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)
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
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)
Then What’s the Problem?
• On small scales, CDM is not in
concordance with observations
Then What’s the Problem?
• On small scales, CDM is not in
concordance with observations
• Central density problem
– Galaxies should have cuspy density profiles
– Galaxy centers are not dense enough
The Central Density Problem
cusp
core
Then What’s the Problem?
• On small scales, CDM 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
Angular Momentum Problem
• Simulations always produce large bulges
• But many disk galaxies lack significant bulges
Abadi et al. (2003)
NGC 5907
Then What’s the Problem?
• On small scales, CDM 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
CDM and the Missing Satellites
CDM predicts large
numbers of subhalos
(~500 for a Milky Waysized galaxy)
Milky Way only has 24
known satellites
Are the simulations
wrong?
Springel et al. 2001
Or do these objects
actually exist despite
the lack of
observational evidence?
CDM and the Missing Satellites
CDM predicts large
numbers of subhalos
(~500 for a Milky Waysized galaxy)
Milky Way only has 24
known satellites
Are the simulations
wrong?
Springel et al. 2001
Or do these objects
actually exist despite
the lack of
observational evidence?
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?
A Recent Breakthrough
• Only 11 Milky Way dwarfs known through 2004
• Since 2005, SDSS has discovered:
 9 new dSphs Willman et al. (2005a), Zucker et al. (2006a,b), Belokurov et al. (2006,2007,2008)
 1 dIrr
Irwin et al. (2007)
 3 ??? dSph/GCs?
Willman et al. (2005b), Belokurov et al. (2007), Walsh et al. (2007)
SDSS Search Technique
Raw SDSS stellar density
SDSS blue stars (g-i < 0.5)
MV = -4.1
D = 44 kpc
New SDSS Dwarfs
Old dwarfs (Leo II)
SDSS
New dwarfs (Willman 1)
Willman et al. (2005)
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
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)
Are They Dwarfs or Globulars?
• Simplest possible mass model:
equilibrium, spherical, isotropic systems
where M follows L
Galaxy
UMa II
UMa I
Segue 1
Leo T
Leo IV
Coma
CVn II
CVn I
Herc
Mass
5.4
1.6
4.6
8.6
1.1
1.4
1.3
2.8
7.8
±
±
±
±
±
±
±
±
±
2.4
0.5
2.8
4.1
1.1
0.5
0.6
0.3
3.1









Luminosity
106 M
107 M
105 M
106 M
106 M
106 M
106 M
107 M
106 M
4.1
1.4
3.4
5.9
8.6
3.7
7.8
2.4
3.7









103 L
104 L
102 L
104 L
103 L
103 L
103 L
105 L
104 L
M/L > 100 M/L!
Are They Dwarfs or Globulars?
• Simplest possible mass model:
equilibrium, spherical, isotropic systems
where M follows L
Galaxy
UMa II
UMa I
Segue 1
Leo T
Leo IV
Coma
CVn II
CVn I
Herc
Mass
5.4
1.6
4.6
8.6
1.1
1.4
1.3
2.8
7.8
±
±
±
±
±
±
±
±
±
2.4
0.5
2.8
4.1
1.1
0.5
0.6
0.3
3.1









Luminosity
106 M
107 M
105 M
106 M
106 M
106 M
106 M
107 M
106 M
4.1
1.4
3.4
5.9
8.6
3.7
7.8
2.4
3.7









103 L
104 L
102 L
104 L
103 L
103 L
103 L
105 L
104 L
3 lower limit on mass
M
M
M
M
M
M
M
M
M
>
>
>
>
>
>
>
>
>
3.5
3.3
3.4
3.8
0
1.9
8.3
1.6
7.8




105 M
106 M
103 M
105 M




105 M
104 M
107 M
104 M
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)
Is Sloan Complete?
• If SDSS is complete:
 Need another ~200 low-mass (starless?)
halos
• If SDSS is not complete:
 Assume radial subhalo distribution from
Via Lactea simulation
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)
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)
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)
Is Sloan Complete?
• If SDSS is complete:
 Need another ~200 low-mass (starless?)
halos
• If SDSS is not complete:
 Assume radial subhalo distribution from
Via Lactea simulation
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)
300-600 dwarfs with L >
1000 L around Milky Way!
Dwarf Galaxy Scaling Relations
Geha et al. (2008)
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)
Better Mass Measurements
Mass follows light
Maximum likelihood
with CDM prior
Strigari et al. (2008)
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)
The Most Metal-Poor Galaxies
• Dwarf galaxies (including the ultra-
faint dwarfs) obey a metallicityluminosity relation
Kirby et al. (2008b)
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
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?
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