N-body model for M51 — I. Multiple encounter versus single passage?
N-body model for M51 — I. Multiple encounter versus single passage?
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DOI：
10.1046/j.1365-8711.2000.03650.x
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发表时间：
2000-12
影响因子：
4.8
通讯作者：
H. Salo;E. Laurikainen
中科院分区：
文献类型：
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作者：
H. Salo;E. Laurikainen
对相互作用的大设计系统M51 （NGC 5194/5195）的相遇历史进行了数值调查，采用三维多球面极坐标编码，其中两个分量都用嵌入刚性解析光晕势的自重力盘来描述。研究了两类模型，(1)近抛物线单通道，(2)界接触暗示了几个盘面交叉。这两种模型都可以近似M51系统的一般形态，并同时拟合预测的速度差和组件的分离。在这两种情况下，导致主螺旋结构的伴星盘面交叉几乎发生在南方，大约400-500千兆尔以前，距离25-30千兆尔，但方向相反。在束缚相遇模型中，也有一个更近的交叉，距离在20-25 kpc之间，大约在50-100 Myr之前。我们的模型解释了M51系统的一些重要的运动学观测，这些观测没有被以前的模型所解释。特别是，我们注意到最近经过的多次遭遇模型产生了显著的面外速度，表现为长轴旋转曲线的s形结构，这也解释了伴星北部的高奇特速度。在这个模型中，由此产生的延伸尾部相对于内部圆盘倾斜40°-50°，导致速度场似乎表明尾部相对于内部圆盘反向旋转。此外，一些形态特征，比如伴星潮汐延伸的方向，与最近一次相遇的模型更吻合。重要的是，任何先前存在的旋臂都会被潮汐触发的旋臂冲刷掉。多重碰撞模型假设轨道倾角高，电流偏率低。通过钱德拉塞卡动力摩擦公式的轨道衰减模拟和包括自洽模拟的原星活动晕的模拟，研究了这种束缚轨道构型的可能起源。即使考虑到几个早期通道的影响，模型的总体特征，包括倾斜的远尾，也被保留了下来。观察到的定义明确的远尾似乎表明，前两次穿越的距离至少比最近两次穿越的距离要远30%。根据我们有限的轨道调查，这样的轨道衰减是可以解释的，只要M51有一个延伸的暗晕，其质量至少是可见盘区质量的几倍（总质量）
A numerical survey of the encounter history of the interacting grand-design system M51 (NGC 5194/5195) is performed with a 3D multiple spherical polar grid code, where both components of the pair are described with self-gravitating discs embedded in rigid analytical halo potentials. Two classes of models are investigated, (1) nearly parabolic single passages, and (2) bound encounters implying several disc-plane crossings. Both types of models can approximate the general morphology of the M51 system and simultaneously fit the projected velocity difference and separation of the components. In both cases the companion disc-plane crossing responsible for the main spiral structure occurred nearly in the south, about 400–500 Myr ago at a distance of 25–30 kpc, but in opposite directions. In the bound encounter model there is also a more recent crossing, at a distance of 20–25 kpc about 50–100 Myr ago. Our models account for some important kinematical observations of the M51 system not explained by the previous models. Especially, we note that the multiple-encounter model with a recent passage produces significant out-of-plane velocities, which manifest as an S-shaped structure of the major axis rotation curve, and which also explain the high peculiar velocities in the north of the companion. In this model the resulting extended tail is tilted 40°–50° with respect to the inner disc, leading to a velocity field that appears to suggest counter-rotation of the tail with respect to the inner disc. Also some morphological features, like the direction of the tidal extensions from the companion, are better matched by a model with a recent encounter. Importantly, any pre-existing spiral arms are washed out by the tidally triggered spiral arms. The multiple-encounter model assumes a high inclination orbit, with a low current eccentricity The possible origin of this type of bound orbital configuration is studied by simulations including the orbital decay via the Chandrasekhar formula for dynamical friction, and also by simulations including a self-consistently modelled live halo for the primary. The gross features of the model, including the tilted far tail, are preserved even when allowing for the effects of several earlier passages. The observed well-defined far tail seems to suggest that the previous passages have been at least 30 per cent more distant than the latest two crossings. According to our limited orbital survey, such an orbital decay can be accounted for, provided that M51 has an extended dark halo containing at least a few times the mass within the visible disc region (total