深入探究NGC 6868 II：电离气体与激发机制
Digging deeper into NGC 6868 II: ionized gas and excitation mechanism
AI总结：
本研究利用Gemini/GMOS观测分析NGC 6868内部电离气体，识别出窄、宽两种气体成分，测得电子温度与密度的空间梯度，结合多类诊断图明确中心区域以AGN电离为主、外围由演化恒星与激波共同电离，证实激波对星系电离平衡有重要作用。
摘要：
我们利用Gemini/GMOS（双子星天文台/双子多目标光谱仪）积分场单元观测数据，研究了星系NGC 6868内部区域（约680×470平方秒差距）的电离气体。通道图揭示出复杂的运动学特征与形态，表明NGC 6868内部存在多种作用过程。通过发射线拟合，我们在数据中识别出两种普遍存在的成分：一种是示踪电离气体盘的窄成分（速度弥散σ约110 km/s），另一种是主要与流入/流出气体相关的宽成分（速度弥散σ约300 km/s）。推导得到的V波段消光空间分布与恒星种群合成得到的结果一致，但整体数值更高。我们首次测量了NGC 6868的电子温度，发现其数值从中心区域的约14000 K到≳20000 K不等，呈现向外升高的温度梯度。电子密度图则呈现相反的变化趋势：宽成分的中心电子密度达N_e约4000 cm⁻³，向视场边缘降低至N_e约100 cm⁻³。利用BPT图，我们发现所有空间像素的电离特征都同时符合活动星系核（AGN）与激波电离的特征。但当我们将这一信息与运动学、温度研究结果相结合，并得到WHAN图的进一步支持后，我们认为：AGN是NGC 6868中心区域的主导电离机制，而外围延展成分则由热的小质量演化恒星与激波共同电离。根据我们的研究结果，激波在该星系的电离平衡中发挥着重要作用。
英文摘要：
We studied the ionized gas in the inner region ($\sim680\times470$ pc$^2$) of the galaxy NGC 6868 using Gemini/GMOS integral field unit observations. Channel maps reveal complex kinematics and morphology, indicating multiple processes at work in NGC 6868. Through emission-line fitting, we identified two ubiquitous components in our data: a narrow ($σ\sim110$ km s$^{-1}$) tracing an ionized gas disc and a broad component ($σ\sim300$ km s$^{-1}$) mainly associated with inflowing/outflowing gas. The derived V-band reddening shows a spatial distribution consistent with that obtained from stellar population synthesis, although with generally higher values. For the first time, we measured the electron temperature in NGC 6868, finding values ranging from $\sim 14000$ K in the central region to $\gtrsim20000$ K with an outward increasing temperature gradient. The electron density map exhibits an inverse relationship, with central values reaching $N_e\sim4000$ cm$^{-3}$ for the broad component decreasing to $N_e\sim100$ cm$^{-3}$ towards the edges of the field of view. Using BPT diagrams, we found that all spaxels are consistent with both AGN and shock ionization. However, when this information is combined with our kinematic and temperature findings, and further supported by the WHAN diagram, we argue that an AGN is the dominant ionisation mechanism in the central region of NGC 6868, while the extended outer component is ionized by a combination of hot low-mass evolved stars and shocks. According to our findings, shocks play a significant role in the ionization balance of this galaxy.
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