1. Ultra-Faint, Ultra-Dark, and Ultra-Handsome
The New Satellites of the Milky Way
Josh Simon
Caltech
Collaborators:
Marla Geha (Yale)
Louie Strigari (Irvine)
Evan Kirby (Santa Cruz)
Anna Frebel (Texas)
Beth Willman (Harvard)
James Bullock (Irvine)

2. Outline I. Intro: CDM has its problems
II. Discovery of the ultra-faint dwarfs
III. Ultra-faint dwarf kinematics and the missing satellite problem
IV. Avenues for future work

3. The Cold Dark Matter Model
CDM model is in concordance with a wide range of astronomical observations
CMB, LSS, SNe Ia, BBN, BAO, Ly-a forest, . . .
NASA/WMAP Science Team; Spergel et al. (2007)
Riess et al. (2007)
Eisenstein et al. (2005)

4. Then What’s the Problem?
On small scales, LCDM is not in concordance with observations

5. Then What’s the Problem?
On small scales, LCDM is not in concordance with observations
Central density problem
Galaxies should have cuspy density profiles
Galaxy centers are not dense enough

6. The Central Density Problem
cusp
core

7. Then What’s the Problem?
On small scales, LCDM is not in concordance with observations
Central density problem
Galaxies should have cuspy density profiles
Galaxy centers are not dense enough
Angular momentum problem
Simulated galaxy disks are too small

8. Angular Momentum Problem
Simulations always produce large bulges
But many disk galaxies lack significant bulges
Abadi et al. (2003)
NGC 5907

9. Then What’s the Problem?
On small scales, LCDM is not in concordance with observations
Central density problem
Galaxies should have cuspy density profiles
Galaxy centers are not dense enough
Angular momentum problem
Simulated galaxy disks are too small
Substructure (missing satellite) problem
Only 1-10% of the predicted number of dwarf galaxies
are actually observed

10. CDM and the Missing Satellites
CDM predicts large numbers of subhalos (~500 for a Milky Way-sized galaxy)
Milky Way only has 24 known satellites
Are the simulations wrong?
Or do these objects actually exist despite the lack of observational evidence?
Springel et al. 2001

11. CDM and the Missing Satellites
CDM predicts large numbers of subhalos (~500 for a Milky Way-sized galaxy)
Milky Way only has 24 known satellites
Are the simulations wrong?
Or do these objects actually exist despite the lack of observational evidence?
Springel et al. 2001

12. What Do These Problem Tell Us?
Two basic sets of possible solutions:
Modifications to CDM
What modifications? Power spectrum, DM particle mass/decay/interaction cross-section?
Astrophysics comes in on small scales
Reionization/feedback/winds prevent stars from forming in most low-mass halos?
Baryonic feedback/outflows adjust the mass distribution at galaxy centers?

13. A Recent Breakthrough Only 11 Milky Way dwarfs known through 2004
Since 2005, SDSS has discovered:
9 new dSphs
1 dIrr
3 ??? dSph/GCs?
Willman et al. (2005a), Zucker et al. (2006a,b), Belokurov et al. (2006,2007,2008)
Irwin et al. (2007)
Willman et al. (2005b), Belokurov et al. (2007), Walsh et al. (2007)

14. SDSS Search Technique Raw SDSS stellar density
SDSS blue stars (g-i < 0.5)
MV = -4.1
D = 44 kpc

15. New SDSS Dwarfs Old dwarfs (Leo II) New dwarfs (Willman 1) SDSS
Willman et al. (2005)

16. Keck Spectroscopic Survey of SDSS Dwarfs
Medium-resolution spectra of 889 stars (448 members) across 9 dwarfs
(CVn I, CVn II, Coma Berenices, Hercules, Leo IV, Leo T, Segue 1, UMa I, UMa II)
Stellar velocities measured from cross-correlation with templates
Templates include GKM giants, KM dwarfs, HB stars
Stellar metallicities measured from EW of the Ca triplet lines (Rutledge et al. 1997) or spectral synthesis (Kirby et al. 2008a)
Velocities can be used to constrain the masses of these extreme systems

17. Kinematics of the SDSS Dwarfs
Incredibly small velocity dispersions to go with their tiny luminosities
Previous lower limit for dSphs (Gilmore et al. 2007)
Simon & Geha (2007)
Martin et al. (2007)
Geha et al. (2008)

18. Are They Dwarfs or Globulars?
Simplest possible mass model: equilibrium, spherical, isotropic systems where M follows L
Galaxy
Mass
Luminosity
UMa II
UMa I
Segue 1
Leo T
Leo IV
Coma
CVn II
CVn I
Herc
5.4 ± 2.4  106 M
1.6 ± 0.5  107 M
4.6 ± 2.8  105 M
8.6 ± 4.1  106 M
1.1 ± 1.1  106 M
1.4 ± 0.5  106 M
1.3 ± 0.6  106 M
2.8 ± 0.3  107 M
7.8 ± 3.1  106 M
4.1  103 L
1.4  104 L
3.4  102 L
5.9  104 L
8.6  103 L
3.7  103 L
7.8  103 L
2.4  105 L
3.7  104 L
M/L > 100 M/L!

