Dynamical friction arises from the interaction of a perturber and the gravitational wake it excites in the ambient medium. This interaction is usually derived assuming that the perturber has a constant velocity. In realistic situations, motion is accelerated as for instance by dynamical friction itself. Here, we study the effect of acceleration on the dynamical friction force. We characterize the density enhancement associated with a constantly accelerating perturber with rectilinear motion in an infinite homogeneous gaseous medium and show that dynamical friction is not a local force and that its amplitude may depend on the perturber's initial velocity. The force on an accelerating perturber is maximal between Mach 1 and Mach 2, where it is smaller than the corresponding uniform motion friction. In the limit where the perturber's size is much smaller than the distance needed to change the Mach number by unity through acceleration, a subsonic perturber feels a force similar to uniform motion friction only if its past history does not include supersonic episodes. Once an accelerating perturber reaches large supersonic speeds, accelerated motion friction is marginally stronger than uniform motion friction. The force on a decelerating supersonic perturber is weaker than uniform motion friction as the velocity decreases to a few times the sound speed. Dynamical friction on a decelerating subsonic perturber with an initial Mach number larger than 2 is much larger than uniform motion friction and tends to a finite value as the velocity vanishes in contrast to uniform motion friction.
Growth of Supermassive Black Hole Seeds in ETG Star-Forming Progenitors: Multiple Merging of Stellar Compact Remnants via Gaseous Dynamical Friction and Gravitational Wave Emission
Accelerated orbital decay of supermassive black hole binaries in merging nuclear star clusters
超大质量黑洞（SMBH）的合并应该会产生宇宙中最强的引力波（GW）源。然而，它们合并的动力学是一个备受争议的话题。在这项研究中，我们使用了一套$N$体模拟遵循合并的两个核星团（NSC），每个托管SMBH在其中心。我们发现，每个SMBH周围的不同的星团的存在下有重要的后果的SMBH二进制的动力学演化：（i）之间的分离SMBH减少了几个数量级的前几个百万年的综合效应的动力摩擦和潮汐剥离的恒星所造成的阻力。事实上，这对于等质量比的双星来说是一个显著的加速，而对于不相等的质量比，例如1：10或1：100，这变得极端，传统的动力学摩擦力本身不允许绑定。(ii)随后的双星硬化是由SMBH双星和恒星之间的引力弹弓驱动的，也取决于SMBH之间的质量比。因此，有了这个额外的阻力，我们发现我们套件中的所有SMBH在哈勃时间内合并。考虑到大约50%的银河系大小的星系主机NSC，我们的结果是令人鼓舞的，即将到来的GW观测与激光干涉仪空间天线- LISA -这将检测SMBH聚结在$10^4-10^7$ $M_{\rm \odot}$质量范围。
From Inspiral to Expansion: The Wake-Driven Torque on Binary Black Holes in Gaseous Medium
Tsinghua University（清华大学）；The Kavli Institute for Astronomy and Astrophysics, Peking University（北京大学科维理天文与天体物理研究所）；Department of Astronomy, School of Physics, Peking University（北京大学物理学院天文学系）；University of California Berkeley（加州大学伯克利分校）
Pairing of Massive Black Holes in Merger Galaxies Driven by Dynamical Friction
出于对大质量黑洞（MBH）对在千秒差距的观测搜索，我们开发了一个半解析模型来描述它们的轨道演化的影响下，恒星和气体动力学摩擦（DF）。这项研究的目的是确定合并残余星系和MBH的性质如何影响形成距离< 1 pc的紧密MBH对的可能性和时间尺度。我们计算了大约40,000种配置，涵盖了广泛的宿主星系的属性，并研究了它们对不等质量MBH对轨道演化的影响。我们发现，在气体质量分数为10^6太阳质量和质量比> 1/4的残余星系中，在哈勃时间内MBH配对的百分比大于80%< 20% and in galaxies hosting MBH pairs with total mass >。在这些星系中，以最快形成紧密的引力束缚MBH为特征的残余星系具有以下一个或多个性质：（1）大的恒星隆起，（2）质量相当的MBH和（3）星系气体盘以接近圆周速度旋转。在这样的星系中，具有最短螺旋时间的MBH，可能是合并MBH的祖先，要么在圆形的逆行轨道上，要么在非常偏心的逆行轨道上。我们的模型还表明，具有相反性质的残余星系，主机缓慢演变的MBH对，是最有可能的主机双活动星系核在千秒差距。
