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The Nature of the Least Luminous Galaxies

Or, Looking for Dark Matter in All the Right Places. The Nature of the Least Luminous Galaxies. Josh Simon Carnegie Observatories. Marla Geha (Yale) Quinn Minor (SUNY Oneonta) Greg Martinez (UC Irvine) Manoj Kaplinghat (UC Irvine) James Bullock (UC Irvine)

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The Nature of the Least Luminous Galaxies

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  1. Or, Looking for Dark Matter in All the Right Places The Nature of the Least Luminous Galaxies Josh Simon Carnegie Observatories Marla Geha (Yale) Quinn Minor (SUNY Oneonta) Greg Martinez (UC Irvine) Manoj Kaplinghat (UC Irvine) James Bullock (UC Irvine) Louie Strigari (Stanford) Beth Willman (Haverford) Evan Kirby (Caltech)

  2. Where To Look For Dark Matter • Galactic Center • Nearby (8 kpc) • Probably high concentration of DM • Horrendous backgrounds S. Guisard/Wang et al.

  3. Where To Look For Dark Matter • Galactic Center • Nearby (8 kpc) • Probably high concentration of DM • Horrendous backgrounds Y. Beletsky

  4. Where To Look For Dark Matter • Galactic halo • Also nearby • Less dark matter, but lots of sky • Backgrounds, but anisotropies may help

  5. Where To Look For Dark Matter • Dwarf galaxies • Farther away (24 – 250 kpc) • Plenty of DM • No backgrounds

  6. Which Galaxies? • The closest and densest • Ursa Minor (69 kpc) • Draco (76 kpc) • Ultra-faints (28-42 kpc) Ursa Minor Strigari et al. (2008a)Martinez et al. (2009)Abdo et al. (2010)

  7. The Ultra-Faint Dwarfs Strigari et al. (2008b)

  8. What Is Segue 1? • Globular cluster? • Globular cluster? • Galaxy? • Galaxy?

  9. 29 pc 59 pc 88 pc A Complete Survey of Segue 1 almost  • Keck/DEIMOS spectroscopy of every photometric member candidate in Segue 1 out to r = 10' (67 pc) • If Segue 1 does not have an extended DM halo, its tidal radius would be ~30 pc

  10. Galaxy or Star Cluster? • Spectra of Segue 1 red giants • [Fe/H] range of 1.7 dex • 2 stars near [Fe/H] = -3.4 Metal-rich Metal-poor Simon et al. (2010) Norris et al. (2010)

  11. +8.2 +1.4 –3.1 –1.1 A Complete Survey of Segue 1 • 71 members (multiple epochs on 33),  = 3.7km s-1 • M1/2 = 5.8  105 M Simon et al. (2010)

  12. Correcting for Binary Stars • Binaries increase  by ~12% • Prior on binary periods has minimal effect Simon et al. (2010) Martinez et al. (2010)

  13. Contamination by the Sgr Stream? Sagittarius stream Orphan stream right position wrong velocity wrong position right velocity Segue 1 Belokurov et al. (2006)

  14. Could Segue 1 be Tidally Disrupting? • No obvious tails/morphological distortion Simon et al. (2010)

  15. Could Segue 1 be Tidally Disrupting? • No obvious tails/morphological distortion • No velocity gradient • Tidal radius from M1/2 ~ 250 pc • Needs pericenter <4 kpc to get rtidal ~ r1/2 • If it is not bound, lifetime is few  107 yr Simon et al. (2010)

  16. Ursa Major II • Very close (32 kpc), apparently high mass (7.9  106 M) +5.6 –3.1 Wolf et al. (2010)

  17. Ursa Major II • Very close (32 kpc), apparently high mass (7.9  106 M) BUT: • Unusually elongated +5.6 –3.1 Muñoz et al. (2010)

  18. Ursa Major II • Very close (32 kpc), apparently high mass (7.9  106 M) BUT: • Power-law density profile +5.6 –3.1 Muñoz et al. (2010)

  19. Ursa Major II • Very close (32 kpc), apparently high mass (7.9  106 M) BUT: • Unusually high  +5.6 –3.1 Simon & Geha (2007)

  20. Ursa Major II • Very close (32 kpc), apparently high mass (7.9  106 M) BUT: • Velocity gradient? +5.6 –3.1 Geha et al. (in prep)

  21. Ursa Major II • Very close (32 kpc), apparently high mass (7.9  106 M) BUT: • Velocity gradient? • 11.5 km s-1 E-W (34.5 km s-1 deg-1) +5.6 –3.1 Geha et al. (in prep)

  22. Willman 1 • Also nearby (38 kpc), large velocity dispersion (4.3 km s-1) Like Segue 1, significant metallicity spread Willman et al. (2010)

  23. Willman 1 • Also nearby (38 kpc), large velocity dispersion (4.3 km s-1) WTF? Willman et al. (2010)

  24. Coma Berenices • Farther away (42 kpc), but very regular Muñoz et al. (2010)

  25. Coma Berenices • Repeat measurements of 13 RGB stars: 1 binary • Indicates that binaries inflate  by ~7% (Minor et al. 2010) • Marginal evidence for a small velocity gradient Geha et al. (in prep)

  26. Conclusions • Assessing the evidence: • Segue 1 - galaxy, no tides, no binaries • UMa II - galaxy, tides • Willman 1 - galaxy, tides?? • Bootes II - galaxy • Coma Berenices - galaxy, no tides, no binaries

  27. Conclusions • Assessing the evidence: • Segue 1 - galaxy, no tides, no binaries • UMa II - galaxy, tides • Willman 1 - galaxy, tides?? • Bootes II - galaxy • Coma Berenices - galaxy, no tides, no binaries Likely good targets: Not recommended: Ursa Minor Willman 1 Draco UMa II Segue 1 Coma Berenices . . .

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