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| Paper: |
The Metal Rich Halo of M31 |
| Volume: |
92, Formation of the Galactic Halo. . . . Inside and Out |
| Page: |
544 |
| Authors: |
Rich, R. Michael; Mighell, Kenneth J.; Neill, James D. |
| Abstract: |
We have reduced archival WFPC2 images of the fields adjacent to globular clusters in M31, revealing the stellar populations in fields with galactocentric distances ranging from 5.3 to 19.4 kpc. This is the first photometry in the M31 halo that reaches solidly below the luminosity of the horizontal branch. We compare the field giant branches with the V,I locus of NGC 6553, a Milky Way globular cluster of Solar metallicity. We find that the majority of the stars in these fields are near the Solar metallicity, with no clear sign of an abundance gradient. This confirms the finding by Rich et al. (1996) that the field adjacent to the M31 globular cluster G1 (RGC=40 kpc) has a red clump. The field adjacent to the cluster G219, at 19.6 kpc, cannot have more than 15% of its population in metal poor stars. The mean V luminosity of blue horizontal branch stars in the G219 field is V=25.35 and for red horizontal branch stars the mean V=25.45; while the zero point uncertainty is 0.10 mag, the formal difference is accurate to 0.01 mag. We argue that the BHB stars are similar to those in the metal poor ([Fe/H] =-2) M31 globular cluster G219, while the red clump stars have [Fe/H] ~ -0.4. We conclude that there is probably very little dependence of the luminosity of the horizontal branch with metallicity [Fe/H]. We also find an excess of stars fainter than the red clump, relative to old Galactic globular clusters; this is also in agreement with the luminosity function of the field adjacent to G1 found by Rich et al. (1996). |
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