A Spitzer IRS Low-Resolution Spectroscopic Search for Buried AGNs in Nearby Ultraluminous Infrared Galaxies: A Constraint on Geometry between Energy Sources and Dust
A Spitzer IRS Low-Resolution Spectroscopic Search for Buried AGNs in Nearby Ultraluminous Infrared Galaxies: A Constraint on Geometry between Energy Sources and Dust
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DOI：
10.1086/513715
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发表时间：
2007-02
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通讯作者：
M. Imanishi;C. Dudley;R. Maiolino;P. Maloney;T. Nakagawa;Guido Risaliti Nao Japan;Nrlgmu;Romé;U. Colorado;Isasjaxa;Arcetri
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作者：
M. Imanishi;C. Dudley;R. Maiolino;P. Maloney;T. Nakagawa;Guido Risaliti Nao Japan;Nrlgmu;Romé;U. Colorado;Isasjaxa;Arcetri
本文报道了用斯皮策红外光谱仪在z < 0.15波段对附近的超发光红外星系（ulrgs）进行低分辨率5-35 μm光谱分析的结果。我们的重点是寻找埋藏的活动星系核（agn）的特征，在完整的ulrgs样本中，光学分类为非塞弗特（liner或H II区）。除了星爆-AGN分离的常规工具6.2、7.7和11.3 μm多环芳烃（PAH）发射特征外，我们还利用9.7和18 μm硅酸尘埃的光学深度特征来推断这些ulrgs核处的能量源和尘埃的几何形状，即能量源是否与尘埃在空间上混合得很好（正常星爆），还是比尘埃更集中（埋在AGN）。在观测到的光学非塞弗特ULIRGs中，至少有30%，甚至可能有50%的红外光谱可以很自然地解释为：(1)能量微不足道，适度遮挡（AV < 20-30等）发射多环芳烃的正常星暴；(2)能量优势，高度尘埃遮挡，集中的能量源，没有多环芳烃发射。我们将后一个组件解释为隐藏的AGN。在LINER地区的ulrgs中，具有隐伏AGN特征的ulrgs比例高于H II地区的ulrgs。大多数发光的埋藏AGN候选者都是在具有冷远红外颜色的ulrgs中发现的。在吸收校正的AGN的固有光度几乎没有不确定性的情况下，它们被发现是1012 L☉的数量级，占了ulrgs的大部分光度。5-35 μm光谱星爆/AGN分类与之前基于3-4 μm光谱的分类基本一致。
We present the results of Spitzer Infrared Spectrograph low-resolution infrared 5-35 μm spectroscopy of nearby ultraluminous infrared galaxies (ULIRGs) at z < 0.15. We focus on the search for the signatures of buried active galactic nuclei (AGNs) in the complete sample of ULIRGs classified optically as non-Seyferts (LINERs or H II regions). In addition to polycyclic aromatic hydrocarbon (PAH) emission features at 6.2, 7.7, and 11.3 μm, the conventional tool of starburst-AGN separation, we use the optical depths of the 9.7 and 18 μm silicate dust absorption features to infer the geometry of energy sources and dust at the nuclei of these ULIRGs, namely, whether the energy sources are spatially well mixed with dust (a normal starburst) or are more centrally concentrated than the dust (a buried AGN). Infrared spectra of at least 30%, and possibly 50%, of the observed optical non-Seyfert ULIRGs are naturally explained by emission consisting of (1) energetically insignificant, modestly obscured (AV < 20-30 mag) PAH-emitting normal starbursts and (2) energetically dominant, highly dust-obscured, centrally concentrated energy sources with no PAH emission. We interpret the latter component as a buried AGN. The fraction of ULIRGs showing some buried AGN signatures is higher in LINER ULIRGs than in H II region ULIRGs. Most of the luminous buried AGN candidates are found in ULIRGs with cool far-infrared colors. Where the absorption-corrected intrinsic AGN luminosities are derivable with little uncertainty, they are found to be of the order of 1012 L☉, accounting for the bulk of the ULIRGs' luminosities. The 5-35 μm spectroscopic starburst/AGN classifications are generally consistent with our previous classifications based on 3-4 μm spectra for the same sample.