INTERNAL KINEMATICS OF THE MOST LUMINOUS HII-REGIONS IN M100

We have used the TAURUS Fabry-Perot mapping spectrometer on the Willia m Herschel telescope (WHT) to produce a complete kinematic map of the disk of M100 in H alpha. Here we show how the internal velocity disper sion (sigma) of the principal emission components of the brightest reg ions varies with their H alpha luminosity. The plot shows ample scatte r, but an upper envelope in sigma is clearly linear (in the log-log pl ane) with a slope of 26, a result which agrees precisely with an earli er graph by Arsenault et al., who selected instead the regions of high est surface brightness. We show that this result, which differs from t he conventional prediction from the virial theorem, is consistent with virialisation if the HII regions are density bounded, and thus offers evidence in support of the density bounding hypothesis for the most l uminous regions in disk galaxies.