Populations of high-luminosity density-bounded H II regions in spiral galaxies: Evidence and implications

We present evidence that the H II regions of high luminosity in disk galaxi es may be density bounded, so that a significant fraction of the ionizing p hotons emitted by their exciting OB stars escapes from the regions. The key piece of evidence is the presence of glitches, local sharp peaks at an app arently invariant luminosity, in the H alpha luminosity functions of the po pulations of H II regions. The apparently invariant luminosity is defined a s the Stromgren luminosity (L-Str), such that L-H alpha = L-Str = 10(38.6) +/- 10(0.1) ergs s(-1) (no other peaks are found in any of the luminosity f unctions) accompanying a steepening of slope for L-H alpha > L-Str. This be havior is readily explicable by a physical model in which (1) the transitio n at L-H alpha = L-Str marks a change from essentially ionization bounding at low luminosities to density bounding at higher values and (2) for this t o occur the law relating stellar mass in massive star-forming clouds to the mass of the placental cloud must be such that the ionizing photon flux pro duced within the cloud is a function that rises more steeply than the mass of the cloud. Supporting evidence for the hypothesis of this transition is also presented: measurements of the central surface brightnesses of H II re gions for L-H alpha < L-Str are proportional to L-H alpha(1/3), as expected for ionization bounding but show a sharp trend to a steeper dependence for L-H alpha > L-Str, and the observed relation between the internal turbulen ce velocity parameter, sigma, and the luminosity, L, at high luminosities c an be well explained if these regions are density bounded. If confirmed, th e density-bounding hypothesis would have a number of interesting implicatio ns. It would imply that the density-bounded regions were the main sources o f the photons that ionize the diffuse gas in disk galaxies. Our estimates, based on the hypothesis, indicate that these regions emit sufficient Lyman continuum photons not only to ionize the diffuse medium but to cause a typi cal spiral to emit significant ionizing flux into the intergalactic medium. The low scatter observed in L-str, less than 0.1 mag rms in the still quit e small sample measured to date, is an invitation to widen the database and to calibrate against primary standards to obtain a precise (similar to 10( 5) L-circle dot) widely distributed standard candle.