在CANDELS GOODS-S场中z>4的微弱活动星系核：寻找宇宙再电离的贡献者
Faint AGNs at z>4 in the CANDELS GOODS-S field: looking for contributors to the reionization of the Universe
摘要：
为了得到活动星系核对宇宙学电离辐射率的贡献，我们在坎德尔南场的$z>4$选择了微弱的活动星系核候选者，坎德尔南场是钱德拉、哈勃望远镜、斯皮策和各种地面望远镜覆盖范围最广的多波长场之一。我们采用了一个相对新颖的标准。作为第一步，高红移星系的选择在近红外$H$波段下降到非常微弱的水平（$H\leq27$）使用可靠的光度红移。这对应于$z>4$的选择标准的基础上的星系休息帧紫外线通量。如果候选活动星系核的X射线通量超过$F_X\sim 1.5\times 10^{-17}$ cgs（0.5-2 keV）的阈值，则从这个母样本中挑选出它们。我们在$z>4$发现了22个活动星系核候选者，并导出了在红移间隔$4<z<6.5$和绝对星等间隔$-22.5\lesssim M_{1450} \lesssim -18.5$中典型的本地塞弗特星系的紫外光度函数的第一个估计。在$z\sim 4-6$上得到的光度函数的暗端比从以前的紫外巡天得到的光度函数的暗端大约暗2/4个星等。然后，我们估计电离辐射率和氢光电离率在相同的红移间隔下合理的假设和讨论后可能的警告，最重要的是大的不确定性参与的测光红移的估计与无特征的，几乎幂律的SED和/或低平均逃逸分数的电离光子从活动星系核宿主星系的来源。我们认为，在合理评估可能的偏差，探测活动星系核人口可以产生在$z=4-6.5$光电离率保持高度电离的星系际介质中观察到的高红移类星体光谱的莱曼- $α$森林一致，提供了一个重要的贡献宇宙再电离。
英文摘要：
In order to derive the AGN contribution to the cosmological ionizing emissivity we have selected faint AGN candidates at $z>4$ in the CANDELS GOODS-South field which is one of the deepest fields with extensive multiwavelength coverage from Chandra, HST, Spitzer and various groundbased telescopes. We have adopted a relatively novel criterion. As a first step high redshift galaxies are selected in the NIR $H$ band down to very faint levels ($H\leq27$) using reliable photometric redshifts. This corresponds at $z>4$ to a selection criterion based on the galaxy rest-frame UV flux. AGN candidates are then picked up from this parent sample if they show X-ray fluxes above a threshold of $F_X\sim 1.5\times 10^{-17}$ cgs (0.5-2 keV). We have found 22 AGN candidates at $z>4$ and we have derived the first estimate of the UV luminosity function in the redshift interval $4<z<6.5$ and absolute magnitude interval $-22.5\lesssim M_{1450} \lesssim -18.5$ typical of local Seyfert galaxies. The faint end of the derived luminosity function is about two/four magnitudes fainter at $z\sim 4-6$ than that derived from previous UV surveys. We have then estimated ionizing emissivities and hydrogen photoionization rates in the same redshift interval under reasonable assumptions and after discussion of possible caveats, the most important being the large uncertainties involved in the estimate of photometric redshift for sources with featureless, almost power-law SEDs and/or low average escape fraction of ionizing photons from the AGN host galaxies. We argue that, under reasonable evaluations of possible biases, the probed AGN population can produce at $z=4-6.5$ photoionization rates consistent with that required to keep highly ionized the intergalactic medium observed in the Lyman-$α$ forest of high redshift QSO spectra, providing an important contribution to the cosmic reionization.
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