We report the first results from deep ACS imaging of 10 classical globular clusters in the far outer regions (15 kpc less than or similar to R-p 100 kpc) of M31. Eight of the clusters, including two of the most remote M31 globular clusters presently known, are described for the first time. Our F606W, F814W color-magnitude diagrams extend similar to 3 mag below the horizontal branch and clearly demonstrate that the majority of these objects are old ( greater than or similar to 10 Gyr), metal-poor clusters. Five have [Fe/H] similar to -2.1, while an additional four have - 1.9 less than or similar to [Fe/H] less than or similar to - 1.5. The remaining object is more metal-rich, with [Fe/H] similar to -0.70. Several clusters exhibit the second-parameter effect. Using aperture photometry, we estimate integrated luminosities and structural parameters for all clusters. Many, including all four clusters with projected radii greater than 45 kpc, are compact and very luminous, with -8.9 less than or similar to M-V less than or similar to -8.3. These four outermost clusters are thus quite unlike their Milky Way counterparts, which are typically diffuse, subluminous (- 6.0 less than or similar to M-V less than or similar to -4.7), and more metal-rich (- 1.8 less than or similar to [Fe/H] less than or similar to -1.3).
|Nifer y tudalennau||4|
|Cyfnodolyn||The Astrophysical Journal|
|Dynodwyr Gwrthrych Digidol (DOIs)|
|Statws||Cyhoeddwyd - 1 Chwef 2007|