The Lifetime of Grand Design

The lifetime of the structure in grand design spiral galaxies is observationally ill-determined, but is essentially set by how accurately the pattern's rotation can be characterized by a single angular pattern speed. This paper derives a generalized version of the Tremaine-Weinberg method for observationally determining pattern speeds, in which the pattern speed is allowed to vary arbitrarily with radius. The departures of the derived pattern speed from a constant then provides a simple metric of the lifetime of the spiral structure. Application of this method to CO observations of NGC 1068 reveal that the pattern speed of the spiral structure in this galaxy varies rapidly with radius, and that the lifetime of the spiral structure is correspondingly very short. If this result turns out to be common in grand-design spiral galaxies, then these features will have to be viewed as highly transient phenomena.