PATTERN SPEEDS OF SPIRAL GALAXIES USING THE TREMAINE-WEINBERG METHOD

Abstract

Long-lasting spiral structure in galaxies is established when density waves form a pattern that rotates at a constant angular frequency Ω_p, the pattern speed. This quantity, critical to understanding spiral structure, galaxy evolution, and the connection, if any, between bars and spirals in galaxies, is not directly measurable. Most determinations of pattern speeds have relied on identifying morphological features at certain radii as resonances, but this depends significantly on knowledge of the behavior of stars and gas at these resonances. The Tremaine-Weinberg (TW) method allows for the determination of pattern speeds based on kinematic observations in a way completely independent of resonance identification, or theories of spiral structure, relying on the identi-fication of a tracer that obeys the continuity equation as it orbits through the pattern.