Disturbed Universe

NGC4622(type Sb) the 'beads on a string' appearance of the optical spiral arms, and the strong dust lanes can be readily seen on the inside edges of the optical arms. a sharp peak near 2/, with a gradual tapering out to r, interpreted to indicate that the spiral structure of our Galaxy is mostly confined to radii between somewhat inside r/2 and somewhat beyond r. Sprial structure may well extend far beyond the solar circle, but it's evidently relatively weak in the production of the above tracers of spiral structure. The nature of the spiral arms

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Introduction

Many different kinds of spiral structure are seen in disk galaxies. Most photogenic are the grand-design two-armed spiral galaxies such as M51, but far more common are ragged or flocculent spirals made up of many short arms. The diversity of spiral galaxies is paralleled by the diversity of theories of spiral structure. Grand-design spirals are often discussed in terms of the Lin-Shu theory discussed here (after Chia-Chiao Lin and Frank H. Shu), which views the spirals as slowly turning wave patterns maintaining their form for many rotation periods. However, classic grand-design spirals like M51 often have close companions, and it is possible that such spirals are actually excited by tidal interactions. Flocculent spirals, on the other hand, are generally thought to evolve over time, with individual spiral arms constantly forming and dissolving.

It seems clear now that the spiral structure of galaxies is a complex problem without any unique and tidy answer. Differential rotation clearly plays a central role. However, we know that the logical paths diverge soon and deservedly toward such separate themes as global instabilities, stochastic spirals, and also the shocks patterns that can arise in shearing gas disks when forced by bars.

Figure: In inner regions of the spiral galaxy M51 observed by HST. Several components of the spiral structure are clearly delineated: massive, hot, young stars in HII regions; narrow dust lanes; and the underlying, smoother old stellar population. \begin{figure}\epsfig{figure=m51HST.ps,width=2.0in,angle=0}\end{figure}

We approach the study of spiral galaxies by considering the dynamics of a thin, flat, rotating sheet of self-gravitating gas. Although disk galaxies contains interstellar gas, they is composed primarily of stars. Consequently, it would be more correct to treat the disk as stellar dynamics problem, and to study the Boltzmann equation for the stellar distribution function. A fluid model simplifies the analysis, and can be partially justified. A continnum description is valid if we are interested only in phenomena with length scales large enough that relevant regions of our fluid will contain large numbers of stars. However, we will also assume that our stellar fluid exerts pressure. This assumption is suspicious because the mean free path for a star is large compared to the dimensions of the system. A self-gravitating pressureless gas is unstable and so has limited usefulness as a model for a galaxy. A disk with pressure can overcome these instabilities in much the same way that a stellar system with random velocity components can, so that the acoustic speed a of the gaseous disk should be regarded as mimicking such a random stellar velocity.

The theoretical explanation of spiral structure in disks has been an active field since Lin & Shu's (1964) seminal paper, which introduced the fluid model. Although stellar dynamical models have subsequently been studied, fluid dynamical models have continued to prove useful. The problem of spiral structure has yet to be fully resolved. Fluid models are relatively simple, they are still relevant and, moreover, they are still not fully understood.