Astron. Astrophys. 355, 949-965 (2000)

Age and metallicity gradients in the Galactic Bulge ^*

A differential study using HST/WFPC2

S. Feltzing ^1,2 and G. Gilmore <sup>2

1</sup> Royal Greenwich Observatory Madingley Road, Cambridge CB3 0EZ, U.K.
² Institute of Astronomy, Madingley Road,Cambridge CB3 0HA, U.K.

Received 6 March 1998 / Accepted 22 December 1999

Abstract

The Galactic Bulge has long been assumed to be a largely old stellar population. However, some recent studies based on observations with the HST WF/PC-1 and WFPC2 of stars in the Galactic Bulge have concluded that the old population may not make up more then 30% of the total. Other studies using HST/WFPC2 differential studies of `Bulge' globular clusters and field stars have found the bulge to be comparable in age to the Galactic Halo. A complication in all these studies is the presence of a substantial population of stars which mimic a young bulge population, but which may be, and are often assumed to be, foreground disk stars whose reddening and distance distributions happen to mimic a young bulge turnoff. We show, using number counts in HST/WFPC2 colour-magnitude diagrams of both field stars in the Bulge and of two `bulge' and one `disk' globular cluster (NGC6528, NGC6553, and NGC5927) that the stars interpreted as young in fact are foreground disk stars. Thus, we confirm that the bulk of the bulge field stars in Baade's Window are old. The existence of a young metal-rich population cannot, however, be ruled out from our data.

We also test for age and metallicity gradients in the Galactic Bulge between the two low extinction windows Baade's window (= = ) and Sagittarius-I ( = , b = ). We use the colour-magnitude diagram of a metal-rich globular cluster as an empirical isochrone to derive a metallicity difference of dex between Baade's window and SGR-I window. This corresponds to a metallicity gradient of dex/kpc, in agreement with recent near-IR CMD studies. Such a steep gradient, if detected, would require the existence of a short scale length inner component to the Bulge, most likely that prominent in the near infra red, which perhaps forms a separate entity superimposed on the larger, optical Bulge as observed in Baade's window.

Key words: Galaxy: abundances Galaxy: center Galaxy: general Galaxy: globular clusters: individual: NGC5927, NGC6528, NGC6553 Galaxy: stellar content

* Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract No. NAS5-26555

Present address: Lund Observatory, Box 43, 221 00 Lund, Sweden

Send offprint requests to: Sofia Feltzing

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Contents

• 1. Introduction
• 2. The data
  • 2.1. Data reductions
  • 2.2. Combining images
• 3. Stellar photometry: deriving colour-magnitude diagrams
  • 3.1. Aperture vs psf-photometry ()
  • 3.2. Long versus short exposures
  • 3.3. Aperture corrections, charge transfer efficiency (CTE) and effective pixel area (EPA)
  • 3.4. Completeness
  • 3.5. Selection of final stellar samples
• 4. Reddening, distances and metallicities
  • 4.1. Comparison with previous studies
  • 4.2. The Bulge fields
    • 4.2.1. SGR-I
    • 4.2.2. Baade's window at R.A.=18 03 11.7 =-29 51 40.7
    • 4.2.3. Fields at (l,b)=(,) and (,)
  • 4.3. The globular clusters
    • 4.3.1. NGC5927
    • 4.3.2. NGC6528
    • 4.3.3. NGC6553
    • 4.3.4. Differential reddening towards NGC6553?
    • 4.3.5. Spectroscopic abundances
• 5. The age of the Galactic Bulge
  • 5.1. Can we resolve the metallicity distribution function?
• 6. Is there a detectable metallicity gradient?
  • 6.1. The metallicity in Baade's window
  • 6.2. The Bulge metallicity gradient
• 7. Discussion
• 8. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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