Crater 2: An Extremely Cold Dark Matter Halo
- Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
- McWilliams Center for Cosmology, Department of Physics, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213 (United States)
- Department of Astronomy, University of Michigan, 311 West Hall, 1085 S. University Avenue, Ann Arbor, MI 48109 (United States)
- Steward Observatory, The University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721 (United States)
- Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA (United Kingdom)
We present results from MMT/Hectochelle spectroscopy of 390 red giant candidate stars along the line of sight to the recently discovered Galactic satellite Crater 2. Modeling the joint distribution of stellar positions, velocities, and metallicities as a mixture of Crater 2 and Galactic foreground populations, we identify ∼62 members of Crater 2, for which we resolve a line-of-sight velocity dispersion of σ{sub v{sub l{sub o{sub s}}}} = 2.7{sub −0.3}{sup +0.3} km s{sup −1} and a mean velocity of 〈v{sub los}〉 = 87.5{sub −0.4}{sup +0.4} km s{sup −1} (solar rest frame). We also resolve a metallicity dispersion of σ{sub [Fe/H]} = 0.22{sub −0.03}{sup +0.04} dex and a mean of 〈[Fe/H]〉 = −1.98{sub −0.1}{sup +0.1} dex that is 0.28 ± 0.14 dex poorer than estimated from photometry. Despite Crater 2's relatively large size (projected halflight radius R {sub h} ∼ 1 kpc) and intermediate luminosity (M {sub V} ∼ −8), its velocity dispersion is the coldest that has been resolved for any dwarf galaxy. These properties make Crater 2 the most extreme low-density outlier in dynamical as well as structural scaling relations among the Milky Way’s dwarf spheroidals. Even so, under assumptions of dynamical equilibrium and negligible contamination by unresolved binary stars, the observed velocity distribution implies a gravitationally dominant dark matter halo, with a dynamical mass of 4.4{sub −0.9}{sup +1.2}×10{sup 6} M {sub ⊙} and a mass-to-light ratio of 53{sub −11}{sup +15} M{sub ⊙}/L{sub V,⊙} enclosed within a radius of ∼1 kpc, where the equivalent circular velocity is 4.3{sub −0.5}{sup +0.5} km s{sup −1}.
- OSTI ID:
- 22872607
- Journal Information:
- Astrophysical Journal, Vol. 839, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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