Spirals in galaxies have long been thought to be caused by gravitational instability in the stellar component of the disk, but discerning the precise mechanism had proved elusive. Tidal interactions, and perhaps bars, may provoke some spiral responses, but spirals in many galaxies must be self-excited. We survey the relevant observational data and aspects of disk dynamical theory. The origin of the recurring spiral patterns in simulations of isolated disk galaxies has recently become clear, and it is likely that the mechanism is the same in real galaxies, although evidence to confirm this supposition is hard to obtain. As transient spiral activity increases random motion, the patterns must fade over time unless the disk also contains a dissipative gas component. Continuing spiral activity alters the structure of the disks in other ways: reducing metallicity gradients and flattening rotation curves are two of the most significant. The overwhelming majority of spirals in galaxies have two- or three-fold rotational symmetry, indicating that the cool, thin disk component is massive. Spirals in simulations of halo-dominated disks instead manifest many arms and, consequently, do not capture the expected full spiral-driven evolution. We conclude by identifying areas where further work is needed.

中文翻译：

---

 星系中的螺旋


长期以来，星系中的螺旋一直被认为是由盘中恒星成分的引力不稳定性引起的，但事实证明，辨别精确的机制是难以捉摸的。潮汐相互作用，也许还有棒，可能会引起一些螺旋反应，但许多星系中的螺旋必须是自激的。我们调查了相关的观测数据和盘动力学理论的各个方面。孤立盘状星系模拟中重复出现的螺旋图案的起源最近已经变得清晰，并且该机制很可能在真实星系中是相同的，尽管很难获得证实这一假设的证据。随着瞬态螺旋活动增加随机运动，图案必须随着时间的推移而消失，除非圆盘还包含耗散气体成分。持续的螺旋活动以其他方式改变了圆盘的结构：减少金属丰度梯度和展平旋转曲线是最重要的两个。星系中的绝大多数螺旋具有二重或三重旋转对称性，这表明冷而薄的盘成分是巨大的。相反，在光环主导的盘的模拟中，螺旋呈现出许多旋臂，因此无法捕捉到预期的完全螺旋驱动的演化。最后，我们确定了需要进一步工作的领域。