19. Are They Dwarfs or Globulars?
Simplest possible mass model: equilibrium, spherical, isotropic systems where M follows L
Galaxy
Mass
Luminosity
3 lower limit on mass
UMa II
UMa I
Segue 1
Leo T
Leo IV
Coma
CVn II
CVn I
Herc
5.4 ± 2.4  106 M
1.6 ± 0.5  107 M
4.6 ± 2.8  105 M
8.6 ± 4.1  106 M
1.1 ± 1.1  106 M
1.4 ± 0.5  106 M
1.3 ± 0.6  106 M
2.8 ± 0.3  107 M
7.8 ± 3.1  106 M
4.1  103 L
1.4  104 L
3.4  102 L
5.9  104 L
8.6  103 L
3.7  103 L
7.8  103 L
2.4  105 L
3.7  104 L
M > 3.5  105 M
M > 3.3  106 M
M > 3.4  103 M
M > 3.8  105 M
M > 0
M > 1.9  105 M
M > 8.3  104 M
M > 1.6  107 M
M > 7.8  104 M

20. Are There Still Satellites Missing?
Comparison to Via Lactea N-body simulation (Diemand et al. 2007)
Including new dwarfs, total MW population projects to 76 dwarf galaxies over the full sky
(Simon & Geha 2007; Koposov et al. 2007)

21. Is Sloan Complete? If SDSS is complete: If SDSS is not complete:
Need another ~200 low-mass (starless?) halos
If SDSS is not complete:
Assume radial subhalo distribution from Via Lactea simulation

22. 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)

23. 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)

24. 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)

25. Is Sloan Complete? If SDSS is complete: If SDSS is not complete:
Need another ~200 low-mass (starless?) halos
If SDSS is not complete:
Assume radial subhalo distribution from Via Lactea simulation

26. If Sloan Is Still Missing Satellites
Observed dwarfs (corrected for sky coverage and incompleteness)
Observed dwarfs (corrected for sky coverage)
Observed dwarfs (not corrected for sky coverage)
Tollerud et al. (2008)
dwarfs with L > 1000 L around Milky Way!

27. Dwarf Galaxy Scaling Relations
Geha et al. (2008)

28. Better Mass Measurements
Drop mass follows light assumption
CDM prior, marginalize over unknown parameters (density profile, velocity anisotropy)
Mass is best constrained within a fixed physical radius of ~0.3 kpc
M0.3 instead of M”total”
Use maximum-likelihood technique to calculate PDF for mass given the data
Strigari et al. (2007)

29. Better Mass Measurements
Mass follows light
Maximum likelihood with CDM prior
Strigari et al. (2008)

30. A Lower Limit for Galaxy Formation
Is galaxy formation impossible below Mvir~108 M (M0.3~107 M)?
Milky Way dwarfs; Strigari et al. (2008)
Lower mass limit?
Massive galaxies from CLF; Eke et al. (2004)
van den Bosch et al. (2007) CLF
Strigari et al. (2008)

31. The Most Metal-Poor Galaxies
Dwarf galaxies (including the ultra-faint dwarfs) obey a metallicity-luminosity relation
Kirby et al. (2008b)

32. The Most Metal-Poor Galaxies
Metallicity distribution in the ultra-faint dSphs is similar to the Milky Way halo . . .
Helmi et al. 2006
Kirby et al. 2008b

33. The Most Metal-Poor Galaxies
. . . As is the abundance pattern
MW disk (Venn04)
MW halo (Venn04)
dSphs (Venn04)
ultra-faint dSphs
Could the MW halo be made up of destroyed ultra-faint dwarfs?

34. Conclusions
Ultra-faint dwarfs should solve the missing satellite problem
If SDSS is complete, observed mass function tells us that reionization suppressed star formation in most halos
If SDSS is not complete, observed luminosity function plus Via Lactea suggests >300 luminous dwarfs
We have plenty to learn about galaxy formation . . .
ALL MW dwarf spheroidals appear to have the same mass
Abundances in the ultra-faint dwarfs are consistent with those of stars in the Milky Way halo