Astron. Astrophys. 326, 528-536 (1997)

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Blue stars in the center of the S0 galaxy NGC 5102 ^*

J.-M. Deharveng ¹, R. Jedrzejewski ², P. Crane ³, M.J. Disney ⁴ and B. Rocca-Volmerange <sup>5

1</sup> Laboratoire d'Astronomie Spatiale du CNRS, Traverse du Siphon, BP 8, F-13376 Marseille Cedex 12, France
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³ European Southern Observatory, Karl Schwarzschild Strasse 2, D-8046 Garching, Germany
⁴ Department of Physics and Astronomy, University of Wales College of Cardiff, P.O. Box 913, Cardiff CF2 3YB, UK
⁵ Institut d'Astrophysique, 98bis Boulevard Arago, F-75014 Paris, France

Received 7 March 1997 / Accepted 22 April 1997

Abstract

The S0 galaxy NGC 5102 has been observed with the Faint Object Camera on-board the Hubble Space Telescope. Individual blue stars have been resolved in the central region and measured photometrically through the F175W and F342w filters. The limits of detection and completeness are not uniform over the field observed because the unresolved galaxy light increases toward the center. Comparison of integrated light with previous IUE observations and study of the light profiles determined from the FOC images confirm the blue color gradient inward. The F175wF342w color is found to get bluer from 1:004 (21 pc) to 0:004 (6 pc) but the gradient does not continue in the nucleus. There is no evidence for a central core. The resolved blue stars are slightly more concentrated toward the center than the integrated light distribution. In the m₁₇₅ vs. m₁₇₅--m₃₄₂ plane the stars are at positions where either young stars or P-AGB stars may be expected. Although neither interpretation is certain, several arguments suggest that the majority of the resolved stars are young and belong to the last generation of a star formation episode that ended ~ 15 Myr ago. According to spectral evolution models, the IUE ultraviolet spectrum is best reproduced if this star formation episode started 500-800 Myr ago.

Key words: galaxies: evolution galaxies: individual: NGC 5102 galaxies: elliptical and lenticular galaxies: stellar content ultraviolet: galaxies

* Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.

Send offprint requests to: J.-M. Deharveng

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Contents

• 1. Introduction
• 2. Observations and data reduction
• 3. Analysis
  • 3.1. Photometry of the resolved stars
  • 3.2. Galaxy light distribution
  • 3.3. The spatial distribution of resolved stars
  • 3.4. The dust patches
• 4. Discussion
  • 4.1. The nature of the resolved blue stars
  • 4.2. The star formation history
  • 4.3. Comparison with NGC 3115
  • 4.4. Interpretation
• 5. Conclusion
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997
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