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Title: The large-scale structure of the halo of the Andromeda galaxy. I. Global stellar density, morphology and metallicity properties

We present an analysis of the large-scale structure of the halo of the Andromeda galaxy, based on the Pan-Andromeda Archeological Survey (PAndAS), currently the most complete map of resolved stellar populations in any galactic halo. Despite the presence of copious substructures, the global halo populations follow closely power-law profiles that become steeper with increasing metallicity. We divide the sample into stream-like populations and a smooth halo component (defined as the population that cannot be resolved into spatially distinct substructures with PAndAS). Fitting a three-dimensional halo model reveals that the most metal-poor populations ([Fe/H]<−1.7) are distributed approximately spherically (slightly prolate with ellipticity c/a = 1.09 ± 0.03), with only a relatively small fraction residing in discernible stream-like structures (f {sub stream} = 42%). The sphericity of the ancient smooth component strongly hints that the dark matter halo is also approximately spherical. More metal-rich populations contain higher fractions of stars in streams, with f {sub stream} becoming as high as 86% for [Fe/H]>−0.6. The space density of the smooth metal-poor component has a global power-law slope of γ = –3.08 ± 0.07, and a non-parametric fit shows that the slope remains nearly constant from 30 kpc to ∼300 kpc. The total stellarmore » mass in the halo at distances beyond 2° is ∼1.1 × 10{sup 10} M {sub ☉}, while that of the smooth component is ∼3 × 10{sup 9} M {sub ☉}. Extrapolating into the inner galaxy, the total stellar mass of the smooth halo is plausibly ∼8 × 10{sup 9} M {sub ☉}. We detect a substantial metallicity gradient, which declines from ([Fe/H]) = –0.7 at R = 30 kpc to ([Fe/H]) = –1.5 at R = 150 kpc for the full sample, with the smooth halo being ∼0.2 dex more metal poor than the full sample at each radius. While qualitatively in line with expectations from cosmological simulations, these observations are of great importance as they provide a prototype template that such simulations must now be able to reproduce in quantitative detail.« less
;  [1] ;  [2] ;  [3] ;  [4] ; ; ;  [5] ; ;  [6] ;  [7] ;  [8] ;  [9] ;  [10] ;  [11] ;  [12] ;  [13]
  1. Observatoire astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, 11 rue de lUniversité, F-67000 Strasbourg (France)
  2. Institute of Astronomy, School of Physics A28, University of Sydney, NSW 2006 (Australia)
  3. NRC Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC, V9E 2E7 (Canada)
  4. Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA (United Kingdom)
  5. Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ (United Kingdom)
  6. Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, British Columbia V8P 5C2 (Canada)
  7. Department of Physics and Atmospheric Science, Dalhousie University, 6310 Coburg Road, Halifax NS B3H 4R2 (Canada)
  8. Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg (Germany)
  9. University of Massachusetts, Department of Astronomy, LGRT 619-E, 710 North Pleasant Street, Amherst, MA 01003-9305 (United States)
  10. RSAA, The Australian National University, Mount Stromlo Observatory, Cotter Road, Weston Creek ACT 2611 (Australia)
  11. Department of Physics and Astronomy, University of California, Los Angeles, PAB, 430 Portola Plaza, Los Angeles, CA 90095-1547 (United States)
  12. Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH (United Kingdom)
  13. Department of Physics, Engineering Physics, and Astronomy Queen's University, Kingston, Ontario K7L 3N6 (Canada)
Publication Date:
OSTI Identifier:
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 780; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States