How bar strength and pattern speed affect galactic spiral structure

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Abstract
The response of a gaseous disc to rotating stellar bars of various strengths and pattern speeds is studied. The arm-to-interarm density contrast and the pitch angle of the induced spiral arms both increase with increasing bar strength. Furthermore, along the sequence there is a widening of a region in which stable periodic orbits are difficult to find until it encompasses the whole annulus between corotation (CR) and the outer Lindblad resonance (OLR). Stars moving in quasi-stochastic orbits originating in this region form an exponential tail in the surface density profile of the stellar disc. As the pattern speed is lowered more structure appears inside CR. For example, a ring forms just inside CR and it is suggested that this could be the origin of the inner rings (r) found near the ends of bars of SB systems. In models with pattern speed so low that an inner Lindblad resonance (ILR) exists, spiral arms form there, and as a result gas is moved inward toward the centre.

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