标题： 类星体的宇宙演化 显示英文标题

标题： Cosmological Evolution of Quasars

摘要： 我们提出了一种宇宙学演化模型来描述类星体（QSO）的演化，假设它们是由吸积黑洞提供能量的。 当围绕大质量黑洞的吸积流的吸积率 ${\dot M}/{\dot M}_{Edd}$ 低于临界值 $\sim 0.3\alpha^2\sim 10^{-2}$（其中 ${\dot M}$是吸积率，${\dot M}_{Edd}\propto M$是具有典型10%效率的爱丁顿率，$\alpha (\le 1)$是无量纲粘滞参数）时，吸积流会从高辐射效率（$\sim 10%$）过渡到低效率、辐射主导的吸积流。 我们将这一过渡与在红移 $\sim 2$ 处观测到的类星体X射线光度演化的断点联系起来。 黑洞通过吸积增长可能自然导致在临界红移$z_c\sim 1-3$处发生这种转变，前提是大多数高红移的QSO在$z\sim 3-4$时表现出接近爱丁顿亮度，并且吸积率在整个哈勃时间里以大致同步的方式下降。 在转变之前，类星体的光度（具有高效率）缓慢下降，在转变之后 $z_c$ 处，类星体光度大约按 $\propto (1+z)^{K(z)}$ 演化，其中 $K(z)$ 在 $K\sim3$ 附近逐渐从 $z=z_c$ 变化到 $z\sim 0$。 结果取决于类星体X射线发射机制的细节。 我们讨论了一些进一步的影响。 显示英文摘要

摘要： We present a model for the cosmological evolution of quasars (QSOs) under the assumption that they are powered by massive accreting black holes. Accretion flows around massive black holes make a transition from high radiative efficiency ($\sim 10%$) to low efficiency, advection-dominated flows when ${\dot M}/{\dot M}_{Edd}$ falls below the critical rate $\sim 0.3\alpha^2\sim 10^{-2}$ where ${\dot M}$ is the mass accretion rate, ${\dot M}_{Edd}\propto M$ is the usual Eddington rate with the nominal 10% efficiency, and $\alpha (\le 1)$ is the dimensionless viscosity parameter. We identify this transition with the observed break at a redshift $\sim 2$ in the QSOs' X-ray luminosity evolution. Growth of black holes through accretion could naturally lead to such a transition at a critical redshift $z_c\sim 1-3$, provided that most of high redshift QSOs appear with near Eddington luminosities at $z\sim 3-4$ and the accretion rates decline over the Hubble time in a roughly synchronous manner. Before the transition, the QSOs' luminosities (with a high efficiency) slowly decrease and after the transition at $z_c$, the QSO luminosities evolve approximately as $\propto (1+z)^{K(z)}$ where $K(z)$ gradually varies from $z=z_c$ to $z\sim 0$ around $K\sim3$. The results depend on the details of the QSO X-ray emission mechanism. We discuss some further implications.