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arXiv 0804.4007astro-ph
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在CDF-N中搜索中红外遮蔽的AGN

Searching for mid-IR obscured AGN in the CDF-N

I. Georgantopoulos, A. Georgakakis, M. Rowan-Robinson, E. Rovilos

备注：
Accepted by A&A
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摘要：

利用\chandra深场北的数据，研究了中红外选择方法寻找被遮蔽活动星系核的效率。它表明，样本的活动星系核候选人的基础上编制的中红外颜色只遭受大量的污染，从正常的星系。X射线叠加分析揭示了这些源的软平均X射线谱，与Gamma~2.1一致。这表明，恒星形成星系，而不是模糊AGN占主导地位的叠加信号。相比之下，结合中红外和光学标准的活动星系核选择方法在寻找严重模糊的活动星系核候选者方面更成功。采用与Fiore等人（2008）提出的方法类似的方法来选择具有高24微米与光通量比（f_24/f_R>1000）的极红物体（R-[3.6]> 3.7 mag）。大约80%的这些源在X射线波长下没有被检测到。在这些源的位置上叠加X射线光子显示出平坦的平均X射线谱（γ ~0.8），这表明康普顿厚源，低光度和中等遮蔽（N_H~ 8 e22）活动星系核，或两者的组合。这些源的中红外颜色和亮度在z~2处与ULIRGs一致，而HST/ACS图像，可用于这些源中的光学较亮的，在其中许多中显示出受干扰的光学形态。上述证据表明，这一人口包括系统的形成过程中。

英文摘要：

The efficiency of mid-infrared selection methods for finding obscured AGN is investigated using data in the \chandra Deep Field North. It is shown that samples of AGN candidates compiled on the basis of mid-infrared colours only suffer substantial contamination from normal galaxies. X-ray stacking analysis reveals a soft mean X-ray spectrum for these sources, consistent with Gamma~2.1. This suggests that star-forming galaxies and not obscured AGN dominate the stacked signal. In contrast AGN selection methods that combine mid-infrared with optical criteria are more successful in finding heavily obscured AGN candidates. A method similar to the one proposed by Fiore et al. (2008) is adopted to select extremely red objects (R-[3.6]>3.7mag) with high 24micron to optical flux ratio (f_24/f_R>1000). About 80% of these sources are not detected at X-ray wavelengths. Stacking the X-ray photons at the positions of these sources shows a flat mean X-ray spectrum (Gamma~0.8), which suggests Compton-thick sources, low-luminosity and moderately obscured (N_H~8e22) AGN, or a combination of the two. The mid-infrared colours and luminosities of these sources are consistent with ULIRGs at z~2, while HST/ACS images, available for the optically brighter of these sources, show disturbed optical morphologies in many of them. The evidence above suggests that this population includes systems in the process of formation.

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