达尔文驯服仙女座矮星系：用遗传算法解开NGC 205的轨道
Darwin Tames an Andromeda Dwarf: Unraveling the Orbit of NGC 205 Using a Genetic Algorithm
摘要：
NGC 205是M31星系的一颗近距离卫星，是距离我们最近的矮椭圆星系。光度和运动学观测表明，NGC 205正经历着与M31相互作用的潮汐畸变。尽管有早期的尝试，NGC 205的轨道和祖先属性并不为人所知。我们进行了优化搜索，这些未知数相结合的遗传算法与限制N体模拟的相互作用。这种方法，再加上光度和运动学观测作为约束条件，允许参数空间的有效探索。我们代表NGC 205作为一个静态的赫恩奎斯特潜在嵌入无质量的测试粒子，作为示踪剂的表面亮度。我们探索3个不同的，最初稳定的测试粒子配置：冷旋转盘，温暖的旋转盘，和热，压力支持的球体。每个模型都再现了NGC 205的部分但非全部观测特征，这使我们推测具有大量压力支持的旋转祖星可以与所有观测值相匹配。此外，似是而非的组合的质量和规模的长度的压力支持的球体祖模型再现所观察到的速度分散分布。对于所有3个模型，与观测值最匹配的轨道将卫星置于M31之后11+/-9 kpc，以非常大的速度移动：300-500 km/s的主要径向轨道。鉴于观测到的径向分量只有54 km/s，这意味着NGC 205有一个很大的切向运动，从西北向东南移动。这些结果表明，NGC 205与NGC 205东北方向观测到的恒星弧无关。此外，NGC 205的速度似乎接近或大于它的逃逸速度，这意味着它可能是第一次通过M31。
英文摘要：
NGC 205, a close satellite of the M31 galaxy, is our nearest example of a dwarf elliptical galaxy. Photometric and kinematic observations suggest that NGC 205 is undergoing tidal distortion from its interaction with M31. Despite earlier attempts, the orbit and progenitor properties of NGC 205 are not well known. We perform an optimized search for these unknowns by combining a genetic algorithm with restricted N-body simulations of the interaction. This approach, coupled with photometric and kinematic observations as constraints, allows for an effective exploration of the parameter space. We represent NGC 205 as a static Hernquist potential with embedded massless test particles that serve as tracers of surface brightness. We explore 3 distinct, initially stable configurations of test particles: cold rotating disk, warm rotating disk, and hot, pressure-supported spheroid. Each model reproduces some, but not all, of the observed features of NGC 205, leading us to speculate that a rotating progenitor with substantial pressure support could match all of the observables. Furthermore, plausible combinations of mass and scale length for the pressure-supported spheroid progenitor model reproduce the observed velocity dispersion profile. For all 3 models, orbits that best match the observables place the satellite 11+/-9 kpc behind M31 moving at very large velocities: 300-500 km/s on primarily radial orbits. Given that the observed radial component is only 54 km/s, this implies a large tangential motion for NGC 205, moving from the NW to the SE. These results suggest NGC 205 is not associated with the stellar arc observed to the NE of NGC 205. Furthermore, NGC 205's velocity appears to be near or greater than its escape velocity, signifying that the satellite is likely on its first M31 passage.
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