Evidence for Low Radiative Efficiency or Highly Obscured Growth of z > 7 Quasars
Evidence for Low Radiative Efficiency or Highly Obscured Growth of z > 7 Quasars
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DOI：
10.3847/2041-8213/ab42e3
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发表时间：
2019-06
期刊：
影响因子：
--
通讯作者：
F. Davies;J. Hennawi;A. Eilers
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文献类型：
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作者：
F. Davies;J. Hennawi;A. Eilers
在所有大质量星系中心观测到的超大质量黑洞（SMBH）被认为是在遥远的过去的类星体阶段通过发光吸积而生长的。以光的形式发射的流入静止质量能量的比例，即辐射效率，被推断为 10%，与薄盘吸积模型的预期一致。但是，为 z > 7 的类星体提供动力的数十亿个太阳质量的超大质量黑洞的存在对这一情况提出了挑战：只要它们遵守爱丁顿极限，就没有足够的时间从恒星残余种子中生长出 z > 7 的超大质量黑洞，除非辐射效率低于 10%。在这里，我们表明，可以使用前景中性星系际气体作为宇宙尺度的电离光子计数器来限制已知最远类星体的辐射效率。根据 ULAS J1120+0641 (z = 7.09) 和 ULAS J1342+0928 (z = 7.54) 的 Lyα 吸收曲线，我们分别确定后中值辐射效率为 0.08% 和 0.1%，并且两个测量值的组合排除了在边缘化未知遮蔽部分后可信度为 99.8% 的典型 10% 效率。这种低辐射效率意味着最早的超大黑洞质量会快速吸积，极大地缓解了宇宙年龄和超大黑洞质量之间的紧张关系。然而，我们测量的效率可能反映了类星体宿主星系在超大质量黑洞生长的大部分过程中几乎完全被尘埃气体遮蔽。假设未遮蔽阶段的效率为 10%，我们发现在 95% 可信度下，被遮蔽的分数将 >82%，这意味着 z > 7 时遮蔽的发光类星体数量比未遮蔽的发光类星体数量大一倍。
The supermassive black holes (SMBHs) observed at the centers of all massive galaxies are believed to have grown via luminous accretion during quasar phases in the distant past. The fraction of inflowing rest mass energy emitted as light, the radiative efficiency, has been inferred to be 10%, in agreement with expectations from thin disk accretion models. But the existence of billion solar-mass SMBHs powering quasars at z > 7 challenges this picture: provided they respect the Eddington limit, there is not enough time to grow z > 7 SMBHs from stellar remnant seeds unless the radiative efficiency is below 10%. Here we show that one can constrain the radiative efficiencies of the most distant quasars known using foreground neutral intergalactic gas as a cosmological-scale ionizing photon counter. From the Lyα absorption profiles of ULAS J1120+0641 (z = 7.09) and ULAS J1342+0928 (z = 7.54), we determine posterior median radiative efficiencies of 0.08% and 0.1%, respectively, and the combination of the two measurements rules out the canonical 10% efficiency at 99.8% credibility after marginalizing over the unknown obscured fraction. This low radiative efficiency implies rapid mass accretion for the earliest SMBHs, greatly easing the tension between the age of the universe and the SMBH masses. However, our measured efficiency may instead reflect nearly complete obscuration by dusty gas in the quasar host galaxies over the vast majority of their SMBH growth. Assuming 10% efficiency during unobscured phases, we find that the obscured fraction would be >82% at 95% credibility, and imply a times larger obscured than unobscured luminous quasar population at z > 7.