2006/12/20 by Dougal Mackey, A. D. Mackey, Avon Huxor +13
Physics and Astronomy · #Andromeda Galaxy #Astronomy #Astrophysics #Astrophysics and Star Formation Studies #Galaxies: Formation, Evolution, Phenomena #Galaxy #Globular cluster #Halo #Horizontal branch #Milky Way #Photometry (optics) #Physics #Stars #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/511977
published as Astrophys.J.655:L85-L88,2007 · 13 pages, 3 figures, 1 table; accepted for publication in ApJ Letters
arxiv created 2006/12/20 · openalex publication_date 2007/01/18 · arxiv updated 2009/12/01 · openalex created_date 2019/06/27 · openalex updated_date 2026/08/05
We report the first results from deep ACS imaging of 10 classical globular clusters in the far outer regions (15 kpc ≲ 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 ~3 mag below the horizontal branch and clearly demonstrate that the majority of these objects are old (≳10 Gyr), metal-poor clusters. Five have [Fe/H] ~ -2.1, while an additional four have -1.9 ≲ [Fe/H] ≲ -1.5. The remaining object is more metal-rich, with [Fe/H] ~ -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 ≲ M V ≲ -8.3. These four outermost clusters are thus quite unlike their Milky Way counterparts, which are typically diffuse, subluminous (-6.0 ≲ M V ≲ -4.7), and more metal-rich (-1.8 ≲ [Fe/H] ≲ -1.3).