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

In this paper we present evidence that the H ii regions of high luminosity in disk galaxies may be density bounded, so that a significant fraction of the ionizing photons emitted by their exciting OB stars escape from the regions. The key piece of evidence is the presence, in the Hα luminosity functions (LFs) of the populations of H ii regions, of glitches, local sharp peaks at an apparently invariant luminosity, defined as the Stromgren luminosity (LStr), LHα = LStr = 10 38.6 (± 10) erg s (no other peaks are found in any of the LFs) accompanying a steepening of slope for LHα> LStr. This behavior is readily explicable via a physical model whose basic premises are: (a) the transition at LHα = LStr marks a change from essentially ionization bounding at low luminosities to density bounding at higher values, (b) for this to 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 produced within the cloud is a function which 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 regions for LHα < LStr are proportional to L 1/3 Hα , expected for ionization bounding, but show a sharp trend to a steeper dependence for LHα > LStr, and the observed relation between the internal turbulence velocity parameter, sigma, and the luminosity, L, at high luminosities, can be well explained if these regions are density bounded. If confirmed, the density-bounding hypothesis would have a number of interesting implications. It would imply that the density-bounded regions were the main sources of the photons which ionize the diffuse gas in disk galaxies. Our estimates, based on the hypothesis, indicate that these regions emit sufficient Lyman continuum not only Instituto de Astrof́ısica de Canarias, E-38200 La Laguna, Tenerife, Spain Consejo Superior de Investigaciones Cient́ıficas, CSIC, Spain University of Hertfordshire, Department of Physical Sciences, Hatfield, Herts AL10 9AB, UK. On leave at Isaac Newton Group of Telescopes, E-38700 Santa Cruz de La Palma, Spain