大设计星系M51的模拟：分析潮汐诱导螺旋结构的个例研究
Simulations of the grand design galaxy M51: a case study for analysing tidally induced spiral structure
摘要：
我们提出了大设计螺旋M51（NGC 5194）的流体动力学模型，以及它与伴星NGC 5195的相互作用。尽管我们的模型很简单，但我们的模拟非常好地捕捉到了M51的螺旋结构，甚至再现了诸如沿着一个螺旋臂的扭结和螺旋臂分叉等细节。我们研究了恒星和气体旋臂之间的偏移，发现在大多数时候（包括今天），恒星和气体之间没有偏移到我们的误差范围内。我们还比较了我们的模拟与最近的观测分析M51。我们计算模式速度与半径的关系，就像观察一样，没有找到单一的全局模式速度。我们还表明，螺旋臂不能很好地拟合对数螺线。我们解释这些发现作为证据，M51并没有表现出准稳定的密度波，将由密度波理论预测。M51的内部结构来自于它与伴星复杂的动力相互作用，导致螺旋臂以短暂的扭结和分叉的形式显示出相当大的结构。与其试图按照具有固定模式速度的全球螺旋模式来模拟这种星系，不如从这样一幅图景开始，即螺旋臂虽然不是简单的物质臂，但却是潮汐引起的运动密度“波”或密度模式的结果，它们随着时间的推移缓慢地结束。
英文摘要：
We present hydrodynamical models of the grand design spiral M51 (NGC 5194), and its interaction with its companion NGC 5195. Despite the simplicity of our models, our simulations capture the present day spiral structure of M51 remarkably well, and even reproduce details such as a kink along one spiral arm, and spiral arm bifurcations. We investigate the offset between the stellar and gaseous spiral arms, and find at most times (including the present day) there is no offset between the stars and gas to within our error bars. We also compare our simulations with recent observational analysis of M51. We compute the pattern speed versus radius, and like the observations, find no single global pattern speed. We also show that the spiral arms cannot be fitted well by logarithmic spirals. We interpret these findings as evidence that M51 does not exhibit a quasi-steady density wave, as would be predicted by density wave theory. The internal structure of M51 derives from the complicated and dynamical interaction with its companion, resulting in spiral arms showing considerable structure in the form of short-lived kinks and bifurcations. Rather than trying to model such galaxies in terms of global spiral modes with fixed pattern speeds, it is more realistic to start from a picture in which the spiral arms, while not being simple material arms, are the result of tidally induced kinematic density `waves' or density patterns, which wind up slowly over time.
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