The Nature of the Least Luminous Galaxies Josh Simon Carnegie Observatories 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) 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) Or, Looking for Dark Matter in All the Right Places

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

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

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

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

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

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

What Is Segue 1? Globular cluster? Galaxy?

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 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 A Complete Survey of Segue 1  almost 29 pc 59 pc 88 pc

Galaxy or Star Cluster? Spectra of Segue 1 red giants –[Fe/H] range of 1.7 dex –2 stars near [Fe/H] = -3.4 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)

A Complete Survey of Segue 1 71 members (multiple epochs on 33),  = 3.7 km s -1 M 1/2 = 5.8  10 5 M  71 members (multiple epochs on 33),  = 3.7 km s -1 M 1/2 = 5.8  10 5 M  +8.2 – –1.1 Simon et al. (2010)

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

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

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

Could Segue 1 be Tidally Disrupting? No obvious tails/morphological distortion No velocity gradient Tidal radius from M 1/2 ~ 250 pc –Needs pericenter <4 kpc to get r tidal ~ r 1/2 If it is not bound, lifetime is few  10 7 yr No obvious tails/morphological distortion No velocity gradient Tidal radius from M 1/2 ~ 250 pc –Needs pericenter <4 kpc to get r tidal ~ r 1/2 If it is not bound, lifetime is few  10 7 yr Simon et al. (2010)

Ursa Major II Very close (32 kpc), apparently high mass (7.9  10 6 M  ) +5.6 –3.1 Wolf et al. (2010)

Ursa Major II Very close (32 kpc), apparently high mass (7.9  10 6 M  ) BUT: Unusually elongated Very close (32 kpc), apparently high mass (7.9  10 6 M  ) BUT: Unusually elongated Muñoz et al. (2010) +5.6 –3.1

Ursa Major II Very close (32 kpc), apparently high mass (7.9  10 6 M  ) BUT: Power-law density profile Very close (32 kpc), apparently high mass (7.9  10 6 M  ) BUT: Power-law density profile Muñoz et al. (2010) +5.6 –3.1

Ursa Major II Very close (32 kpc), apparently high mass (7.9  10 6 M  ) BUT: Unusually high  Very close (32 kpc), apparently high mass (7.9  10 6 M  ) BUT: Unusually high  Simon & Geha (2007) +5.6 –3.1

Ursa Major II Very close (32 kpc), apparently high mass (7.9  10 6 M  ) BUT: Velocity gradient? Very close (32 kpc), apparently high mass (7.9  10 6 M  ) BUT: Velocity gradient? Geha et al. (in prep) +5.6 –3.1

Ursa Major II Very close (32 kpc), apparently high mass (7.9  10 6 M  ) BUT: Velocity gradient? 11.5 km s -1 E-W (34.5 km s -1 deg -1 ) Very close (32 kpc), apparently high mass (7.9  10 6 M  ) BUT: Velocity gradient? 11.5 km s -1 E-W (34.5 km s -1 deg -1 ) Geha et al. (in prep) +5.6 –3.1

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

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

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

Coma Berenices Geha et al. (in prep) 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 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

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 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

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 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...