Stationary models for fast and slow logarithmic spiral patterns in disc galaxies
Stationary models for fast and slow logarithmic spiral patterns in disc galaxies
复制标题
盘状星系中快速和慢速对数螺旋模式的固定模型
DOI：
10.1046/j.1365-8711.2002.05142.x
复制
发表时间：
2002
影响因子：
4.8
通讯作者：
Z. Fan
中科院分区：
文献类型：
--
作者：
Y. Lou;Z. Fan
最近的小波分析的多波长图像数据的附近的螺旋星系NGC 6946揭示了一个多臂螺旋结构，持续到外部差分旋转盘区域。在偏振射电连续谱发射和红光中，延伸的旋臂似乎彼此交错，而在总射电连续谱、H ii区的Hα和中性氢的发射中，旋臂都跟踪红光旋臂，尽管程度稍低。现在的关键问题是如何维持扩展的慢磁流体动力学（MHD）密度波功能在一个薄的磁化磁盘与平坦的旋转曲线。
在这里，我们描述了一个理论模型来检查固定的非轴对称对数螺旋配置构造从一个磁化的奇异等温盘（MSID）的背景平衡与平坦的旋转曲线和非力自由的方位磁场。解析地发现，可能存在两种类型的静止螺旋MSID配置，物理上对应于快和慢螺旋MHD密度波的两种可能性。这种稳定的MHD密度波只有在适当的MSID转速下才有可能。
对于快速MSID组态，磁场和气体密度的对数螺旋增强要么在紧缠绕区同相，要么在开放螺旋结构中以空间相位差Δ π/2移动。对于慢MSID配置，对数螺旋增强的磁场和气体密度的相位在紧绕制度或移动与空间相位差开放的螺旋结构，并坚持在一个平坦的旋转曲线。对于NGC 6946，几个相关方面的慢MSID方案与固定对数旋臂进行了讨论。这两个精确解也可用来检验有关的数值MHD程序。
A recent wavelet analysis on multiwavelength image data of the nearby spiral galaxy NGC 6946 revealed a multi-arm spiral structure that persists well into the outer differentially rotating disc region. The extended spiral arms in polarized radio-continuum emission and in red light appear interlaced with each other, while the spiral arms in emissions of total radio continuum, of Hα from H ii regions, and of neutral hydrogen all trace the red-light spiral arms, although to a somewhat lesser extent. The key issue now becomes how to sustain extended slow magnetohydrodynamic (MHD) density wave features in a thin magnetized disc with a flat rotation curve.
We describe here a theoretical model to examine stationary non-axisymmetric logarithmic spiral configurations constructed from a background equilibrium of a magnetized singular isothermal disc (MSID) with a flat rotation curve and with a non-force-free azimuthal magnetic field. It is found analytically that two types of stationary spiral MSID configurations may exist, physically corresponding to the two possibilities of fast and slow spiral MHD density waves. Such stationary MHD density waves are possible only at proper MSID rotation speeds.
For the fast MSID configuration, logarithmic spiral enhancements of magnetic field and gas density are either in phase in the tight-winding regime or shifted with a spatial phase difference ≳π/2 for open spiral structures. For the slow MSID configuration, logarithmic spiral enhancements of magnetic field and gas density are either out of phase in the tight-winding regime or shifted with a spatial phase difference for open spiral structures and persist in a flat rotation curve. For NGC 6946, several pertinent aspects of the slow MSID scenario with stationary logarithmic spiral arms are discussed. The two exact solutions can be also utilized to test relevant numerical MHD codes.