The Pattern Speeds of M51, M83, and NGC 6946 Using CO and the Tremaine-Weinberg Method

In spiral galaxies in which the molecular phase dominates the ISM, the molecular gas as traced by CO emission will approximately obey the continuity equation on orbital timescales. The Tremaine-Weinberg method can then be used to determine the pattern speed of such galaxies. We have applied the method to single-dish CO maps of three nearby spirals, M51, M83, and NGC 6946, to obtain estimates of their pattern speeds: 38 ± 7, 45 ± 8, and 39 ± 8 km s-1 kpc-1, respectively, and we compare these results to previous measurements. We also analyze the major sources of systematic errors in applying the Tremaine-Weinberg method to maps of CO emission.