RADIAL BRIGHTNESS PROFILES OF EXTRAGALACTIC H-II REGIONS

We present a systematic study of the radial brightness profiles in H a lpha of a set of HII regions in 6 late-type spiral galaxies, complete down to stated limiting luminosities. We have sufficient linear resolu tion in our observations to be able to fit models in which the effects of the radial fall off in density and in mean degree of ionization ca n be distinguished. We find a sharp break in the plot of internal radi al brightness gradient against luminosity at a luminosity in H alpha o f 10(38.6) erg/s in all these objects. For lower luminosities the grad ients are almost invariant, showing only a slight tendency to steepen with luminosity; at this critical value there is a jump to higher grad ients, and at higher luminosities the gradients steepen progressively. This is consistent with the regions showing a transition from ionizat ion-bounded to density-bounded behaviour, in conformity with evidence from HII region luminosity functions presented in previous studies.