Dynamics in galaxy centers

天文学家现在认为，大多数星系的中心都有极大的黑洞，黑洞的质量是太阳的数百万到数十亿倍。这些黑洞只能通过它们对恒星、尘埃和气体等邻居的影响才能被探测到。研究这种影响，特别是解释天文观测的唯一方法是通过复杂的计算机模拟。该项目将开发几种新的方法来理解黑洞附近恒星的行为，强调可观察到的后果。许多详细的工作将由初级科学家进行，因此他们将接受广泛的理论天体物理学高级培训。重要的粒子效应包括来自太靠近黑洞的恒星的耀斑，来自围绕黑洞的气体吸积盘提供的活跃星系核的耀斑，以及来自黑洞引力对附近恒星运动学的影响。如上所述，这项研究将开发新的数值工具来研究黑洞附近恒星系统的动力学，此外还将研究吸积盘中翘曲的演化，并在类星体的光谱中寻找相对论效应。具体地说，这项工作将：(I)探索用于引力N体问题的新的轨道积分器，可能允许更大的时间步长；(Ii)研究翘曲吸积盘的行为，其中自重、粘性应力和透镜-Thirring效应都是确定盘形状的重要因素；(Iii)开发“N线”程序(通过将每颗恒星表示为偏心线来平均轨道相位)，并用它来研究黑洞附近恒星系统和M31中心偏心恒星盘的松弛和稳定性；(4)实施“N环”代码(将每一颗恒星表示为一个环形圆盘)，并用它来模拟银河系中心附近的恒星；(5)在大样本类星体中寻找宽窄发射线之间速度偏差的相对论效应。该项目将有助于对理论天体物理学方面的研究生和博士后研究员进行高级培训，并将有助于解释现有和计划中的旗舰望远镜项目的结果。

Astronomers now believe that the centers of most galaxies harbor extremely large black holes, containing millions to billions of times the mass of our Sun. These black holes can be detected only through their influence on their neighbors, including stars, dust, and gas. The only way to study this influence, and especially to interpret the astronomical observations, is through complex computer simulations. This project will develop several new approaches to understanding the behavior of stars near black holes, emphasizing the observable consequences. Much of the detailed work will be carried out by junior scientists who will thus undergo extensive advanced training in theoretical astrophysics.Particular effects of importance include flares from stars that wander too close to the black hole, from active galactic nuclei fed by gaseous accretion disks orbiting the black hole, and from the influence of the black hole's gravity on the kinematics of nearby stars. As noted, this research will develop novel numerical tools to study the dynamics of stellar systems near black holes, and in addition will study the evolution of warps in accretion disks and look for relativistic effects in the spectra of quasars. Specifically, this work will: (i) explore novel orbit integrators for the gravitational N-body problem, possibly allowing much larger time-steps; (ii) investigate the behavior of warped accretion disks, where self-gravity, viscous stresses, and the Lense-Thirring effect, are all important in determining the disk shape; (iii) develop an "N-wire" code (which averages over orbital phase by representing each star as an eccentric wire) and use it to study relaxation and stability of stellar systems near a black hole and the eccentric stellar disk at the center of M31; (iv) implement an "N-ring" code (which represents each star as an annular disk) and use it to model stars near the Galactic center; and (v) look for relativistic effects in the velocity offsets between broad and narrow emission lines in a large sample of quasars. The project will contribute to the advanced training of graduate students and postdoctoral fellows in theoretical astrophysics, and will help to interpret results from existing and planned flagship telescope projects.