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Issue A&A Volume 386, Number 3, May II 2002 Page(s) 801 - 815 Section Extragalactic astronomy DOI 10.1051/0004-6361:20020212

A&A 386, 801-815 (2002)
DOI: 10.1051/0004-6361:20020212

The origin of the ionization of the diffuse ionized gas in spirals

II. Modelling the distribution of ionizing radiation in NGC 157

A. Zurita^{1, 2}, J. E. Beckman^{2, 3}, M. Rozas⁴ and S. Ryder<sup>5

1</sup>  Isaac Newton Group of telescopes, La Palma, Spain
    e-mail: azurita@ing.iac.es
²  Instituto de Astrofísica de Canarias, C. Vía Láctea s/n, 38200-La Laguna, Tenerife, Spain
³  Consejo Superior de Investigaciones Científicas (CSIC), Spain
    e-mail: jeb@ll.iac.es
⁴  Observatorio Nacional de S. Pedro Mártir, UNAM, Ensenada, México
    e-mail: maite@astrosen.unam.mx
⁵  Anglo-Australian Observatory, Epping, NSW 1710, Australia
    e-mail: sdr@aaoepp.aao.GOV.AU

(Received 14 August 2001 / Accepted 31 January 2002 )

Abstract
In this paper we make a quantitative study of the hypothesis that the diffuse H emitted from the discs of spiral galaxies owes its origin to the ionizing pho tons escaping from H II regions. The basis of the models is the assumption that a fraction of the Lyman con ti nuum (Lyc) luminosity from the OB stars within each H II region escapes from the region, leaking into the diffuse gas. A basic input element of any such model is a position and luminosity catalogue in H of the H II regions in the galaxy under examination, down to a low limiting luminosity, and we have previously produced a catalogue of this type for NGC 157. An initial family of models can then be ge ne ra ted in which the Lyc escaping from an H II region is parametrized in terms of the observed H luminosity of the region and the escaping fluxes allowed through the di ffu se disc gas. These models can then be refined using a measured map of H I surface density to effect the down-conversion of the Lyc to H . For NGC 157 an H I map was available. Although its mo de ra te angular resolution did limit the accuracy with which we could test our models, the predicted diffuse H surface brightness distributions from our mo dels were compared with the observed distributions showing that, in general terms, the hypothesis of density bounding for the H II regions allows us to predict well the spatial distribution of the diffuse ionized gas. In the model yielding the best fit to the data, the regions of lower luminosity lose a constant fraction of their ionizing flux to their surroundings, while for H II region luminosities above a specific transition va lue the ionizing escape fraction is a rising function of the H luminosity.

Key words: galaxies: general -- galaxies: individual: NGC 157 -- galaxies: ISM -- galaxies: spiral -- ISM: general -- ISM: HII regions

Offprint request: A. Zurita, azurita@ing.iac.es

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