The Pattern Speeds of M51, M83 and NGC 6946 Using CO and the   Tremaine-Weinberg Method
  In spiral galaxies where 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 km/s/kpc, 45 +/- 8 km/s/kpc and 39 +/- 8 km/s/kpc, 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. 
P. Zimmer, R. J. Rand, J. T. McGraw