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arXiv 1202.3516astro-ph.CO
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超大质量黑洞的宇宙演化。II。自旋演化尺度缩小的证据

Cosmological Evolution of Supermassive Black Holes. II. Evidence for Downsizing of Spin Evolution

Yan-Rong Li, Jian-Min Wang, Luis C. Ho

摘要:

自旋是描述超大质量黑洞的一个重要参数,但其约束性较差。利用SMBH数密度的连续性方程,我们明确地得到了吸积辐射效率的质量依赖的宇宙学演化,它作为SMBH自旋的代理。我们的计算利用了活动星系和非活动星系的SMBH质量函数(在本系列的第一篇论文中导出),活动星系核(AGN)的热光度函数,校正了康普顿厚源的贡献,以及观测到的爱丁顿比分布。我们发现,在高红移(z>~1)处,辐射效率通常随黑洞质量的增加而增加,大致为η \propto M_bh ^[0.5],而在较低的红移处,这种趋势相反,因此质量最低的黑洞获得了最高的效率。M_bh>~10^8.5M_sun的黑洞在高红移时保持高达η~0.3-0.4的辐射效率,接近快速旋转系统的最大值,但它们的效率在z~0时急剧下降(下降一个数量级)。质量较低的黑洞的模式稍微复杂一些,但性质相似。假设标准的吸积盘模型适用,我们建议的吸积历史的SMBHs及其伴随的自旋演变在两个不同的制度:一个早期阶段的长期吸积,可能驱动的主要合并,在此期间的黑洞旋转,然后切换到一个时期的随机,情节吸积,受轻微的合并和内部长期的过程,在此期间,孔自旋下降。过渡时期取决于质量,反映了活动星系核群体中“宇宙缩小”的其他证据;对于高质量黑洞,它发生在z~2处,而对于低质量系统,它发生在z~1处。

英文摘要:

The spin is an important but poorly constrained parameter for describing supermassive black holes (SMBHs). Using the continuity equation of SMBH number density, we explicitly obtain the mass-dependent cosmological evolution of the radiative efficiency for accretion, which serves as a proxy for SMBH spin. Our calculations make use of the SMBH mass function of active and inactive galaxies (derived in the first paper of this series), the bolometric luminosity function of active galactic nuclei (AGNs), corrected for the contribution from Compton-thick sources, and the observed Eddington ratio distribution. We find that the radiative efficiency generally increases with increasing black hole mass at high redshifts (z>~1), roughly as η\propto M_bh^0.5, while the trend reverses at lower redshifts, such that the highest efficiencies are attained by the lowest mass black holes. Black holes with M_bh>~10^8.5M_sun maintain radiative efficiencies as high as η~0.3-0.4 at high redshifts, near the maximum for rapidly spinning systems, but their efficiencies drop dramatically (by an order of magnitude) by z~0. The pattern for lower mass holes is somewhat more complicated but qualitatively similar. Assuming that the standard accretion disk model applies, we suggest that the accretion history of SMBHs and their accompanying spins evolve in two distinct regimes: an early phase of prolonged accretion, plausibly driven by major mergers, during which the black hole spins up, then switching to a period of random, episodic accretion, governed by minor mergers and internal secular processes, during which the hole spins down. The transition epoch depends on mass, mirroring other evidence for "cosmic downsizing" in the AGN population; it occurs at z~2 for high-mass black holes, and somewhat later, at z~1, for lower-mass systems.

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