The Ultra-Faint Milky Way Satellites

1. Implications for the Missing Satellite Problem The Ultra-Faint Milky Way Satellites Josh Simon (Caltech) Marla Geha (HIA)

2. UMa I Coma Berenices Leo T = = = (you try saying it!) (blue eyes) (same shape) Hercules CVn I = = (the leader) (the odd one out) Leo IV CVn II = = (no followup observations) (roundest) A Guide to the New Dwarfs UMa II = (faintest) The Globular Cluster - Dwarf Galaxy Connection

3. CDM Has a Missing Satellite Problem CDM predicts large numbers of subhalos (~100-1000 for a Milky Way-sized galaxy) Milky Way only has 23 known satellites What happened to the rest of them? Springel et al. 2001 The Globular Cluster - Dwarf Galaxy Connection

4. CDM Has a Missing Satellite Problem CDM predicts large numbers of subhalos (~100-1000 for a Milky Way-sized galaxy) Milky Way only has 23 known satellites What happened to the rest of them? Springel et al. 2001 The Globular Cluster - Dwarf Galaxy Connection

5. What Does This Problem Tell Us? • Two basic sets of possible solutions: • Modifications to CDM • What modifications? Power spectrum, DM particle mass/decay/interaction cross-section? • Astrophysics prevents stars from forming in most low-mass halos • What astrophysics? Reionization, feedback, winds? The Globular Cluster - Dwarf Galaxy Connection

6. A Recent Breakthrough • Only 11 Milky Way dwarfs known through 2004 • Since 2005, SDSS has discovered: • 8 new dSphs • 1 dIrr • 3 ??? dSph/GCs? Willman et al. (2005a), Zucker et al. (2006a,b), Belokurov et al. (2006,2007) Irwin et al. (2007) Willman et al. (2005b), Belokurov et al. (2007), Walsh et al. (2007) Belokurov et al. (2006) The Globular Cluster - Dwarf Galaxy Connection

7. Keck Spectroscopic Survey of SDSS Dwarfs • Medium-resolution spectra of 841 stars (424 members) across 8 dwarfs (CVn I, CVn II, Coma Berenices, Hercules, Leo IV, Leo T, UMa I, UMa II) • Measured stellar velocities + metallicities • Velocities can be used to constrain the masses and densities of these extreme systems The Globular Cluster - Dwarf Galaxy Connection

8. Previous lower limit for dSphs (Gilmore et al. 2007) • Simon & Geha (2007) • UMa II (tidally disrupting; SG07) • Martin et al. (2007)  Keck Spectroscopic Survey of SDSS Dwarfs • Medium-resolution spectra of 841 stars (424 members) across 8 dwarfs (CVn I, CVn II, Coma Berenices, Hercules, Leo IV, Leo T, UMa I, UMa II) • Measured stellar velocities + metallicities The Globular Cluster - Dwarf Galaxy Connection

9. Velocity Dispersion Profiles • (r) declines with r in globular clusters • (r) = const is typical for dSphs (see Walker et al. 2007b) • Evidence for extended dark matter halos around the ultra-faint dwarfs Simon & Geha (in prep) The Globular Cluster - Dwarf Galaxy Connection

10. Velocity Dispersion Profiles • (r) declines with r in globular clusters • (r) = const is typical for dSphs (see Walker et al. 2007b) • Evidence for extended dark matter halos around the ultra-faint dwarfs Simon & Geha (in prep) The Globular Cluster - Dwarf Galaxy Connection

11. Simon & Geha (2007) • UMa II (tidally disrupting; SG07) • Martin et al. (2007)  Masses of the Ultra-Faint Dwarfs • Simplest possible model: equilibrium, spherical, isotropic systems where M follows L (although see L. Strigari’s talk, Strigari et al. in prep) The Globular Cluster - Dwarf Galaxy Connection

12. Mateo Plot • Do all dwarfs live in similar halos? • Is there a minimum mass for dwarfs? Simon & Geha (2007) The Globular Cluster - Dwarf Galaxy Connection

13. ? ? The Missing Satellites Does the correction from the observed number of satellites to the true number go in the horizontal or vertical direction? Moore et al. (1999), also see Klypin et al. (1999) The Globular Cluster - Dwarf Galaxy Connection

14. Are There Still Satellites Missing? Comparison to Via Lactea N-body simulation (Diemand et al. 2007) Including new dwarfs, total MW population projects to 51 dwarf galaxies over the full sky (Simon & Geha 2007; Koposov et al. 2007) The Globular Cluster - Dwarf Galaxy Connection

15. Are There Still Satellites Missing? Comparison to Via Lactea N-body simulation (Diemand et al. 2007) What if dwarfs only populate the most massive halos? (Stoehr et al. 2002) Simon & Geha (2007) The Globular Cluster - Dwarf Galaxy Connection

16. Are There Still Satellites Missing? Comparison to Via Lactea N-body simulation (Diemand et al. 2007) How about the subhalos that were most massive before being accreted by the Milky Way? (Kravtsov et al. 2004) Simon & Geha (2007) The Globular Cluster - Dwarf Galaxy Connection

17. Are There Still Satellites Missing? Comparison to Via Lactea N-body simulation (Diemand et al. 2007) Halos that reached vcirc ~ 8 km/s before reionization . . . (Bullock et al. 2000; Ricotti & Gnedin 2005; Moore et al. 2006) Simon & Geha (2007) The Globular Cluster - Dwarf Galaxy Connection

18. Summary • Ultra-faint dwarfs are extreme systems in every sense • Lowest luminosities (2600 L < L < 124000 L), lowest metallicities ([Fe/H] < -2) • Smallest velocity dispersions (3 km/s < s < 8 km/s), lowest masses (106 - 107 M), largest mass-to-light ratios (M/L = 100 - 1000 M/L) • Is there still a missing satellite problem? • Ultra-faint dwarfs alleviate but don’t remove the substructure problem . . . • UNLESS dwarf galaxy formation was strongly suppressed by reionization at z  12 The Globular Cluster - Dwarf Galaxy Connection