CHASING HIGHLY OBSCURED QSOs IN THE COSMOS FIELD-猫眼课题宝

DOI：

10.1088/0004-637x/693/1/447

发表时间：

2008-10

期刊：

The Astrophysical Journal

影响因子：

通讯作者：

中科院分区：

其他

文献类型：

作者：

F. Fiore;S. Puccetti;M. Brusa;M. Salvato;G. Zamorani;T. Aldcroft;H. Aussel;H. Brunner;P. Capak;N. Cappelluti;F. Civano;A. Comastri;M. Elvis;C. Feruglio;A. Finoguenov;A. Fruscione;R. Gilli;G. Hasinger;A. Koekemoer;J. Kartaltepe;O. Ilbert;C. Impey;E. Floc’h;S. Lilly;V. Mainieri;A. Martínez-Sansigre;H. McCracken;N. Menci;A. Merloni;T. Miyaji;D. Sanders;M. Sargent;E. Schinnerer;N. Scoville;J. Silverman;V. Smolčić;A. Steffen;P. Santini;Y. Taniguchi;D. Thompson;D. Thompson;J. Trump;C. Vignali;M. Urry;Lin Yan

根据宇宙X射线背景的活动星系核(AGN)合成模型和从局部凸起测量的“残余”超大质量黑洞质量函数，预测了大量严重遮挡的康普顿厚的活动星系核(AGN)。然而，即使是最深入的X射线调查也不能有效地寻找这些难以捉摸的活动星系核。替代的选择标准，结合了中红外和近红外，以及光学光度学，已经成功地精确定位了大量康普顿厚度的活动星系核。我们利用宇宙演化调查(COSMOS)领域中大片区域(钱德拉深场覆盖面积的10倍以上)的钱德拉和斯皮策深度覆盖，将对高度遮挡、康普顿厚度的活动核的搜索扩展到更高的光度。这些来源的表面密度很低，因此只有通过大面积调查才能提供大样本，比如宇宙调查。分析了能量密度大于2 4μm的宇宙MIPS源的X射线性质，其能量密度大于5 5 0μJy。对于钱德拉图像中没有直接探测到的MIPS源，我们在软X射线和硬X射线波段产生堆叠图像。为了估计康普顿厚的活动星系核在MIPS源群体中的比例，我们将观测到的堆积计数率和硬度比与详细的蒙特卡罗模拟预测的结果进行了比较，其中包括遮蔽的活动星系核和恒星形成星系。用这种方法估算的康普顿厚类星体(对数L(2-10keV)=44-45erg S−1，或对数λLλ(5.8Mpc m)=44.79-46.18erg S−1，对于典型的红外-X射线光度比)在红移盒1.2-2.2中的体积密度为(4.8±1.1)×10-10Mpc-6Mpc-3。这个密度是相同红移和光度区间中所有X射线选择类星体的∼44%，这与最新的宇宙X射线背景活动星系核合成模型的预期是一致的(Gilli等人)。)。在z=0.7-1.2时，低亮度康普顿厚活动星系核(对数L(2-10keV)=43.5-44)的密度为(3.7±1.1)×10−5 MpC−3，相当于X射线选择活动星系核的∼6 7%。在高光度和低光度下，红外选择的康普顿厚度的活动星系核与X射线选择的、未遮挡的和中等遮挡的活动星系核的比例之间的比较表明，康普顿厚的活动星系核遵循与康普顿薄活动星系核相似的光度依赖关系，在高亮度时变得相对稀有。我们估计活动星系核(无遮挡、中度遮挡和康普顿厚)在MIPS源总体中所占的比例为49±10%，这一值显著高于之前在类似24μm通量下的估计值。我们讨论了我们的发现如何约束AGN反馈模型。

A large population of heavily obscured, Compton-thick active galactic nuclei (AGNs) is predicted by AGN synthesis models for the cosmic X-ray background and by the “relic” supermassive black hole mass function measured from local bulges. However, even the deepest X-ray surveys are inefficient to search for these elusive AGNs. Alternative selection criteria, combining mid-infrared with near-infrared, and optical photometry, have instead been successful in pinpointing a large population of Compton-thick AGNs. We take advantage of the deep Chandra and Spitzer coverage of a large area (more than 10 times the area covered by the Chandra deep fields, CDFs) in the Cosmic Evolution Survey (COSMOS) field to extend the search of highly obscured, Compton-thick active nuclei to higher luminosity. These sources have low surface density, and therefore large samples can be provided only through large area surveys, like the COSMOS survey. We analyze the X-ray properties of COSMOS MIPS sources with 24 μm fluxes higher than 550 μJy. For the MIPS sources not directly detected in the Chandra images, we produce stacked images in soft and hard X-rays bands. To estimate the fraction of Compton-thick AGN in the MIPS source population, we compare the observed stacked count rates and hardness ratios to those predicted by detailed Monte Carlo simulations, including both obscured AGN and star-forming galaxies. The volume density of Compton-thick QSOs (log L(2–10 keV) = 44–45 erg s−1, or logλLλ(5.8 μm) = 44.79–46.18 erg s−1 for a typical infrared to X-ray luminosity ratio) evaluated in this way is (4.8 ± 1.1) × 10−6 Mpc−3 in the redshift bin 1.2–2.2. This density is ∼44% of all X-ray-selected QSOs in the same redshift and luminosity bin, and it is consistent with the expectation of the most up-to-date AGN synthesis models for the cosmic X-ray background (Gilli et al. ). The density of lower luminosity Compton-thick AGNs (log L(2–10 keV) = 43.5–44) at z = 0.7–1.2 is (3.7 ± 1.1) × 10−5 Mpc−3, corresponding to ∼67% of X-ray-selected AGNs. The comparison between the fraction of infrared-selected, Compton-thick AGNs to the X-ray selected, unobscured, and moderately obscured AGNs at high and low luminosity suggests that Compton-thick AGNs follow a luminosity dependence similar to that discovered for Compton-thin AGNs, becoming relatively rarer at high luminosities. We estimate that the fraction of AGNs (unobscured, moderately obscured, and Compton thick) to the total MIPS source population is 49 ± 10%, a value significantly higher than that previously estimated at similar 24 μm fluxes. We discuss how our findings can constrain AGN feedback models.

CHASING HIGHLY OBSCURED QSOs IN THE COSMOS FIELD

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