Astron. Astrophys. 326, 597-607 (1997)

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Stellar populations and inhomogeneities
of the galactic plane from DENIS star counts ^*

S. Ruphy ¹, N. Epchtein ¹, M. Cohen ², E. Copet ¹, B. de Batz ¹, J. Borsenberger ³, P. Fouqué ^{1, 5}, S. Kimeswenger ⁴, F. Lacombe ¹, T. Le Bertre ¹, D. Rouan ¹ and D. Tiphène <sup>1

1</sup> Observatoire de Paris, F-92195 Meudon, France
² Radio Astronomy Laboratory, University of California, Berkeley, CA 94720, USA
³ Institut d'Astrophysique de Paris, 98 bis Bd. Arago, F-75014 Paris, France
⁴ Institut für Astronomie der Leopold-Franzens Universität, Technikerstra e 25, A-6020 Innsbruck, Austria
⁵ European Southern Observatory, Casilla 19001, Santiago 19, Chile

Received 17 April 1997 / Accepted 23 May 1997

Abstract

This paper reports on the analysis of 3-colour (IJK_s ) near infrared star counts obtained during the commissioning phase of the Deep Near Infrared Southern Sky Survey (DENIS) project. Observing methods, data processing, source extraction and calibration are briefly outlined. Nine strips of 12' of RA x 30 in declination crossing the galactic plane at different longitudes ranging from 217 to 385 , covering approximately 50 square degrees are analyzed. One strip has been measured in 3 colours, the others in J and K_s only. More than 250 000 objects are detected and calibrated. The completeness limits are 17.5, 15.5 and 13.5 in the I, J and K_s bands, respectively. Colour-colour diagrams are shown to be efficient tool for breaking out dwarf and giant star populations especially at low latitude, where the interstellar extinction strongly affects the far away red giants. Source counts are compared to the SKY model developed by Wainscoat et al. (1992). This small sample of DENIS data is tentatively used to investigate the existence and the spatial distribution of the stellar populations associated with the molecular ring at R = 4 kpc.

Key words: galaxy: structuregalaxy: stellar contentinfrared: stars

* Based on observations collected at the European Southern Observatory, La Silla, Chile

Send offprint requests to: epchtein@obspm.fr

Contents

• 1. Introduction
• 2. Observations
• 3. Data reduction
  • 3.1. Darks, bias and flat fielding
  • 3.2. Source extraction
  • 3.3. Astrometric calibration
  • 3.4. Photometric calibration
• 4. Results
  • 4.1. Completeness limit
  • 4.2. Separation of dwarfs and giants
    • 4.2.1. I-J/J-K_s diagrams in the anticenter direction
    • 4.2.2. K_s /J-K_s magnitude-colour diagrams at different longitudes along the galactic plane
  • 4.3. Comparisons with the SKY model
    • 4.3.1. Comparisons SKY/DENIS at l=303
    • 4.3.2. Modeling of the molecular ring
• 5. Conclusion
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997
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