Astron. Astrophys. 363, 9-28 (2000)

The origin of the ionization of the diffuse interstellar medium in spiral galaxies

I. Photometric measurements

A. Zurita ¹, M. Rozas ¹ and J.E. Beckman <sup>1,2

1</sup> Instituto de Astrofísica de Canarias, C/ Vía Lãctea s/n, 38200 La Laguna, Tenerife, Spain (azurita, mrozas@ll.iac.es)
² Consejo Superior de Investigaciones Científicas (CSIC), Spain (jeb@ll.iac.es)

Received 4 April 2000 / Accepted 6 June 2000

Abstract

We present a complete study of the diffuse ionized gas (DIG) in a sample of six spiral galaxies (NGC 157, NGC 3359, NGC 3631, NGC 6764, NGC 6951, NGC 7479) using very high quality H images which allow analysis down to very low surface brightness (between 0.3 and 2 pc cm^-6). Separation of the diffuse H emission from that of the H  II regions was performed using the most reliable method: subtracting from the integrated H flux of a complete galaxy the contribution from its fully catalogued population of H  II regions. The integrated luminosity of the DIG is considerable (10⁴⁰-10⁴² erg s^-1) and is a high fraction of the total H emission of each galaxy. Lower and upper limits to the DIG emission in H were derived. The lower limits vary from 25% to 50% and the upper limits from 45% to 70%, in our observed sample.

Previous studies of a very small number of objects, showed that there is a spatial correlation between the DIG and the H  II regions in spiral galaxies suggesting that the DIG is photoionized by Lyman continuum photons (Lyc) which leak from H  II regions. Here we go further: we show that the correlation of the DIG is stronger with the most luminous H  II regions and we propose a specific model for the ionization of the DIG: we show that the luminosity, in Lyc photons, leaking from the most luminous H  II regions is enough to ionize the diffuse gas in a model where the H  II regions with luminosities greater than  erg s^-1 are density bounded (Beckman et al. 2000). This model predicts that a fraction of this flux escapes from the galaxy into the surrounding medium.

Key words: galaxies: general galaxies: ISM galaxies: spiral ISM: H ii regions

Send offprint requests to: A. Zurita

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Contents

• 1. Introduction
• 2. The data
  • 2.1. The sample of galaxies
  • 2.2. The observations and data reduction
  • 2.3. Producing the H  II region catalogues
• 3. Integrated H luminosities of the galaxies and their DIG
  • 3.1. Total H luminosity
  • 3.2. The H luminosity of the DIG
    • 3.2.1. Error estimates
• 4. Distribution of the diffuse ionized gas
• 5. Sources of ionization: density bounded H II regions
  • 5.1. Estimating the escaping ionizing photon flux
• 6. Conclusions: global parameters of the DIG emission and comparison with schematic model predictions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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