NASA Logo

NTRS

NTRS - NASA Technical Reports Server

Back to Results
Populations of High-Luminosity Density-Bounded HII Regions in Spiral Galaxies? Evidence and ImplicationsIn 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 Ha 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), LH(sub alpha) = Lstr = 10(sup 38.6) (+/- 10(sup 0.1)) erg/ s (no other peaks are found in any of the LFs) accompanying a steepening of slope for LH(sub alpha) greater than Lstr This behavior is readily explicable via a physical model whose basic premises are: (a) the transition at LH(sub alpha) = 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 LH(sub alpha) less than Lstr are proportional to L(sup 1/3, sub H(sub alpha)), expected for ionization bounding, but show a sharp trend to a steeper dependence for LH(sub alpha) greater than 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 to ionize the diffuse medium, but to cause a typical spiral to emit significant ionizing flux into the intergalactic medium. The low scatter observed in Lstr, less than 0.1 mag rms in the still quite small sample measured to date, is an invitation to widen the data base, and to calibrate against primary standards, with the aim of obtaining a precise, approx. 10(exp 5) solar luminosity widely distributed standard candle.
Document ID
20000034928
Acquisition Source
Jet Propulsion Laboratory
Document Type
Preprint (Draft being sent to journal)
Authors
Beckman, J. E.
(Instituto de Astrofisica de Canarias Tenerife, Spain)
Rozas, M.
(Instituto de Astrofisica de Canarias Tenerife, Spain)
Zurita, A.
(Instituto de Astrofisica de Canarias Tenerife, Spain)
Watson, R. A.
(Instituto de Astrofisica de Canarias Tenerife, Spain)
Knapen, J. H.
(Hertfordshire Univ. Hatfield, United Kingdom)
Date Acquired
September 8, 2013
Publication Date
January 1, 2000
Subject Category
Astrophysics
Funding Number(s)
CONTRACT_GRANT: PB91-0525
CONTRACT_GRANT: PB94-1107
Distribution Limits
Public
Copyright
Other
Document Inquiry

Available Downloads

There are no available downloads for this record.
No Preview Available