Astron. Astrophys. 324, 51-64 (1997)

Molecular absorption and its time variations in Centaurus A

T. Wiklind ¹ and F. Combes <sup>2

1</sup> Onsala Space Observatory, S-43992 Onsala, Sweden
² DEMIRM, Observatoire de Paris, 61 Av. de l'Observatoire, F-75014 Paris, France

Received 26 November 1996 / Accepted 20 January 1997

Abstract

New high quality absorption spectra of J =1 0 , HCN, HNC and the J =2 1 line of CS towards the center of the radio core of Cen A (NGC 5128) are presented. In addition the absorption profile of the J =1 0 line of has been detected for the first time, revealing that most absorption features have low opacities. The new spectrum allows a comparison with results obtained more than 7 years earlier. No significant change in the spectrum is found. From this remarkable result, constraints can be put on the minimum size of the radio source in the millimeter range ( 500 AU). A comparison of abundance ratios between different absorption components suggests that the absorbing gas is essentially low density gas, with both low excitation and kinetic temperatures. Relative molecular abundances are compatible with those of the Milky Way. A parametrization of the high signal-to-noise spectrum of is presented for future reference when looking for spectral changes.

Key words: ISM: molecules galaxies: Cen A; ISM radio continuum: galaxies radio lines: galaxies

Send offprint requests to: T. Wiklind, tommy@oso.chalmers.se

SIMBAD Objects

Contents

• 1. Introduction
• 2. Observations
• 3. Results
  • 3.1. The spectra
  • 3.2. The hyperfine components of HCN
  • 3.3. Parametrization of the (1-0) spectrum
  • 3.4. Variations in the absorption lines?
• 4. Column densities and abundance ratios
  • 4.1. Basic assumptions
  • 4.2. Qualitative results
• 5. Discussion
  • 5.1. Type of molecular clouds
  • 5.2. Location of the absorbing molecular gas
• 6. Summary
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

helpdesk.link@springer.de