Rotation and Radial Motions for M101, M33, M51 and M81
van Maanen, A., Internal Motion in Four Spiral Nebulae. PASP, 33, 200 (1921) -
NADS
See Messier 33 Internal Motions for photos and analysis
of van Maanen's study on M33 along with selected results from recent VLA motion studies
for the same object.