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arXiv 1102.4714astro-ph.CO
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4 Ms Chandra深场南z~5处的康普顿厚AGN

A Compton-thick AGN at z~5 in the 4 Ms Chandra Deep Field South

R. Gilli, J. Su, C. Norman, C. Vignali, A. Comastri, P. Tozzi, P. Rosati, M. Stiavelli, W. N. Brandt, Y. Q. Xue, B. Luo, M. Castellano, A. Fontana, F. Fiore, V. Mainieri, A. Ptak

摘要:

本文报道了在4 M钱德拉深场南发现了一个Z=4.76的康普顿厚度的活动星系核。这个物体被选为HST/ACS图像中的V波段脱落物,以前被认为是光谱学中的活动星系核。4 Ms Chandra观测显示在V波段脱落位置处有显著的(~ 4.2 sigma)X射线探测。X射线源显示出HR=0.23 ± 0.24的硬度比,对于z~5处的源,这高度暗示了康普顿厚吸收。在~0.9-4 keV的能量范围内,可以看到源X射线光谱高于背景水平,即,在~5-23 keV的静止帧能量范围内。当光子指数固定为Gamma=1.8时,测得的柱密度为N_H=1.4^{+0.9}_{-0.5} x 10^{24} cm^{-2},为康普顿厚度。据我们所知,这是迄今为止发现的最遥远的、被X射线光谱分析证实的活动星系核。在2-10 keV波段的固有(去吸收)静止坐标系光度为~2.5 x 10^{44} erg s^{-1},这使该天体属于2型类星体。光谱能量分布表明,大质量恒星的形成与遮蔽的黑洞吸积有关。这个系统可能是在黑洞和早期恒星质量组装的一个主要的共同时期被捕获的。在高红移的严重模糊AGN的数量密度的措施将是至关重要的,从一开始重建BH/星系的演化历史。

英文摘要:

We report the discovery of a Compton-thick Active Galactic Nucleus (AGN) at z=4.76 in the 4 Ms Chandra Deep Field South. This object was selected as a V-band dropout in HST/ACS images and previously recognized as an AGN from optical spectroscopy. The 4 Ms Chandra observations show a significant (~4.2sigma) X-ray detection at the V-band dropout position. The X-ray source displays a hardness ratio of HR=0.23+-0.24, which, for a source at z~5, is highly suggestive of Compton-thick absorption. The source X-ray spectrum is seen above the background level in the energy range of ~0.9-4 keV, i.e., in the rest-frame energy range of ~5-23 keV. When fixing the photon index to Gamma=1.8, the measured column density is N_H=1.4^{+0.9}_{-0.5} x 10^{24} cm^{-2}, which is Compton-thick. To our knowledge, this is the most distant heavily obscured AGN, confirmed by X-ray spectral analysis, discovered so far. The intrinsic (de-absorbed), rest-frame luminosity in the 2-10 keV band is ~2.5 x 10^{44} erg s^{-1}, which places this object among type-2 quasars. The Spectral Energy Distribution shows that massive star formation is associated with obscured black hole accretion. This system may have then been caught during a major co-eval episode of black hole and stellar mass assembly at early times. The measure of the number density of heavily obscured AGN at high redshifts will be crucial to reconstruct the BH/galaxy evolution history from the beginning.

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