Astron. Astrophys. 327, 909-920 (1997)

Photoionization of very high excitation gas in the Circinus galaxy and other active galactic nuclei

Luc Binette ^{1, 2}, Andrew S. Wilson ^{3, 4}, Alex Raga ⁵ and Thaisa Storchi-Bergmann <sup>2

1</sup> European Southern Observatory, Casilla 19001, Santiago 19, Chile (e-mail: lbinette@eso.org)
² Instituto de Fisica, UFRGS, Campus do Vale, CxP 15051, 91501-970 Porto Alegre, RS, Brasil
³ Astronomy Department, University of Maryland, College Park, MD 20742, USA
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA
⁵ Instituto de Astronoma, UNAM, Ap. 70264, 04510 México, DF, México

Received 7 January 1997 / Accepted 18 June 1997

Abstract

Recent measurements of the Circinus galaxy with ISO by Moorwood et al. (1996) have confirmed the extremely high excitation of the emission-line spectrum in this active galaxy. Moorwood et al. modelled the spectrum in terms of photoionization of gas by a hard X-ray continuum plus a UV bump near 70 eV. We present new photoionization calculations which differ markedly in geometry and in the adopted spectrum of the ionizing radiation. In our model, all the zones of high excitation coexist within the internal structure of matter-bounded clouds of 1 pc thickness with a total cross section of 165 pc². A strong UV bump is not required to explain the line ratios in this picture. Our calculations consider the effects of radiation pressure exerted by photoelectric absorption which become very important at the high ionization parameters required by the line ratios. We propose that the radiation pressure generates a strong density gradient within the photoionized structure and is responsible for accelerating the matter-bounded gas, explaining the systematic blueshift observed for the coronal lines.

Key words: galaxies: active galaxies: Seyfert line: formation galaxies: ISM galaxies: Circinus galaxy

Send offprint requests to: Andrew Wilson

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Contents

• 1. Introduction
• 2. The model
  • 2.1. The computer code
  • 2.2. Two powerlaws to represent the EUV and the X-ray domaines
  • 2.3. Gas pressure vs radiation pressure
  • 2.4. Matter-bounded vs ionization-bounded clouds
  • 2.5. Strong density gradient as a result of radiation pressure
• 3. Model parameters inferred from Circinus
  • 3.1. A high ionization parameter
  • 3.2. The density
  • 3.3. Solar metallicity and dust-free gas
  • 3.4. Line ratios from the high excitation MB cloud
• 4. Results and comparison with Circinus
  • 4.1. Comparison with coronal line ratios
  • 4.2. `Onion' ring vs cloud geometry
  • 4.3. The iron lines
  • 4.4. The energy distribution: UV bump or no UV bump?
• 5. The intermediate and low excitation lines
  • 5.1. Rarity of clouds with coronal line emission
  • 5.2. The low excitation IB component
    • 5.2.1. The parametrized A -sequence
    • 5.2.2. Comparison with Paper I
• 6. Conclusions
• Acknowledgements
• Appendix
• Appendix A: density gradients from radiation pressure
• A.1. Density behaviour in the hydrostatic case
• A.2. Implications of strong density stratification
• A.3. Implementation of radiation pressure in MAPPINGS IC
• References

© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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