The internal kinematics of complete sets of HII regions in disc galaxies

We analyse spectral images across the full emission line in H-alpha of the discs of M100 and M101, extracting for each HII region its spectrum, and fo r the more luminous and extended, spectra over a range of circular aperture s. The principal components of these spectra are presented in log-log plots of luminosity vs. turbulent line-width. These plots are apparently scatter diagrams, with no convincing linear fits, e.g. with slope similar to 4 as predicted for a population of virialized systems. Both plots do show, howev er, clean linear lower envelopes in turbulent width. We interpret these as the loci of HII regions in virial equilibrium. The emissions with greater w idths (the great majority), show the effects of not yet fully damped kinema tic inputs from stellar winds and supernovae. As expected, a greater fracti on of the most luminous regions, which are the most massive, lie on or near the virial envelopes. The slopes of these envelopes are in fact 2.6, not 4 , which is explicable if the most luminous regions are density bounded, so that a fraction of their ionizing luminosity, (increasing with luminosity a bove a critical value) is not registered in H-alpha. The regions on the env elopes are also those of greatest surface brightness, as predicted for dens ity bounded regions. The masses of the regions on the envelopes, computed v ia the virial expression, agree well with masses estimated via their emissi on measures and assuming density bounding.