Astron. Astrophys. 324, 65-79 (1997)

Probing the Galaxy

I. The galactic structure towards the galactic pole

Y.K. Ng ^{1, 2, 5}, G. Bertelli ^{3, 4}, C. Chiosi ³ and A. Bressan <sup>5

1</sup> Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands
² IAP, CNRS, 98 bis Boulevard Arago, F-75014 Paris, France
³ Department of Astronomy, Vicolo dell'Osservatorio 5, I-35122 Padua, Italy
⁴ National Council of Research, CNR - GNA, Rome, Italy
⁵ Padova Astronomical Observatory, Vicolo dell'Osservatorio 5, I-35122 Padua, Italy
([bertelli,bressan,chiosi,yuen]@astrpd.pd.astro.it)

Received 9 November 1995 / Accepted 20 January 1997

Abstract

Observations of (B-V) colour distributions towards the galactic poles are compared with those obtained from synthetic colour-magnitude diagrams to determine the major constituents in the disc and spheroid. The disc is described with four stellar sub-populations: the young, intermediate, old and thick disc populations, which have respectively scale heights of 100 pc, 250 pc, 0.5 kpc, and 1.0 kpc. The spheroid is described with stellar contributions from the bulge and halo. The bulge is not well constrained with the data analyzed in this study. A non-flattened power-law describes the observed distributions at fainter magnitudes better than a deprojected -law. Details about the age, metallicity, and normalizations are listed in Table 1.

The star counts and the colour distributions from the stars in the intermediate fields towards the galactic anti-centre are well described with the stellar populations mentioned above. Arguments are given that the actual solar offset is about 15 pc north from the galactic plane.

Key words: stars: HR-diagram, statistics Galaxy: stellar content, structure

SIMBAD Objects

Contents

• 1. Introduction
• 2. Analysis
  • 2.1. Method
  • 2.2. Limits
  • 2.3. Normalization
• 3. Results
  • 3.1. The north galactic pole
  • 3.2. The south galactic pole
  • 3.3. Intermediate latitude fields
    • 3.3.1.
    • 3.3.2. l = 167 5, b = +47 4
• 4. Discussion
  • 4.1. Age and metallicity
  • 4.2. Initial mass function
  • 4.3. The solar offset
  • 4.4. Stellar populations in the disc
    • 4.4.1. The old disc population
    • 4.4.2. The age-metallicity of the thick disc
    • 4.4.3. The thick disc population: scale height
    • 4.4.4. Indications for a possible merger
    • 4.4.5. Low mass stars
  • 4.5. Stellar populations in the spheroid
    • 4.5.1. Flattening of the spheroid
    • 4.5.2. Power-law or R - law
    • 4.5.3. Metal-rich stars
• 5. Conclusions and future work
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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