2001/01/24 by Patrick R. Durrell, William E. Harris, C. J. Pritchet +1 · 17 citations
Physics and Astronomy · #Astronomy #Astrophysics #Astrophysics and Star Formation Studies #Bulge #Galactic halo #Galaxies: Formation, Evolution, Phenomena #Galaxy #Globular cluster #Halo #Horizontal branch #Metallicity #Milky Way #Photometry (optics) #Physics #Population #Red-giant branch #Stars #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/320403
to appear in the Astronomical Journal; 43 pages, including 15 figures
arxiv created 2001/01/24 · openalex publication_date 2001/05/01 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
We have conducted a wide-field CCD-mosaic study of the resolved red giant branch stars of M31, in a field located 20 kpc from the nucleus along the southeast minor axis. In our ( I , V - I ) color-magnitude diagram, red giant branch (RGB) stars in the top three magnitudes of the M31 halo are strongly present. We use photometry of a more distant control field to subtract field contamination and then to derive the "cleaned" luminosity function and metallicity distribution for this outer-halo region of M31. From the color distribution of the foreground Milky Way halo stars, we find a reddening E ( V - I ) = 0.10 ± 0.02 for this field, and from the luminosity of the RGB tip we determine a distance modulus ( m - M ) 0 = 24.47 ± 0.12 (=783 ± 43 kpc). The metallicity distribution function (MDF) is derived from interpolation within an extensive new grid of RGB models (Vandenberg et al.). We find that the MDF is dominated by a moderately high-metallicity population ([ m /H] ∼ -0.5) that has previously been found in more interior M31 halo/bulge fields, and is very much more metal-rich than the [ m /H] ∼ -1.5 level which characterizes the Milky Way halo. In addition, a significant (∼30%–40%, depending on AGB star contribution) metal-poor population is also present. To first order, the total shape of the MDF resembles that predicted by a simple, single-component model of chemical evolution starting from primordial gas with an effective yield y = 0.0055. It strongly resembles the MDF recently found by Harris et al. for the outer halo of the giant elliptical NGC 5128, though NGC 5128 has an even lower fraction of low-metallicity stars. Intriguingly, in both NGC 5128 and M31, the metallicity distribution of the globular clusters in M31 does not match the halo stars , in the sense that the clusters are far more heavily weighted to metal-poor objects. We suggest similarities in the formation and early evolution of massive, spheroidal stellar systems.