Astron. Astrophys. 345, L17-L21 (1999)

Letter to the Editor

On the origin of [O IV ] emission in Wolf-Rayet galaxies

Daniel Schaerer ¹ and Grayna Stasiska <sup>2

1</sup> Laboratoire d'Astrophysique, Observatoire Midi-Pyrénées, 14, Av. E. Belin, F-31400 Toulouse, France (schaerer@obs-mip.fr)
² DAEC, Observatoire de Meudon, Meudon, France (grazyna@obspm.fr)

Received 9 March 1999 / Accepted 29 March 1999

Abstract

We propose that the emission of the high excitation [O IV ] 25.9 µm line observed with ISO in NGC 5253 and II Zw 40 is due to the presence of hot Wolf-Rayet (WR) stars in these objects. We construct a consistent evolutionary synthesis and photoionization model which successfully reproduces the constraints on their massive star content and the relevant optical and IR emission lines including [O IV ] 25.9 µm. Our explanation for the origin of [O IV ] is supported empirically by: 1)  the simultaneous presence of nebular He II and [O IV ] in these objects, and 2)  the close relation between nebular He II and WR stars in extragalactic HII regions. Photoionization by hot WR stars is mainly expected to be of importance in young low metallicity galaxies. Alternate mechanisms are likely at the origin of [O IV ] 25.9 µm emission in other objects.

Key words: galaxies: starburst galaxies: stellar content ISM: H ii regions stars: Wolf-Rayet infrared: galaxies galaxies: individual: NGC 5253 galaxies: individual: II Zw 40

Send offprint requests to: D. Schaerer

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Contents

• 1. Introduction
• 2. On the association of [O IV ] 25.9 µm with He II
• 3. Stellar population
• 4. Photoionization models
• 5. Comparison with observations of NGC 5253 and II Zw 40
• 6. Summary and discussion
• Acknowledgements
• References

© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999
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