[论文解读] On the LINER nuclear obscuration, Compton-thickness and the existence of the dusty torus; Clues from Spitzer/IRS spectra

O. González-Martín, J. Masegosa+8 more|arXiv (Cornell University)|Jan 15, 2015

Galaxies: Formation, Evolution, Phenomena参考文献 98被引用 23

一句话总结

本研究利用斯皮策/IRS对40个LINERs的中红外光谱，研究其核心遮蔽特性及尘埃环的存在。结果发现，光度低于~10^42 erg/s的暗弱LINERs可能缺乏尘埃环，这些源中康普顿厚的X射线遮蔽可能源于尘埃亚稳化半径内的无尘气体，从而调和了缺乏尘埃环特征与高X射线柱密度之间的矛盾。

ABSTRACT

Most of the optically classified low ionisation narrow emission-line regions (LINERs) nuclei host an active galactic nuclei (AGN). However, how they fit into the unified model (UM) of AGN is still an open question. The aims of this work are to study at mid-infrared (mid-IR) (1) the Compton-thick nature of LINERs; and (2) the disappearance of the dusty torus in LINERs predicted from theoretical arguments. We have compiled all the available low spectral resolution mid-IR spectra of LINERs from the IRS/Spitzer (40 LINERs). We have complemented this sample with Spitzer/IRS spectra of PGQSOs, S1s, S2s, and SBs nuclei. We have studied the AGN versus the starburst content in our sample using different indicators: the EW(PAH 6.2um), the strength of the silicate feature at 9.7um, and the steepness of the mid-IR spectra. In 25 out of the 40 LINERs (i.e., 62.5%) the mid-IR spectra are not SB-dominated, similar to the comparison S2 sample (67.7%). The average spectra of both SB-dominated LINERs and S2s are very similar to the average spectrum of the SB class. Moreover, the average spectrum of AGN-dominated LINERs with Lx(2-10keV)>10E+41 erg/s is similar to the average mid-IR spectrum of AGN-dominated S2s. However, faint LINERs show flat spectra different from any of the other optical classes, suggesting the disappearance of the dusty torus. The correlation between nuLnu(12um) and Lx(2-10keV) for AGN nicely extends toward low luminosities only if SB-dominated LINERs are excluded and Lx(2-10keV) is corrected in Compton-thick LINER candidates. We discuss the nature of faint LINERs by comparing it with the spectra of several emission mechanisms like jet, ADAF, planetary nebulae, or post-AGB stars. We suggest that the material producing the Compton-thick X-ray obscuration is free of dust, to reconcile the Compton-thick nature of a large fraction of LINERs with the lack of dusty-torus signatures.

研究动机与目标

• 确定LINERs是否如统一模型所预测的那样拥有尘埃环。
• 利用中红外与X射线光度相关性，评估LINERs的康普顿厚度。
• 研究暗弱LINERs中红外发射的起源，特别是尘埃环特征的缺失。
• 调和LINERs中高X射线遮蔽与缺乏中红外尘埃环特征之间的矛盾。

提出的方法

• 从斯皮策/IRS收集40个LINERs的低分辨率中红外光谱，辅以PG QSOs、S1s、S2s和星暴核的样本。
• 利用PAH 6.2 µm等效宽度、硅酸盐9.7 µm光学深度和中红外光谱斜率，将光谱分类为AGN主导或星暴主导。
• 比较不同光学分类的平均中红外光谱，识别发射特征的差异。
• 分析12 µm光度（νLν(12 µm)）与2–10 keV X射线光度（LX）之间的相关性，以评估AGN光度关系。
• 对康普顿厚候选LINERs的X射线光度进行吸收修正，以检验其与X射线-中红外相关性的自洽性。
• 通过SED建模和光谱比较，评估类太阳末期星和行星状星云在中红外发射中的作用。

实验结果

研究问题

• RQ1 理论预测暗弱LINERs（L_bol < 10^42 erg/s）缺乏尘埃环，这一预测是否成立？
• RQ2 在吸收修正后，LINERs中康普顿厚X射线遮蔽是否与X射线-中红外光度相关性一致？
• RQ3 暗弱LINERs中观测到的平坦中红外谱的起源是什么？其与AGN主导的S2s有何不同？
• RQ4 LINERs中的中红外发射是否可由椭圆星系宿主、星暴、喷流或类太阳末期星和行星状星云的贡献来解释？
• RQ5 LINERs中的X射线遮蔽是由尘埃环物质引起，还是由尘埃亚稳化半径内的无尘气体引起？

主要发现

• 62.5%的LINERs（25/40）在中红外光谱中未表现出星暴主导的迹象，与类型2赛弗特星系中67.7%的比例相似。
• AGN主导的LINERs的平均中红外光谱在6–28 µm波段比星暴主导的LINERs、S2s或星暴源更平坦。
• 光度较低的LINERs（LX(2–10 keV) < 10^41 erg/s）表现出与所有其他光学分类均不同的平坦中红外光谱，暗示缺乏尘埃环发射。
• 仅当对康普顿厚LINER候选者进行吸收修正后，X射线-中红外光度相关性才在AGN中成立，表明其真实X射线光度约为观测值的70倍。
• 仅在排除星暴主导的LINERs后，X射线-中红外相关性才在低光度下成立，支持暗弱LINERs中红外发射具有不同起源。
• 在具有康普顿厚X射线遮蔽的暗弱LINERs中，尘埃环特征的缺失表明遮蔽物质为无尘气体，且很可能位于尘埃亚稳化半径内。