. We probe the physical conditions in the core of Arp 299A and try to put constraints to the nature of its nuclear power source. Methods. We used Herschel Space Observatory far-infrared and submillimeter observations of H2O and OH rotational lines in Arp 299A to create a multi-component model of the galaxy. In doing this, we employed a spherically symmetric radiative transfer code. Results. Nine H2O lines in absorption and eight in emission as well as four OH doublets in absorption and one in emission, are detected in Arp 299A. No lines of the 18O isotopologues, which have been seen in compact obscured nuclei of other galaxies, are detected. The absorption in the ground state OH doublet at 119 μm is found redshifted by ~175 km/s compared to other OH and H2O lines, suggesting a low excitation inflow. We find that at least two components are required in order to account for the excited molecular line spectrum. The inner component has a radius of 20-25 pc, a very high infrared surface brightness (> 3e13 Lsun/kpc^2), warm dust (Td > 90 K), and a large H2 column density (NH2 > 1e24 cm^-2). The outer component is larger (50-100 pc) with slightly cooler dust (70-90 K). In addition, a much more extended inflowing component is required to also account for the OH doublet at 119 μm. Conclusions. The Compton-thick nature of the core makes it difficult to determine the nature of the buried power source, but the high surface brightness indicates that it is either an active galactic nucleus and/or a dense nuclear starburst. The high OH/H2O ratio in the nucleus indicates that ion-neutral chemistry induced by X-rays or cosmic-rays is important. Finally we find a lower limit to the 16O/18O ratio of 400 in the nuclear region, possibly indicating that the nuclear starburst is in an early evolutionary stage, or that it is fed through a molecular inflow of, at most, solar metallicity.

研究动机与目标

• 探究Arp 299A核心区域的物理条件，该星系为具有致密遮蔽核的明亮红外星系。
• 约束被遮蔽核区能源的性质，区分活动星系核（AGN）与核区星暴活动的贡献。
• 研究分子气体流入在驱动核区活动中的作用。
• 测定同位素比值，特别是16O/18O，以推断核区气体储层的演化阶段或金属丰度。

提出的方法

• 利用赫歇尔空间望远镜对Arp 299A中H2O与OH转动能级谱线进行远红外与亚毫米波段的谱线观测。
• 采用球对称辐射转移模型代码，对观测到的谱线轮廓与强度进行多组分拟合。
• 分析吸收与发射谱线轮廓，包括红移测量以推断动力学特征。
• 将观测到的谱线流量与连续谱水平与合成模型进行比较，推导出柱密度、温度与丰度。
• 由于未探测到18O同位素物种，据此设定16O/18O比值的下限。
• 对不同空间组分的尘埃温度、H2柱密度与分子丰度进行建模。

实验结果

研究问题

• RQ1 Arp 299A核区分子气体的动力学状态如何？是否存在气体流入的迹象？
• RQ2 致密核区高红外表面亮度的物理成因是什么？是由AGN、星暴，还是两者共同驱动？
• RQ3 核区的同位素16O/18O比值是多少？该比值对气体金属丰度或演化阶段有何启示？
• RQ4 H2O与OH丰度如何与核区辐射场及离子-中性分子化学过程相关联？
• RQ5 康普顿光学厚度在遮蔽中心能源方面起什么作用？它如何影响该能源的可探测性？

主要发现

• 探测到九条H2O谱线的吸收与八条H2O谱线的发射，以及四组OH双峰谱线的吸收与一组OH双峰谱线的发射，表明核区具有高激发态特征。
• 基态OH 119 μm双峰谱线红移约175 km s⁻¹，表明存在低激发态的分子气体流入。
• 内层组分半径为20–25 pc，尘埃温度T_d > 90 K，H2柱密度 >10²⁴ cm⁻²，红外表面亮度 ≥3×10¹³ L☉ kpc⁻²。
• 核区H2O与OH丰度较高，相对于H核，H2O ≈ 1–5×10⁻⁶，OH ≈ 0.5–5×10⁻⁵。
• 推断16O/18O比值 >400，表明核区星暴处于早期演化阶段，或由最多为太阳金属丰度的未处理气体供给。
• 未探测到18OH与H₂¹⁸O谱线，暗示核区主导的离子-中性分子化学过程由X射线或宇宙射线驱动。