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M.R. Merrifield (University of Nottingham), R.J. Rand, S.E. Meidt (University of New Mexico)
The evolution of spiral arms and other patterns in galaxies is difficult to determine because there is no immediate link between the motion of the pattern and the observable velocity field in the underlying galaxy. In 1984, Tremaine & Weinberg came up with an elegant method for relating the observed velocity field to a global constant pattern speed via the continuity equation. Here, we present a generalization of the technique, which lets the pattern speed vary arbitrarily with radius, thus allowing for the possibility that the pattern may be evolving with time in at least this straightforward manner.
We have made an initial application of this method to the grand design spiral arms in NGC1068. The results indicate that this structure is winding up very rapidly, on a timescale comparable to the rotation period of the galaxy. If this result turns out to hold generally for such structures in spiral galaxies, then they will have to be seen as transient features rather than long-lived steady density waves. Clearly, this conclusion has implications both for the interpretation of the Hubble Sequence, and for the significance of physical processes such as dynamical heating by spiral arms, which depend on the duty cycle of the structure.
A further application to the double bar system in NGC2950 indicates that its inner and outer bars must be rotating in opposite directions, and that the inner bar's shape must be changing significantly as it rotates within the larger bar. Since nested bars have been suggested as a conduit for feeding fuel on to AGN, the inter-relation between the bars in NGC2950 and the lifetimes of such structures also have broader physical implications.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.