H-ALPHA FABRY-PEROT OBSERVATIONS OF THE DENSITY-WAVE PATTERN IN M51

The Maryland-Caltech wide-field Fabry-Perot camera has been used to image Ha emission in the grand design spiral galaxy M51. The velocity field of the ionized gas has been determined over most of the optical disk at an angular resolution of 2''-8'' and a relative velocity accuracy of 1-2 km s-1 rms. The well-known kinematic perturbations due to density-wave streaming are clearly resolved in both arm and interarm regions. Determination of the radius at which the gas and spiral pattern corotate using reversals in the streaming direction, frequently proposed as the preferred method of locating the corotation resonance, is hampered by the observed complexity of the density-wave response. Nonetheless, new evidence is presented that supports previous determinations using other methods that corotation of the inner spiral is located near a galactocentric radius of 6 kpc, and that the outer optical arms cannot be part of the same spiral pattern as the inner arms.