dwarf spheroidal galaxies and the physical properties of dark ......• dwarf spheroidal galaxies...
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Dwarf spheroidal galaxies and the physical
properties of dark matter
Mark Wilkinson
University of Leicester
Gerry Gilmore (Cambridge) Jan Kleyna (Hawaii)Andreas Koch (UCLA) Wyn Evans (Cambridge)Eva Grebel (Heidelberg)Rosemary Wyse (JHU)Justin Read (Zurich)Vasily Belokurov (Cambridge)Dan Zucker (Cambridge)
Collaborators
Local Group satellites
Grebel (1999)
Satellites and dark matter
Moore
Hot Warm Cold
• Low luminosity, gas-poor satellites of Milky Way and M31
•
• No well-defined tidal radii
• Individual stellar velocities measurable with ≥ 4m telescopes
•
• Core radii
• Inferred mass-to-light ratios in range
Dwarf spheroidal galaxies
Odenkirchen et al. (2001)
L = 3× 104L! − 2× 107L!
σ0 ∼ 7− 12 km s−1
r0 ≈ 130− 500 pc
3− 300 M!/L!
Velocity dispersion profilesDraco Ursa Minor Sextans
0 2 4
Carina
0 2 4
Leo I
0 2 4
Leo II
0 5 10 15
Omega Centauri
0 5 10 15
Mass follows light
Gilmore et al. (2007)
Velocity dispersion profiles II
Walker et al. (2007)
dSph dispersion profiles generally remain flat to large radii
Are dSphs in equillibrium?
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Koch et al. (2007)
• Velocity gradients are signature of tidal disturbance
• No dSph shows evidence of significant velocity gradient in inner regions ⇒ bulk mass estimates are robust
dSphs: the case for cored haloesJeans equations give simple relation between kinematics, the light distribution and the underlying mass distribution
We can either:
1. Assume a parameterised mass model and velocity anisotropy and fit dispersion profile
or
2. Use Jeans equations to determine mass profile from projected velocity dispersion profile and a fit to the light distribution
M(r) = −r2
G
(1ν
d νσ2r
d r+ 2
βσ2r
r
)
β(r)M(r)
Fitting dSph dispersion profiles: Leo I• Assume either NFW halo
( ) or more general profile ( )
• Best-fit dispersion obtained for cored profiles with roughly isotropic velocity dispersions
• Significant velocity anisotropy not favoured (but not excluded)
• Enclosed mass in both cored and cusped haloes
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∼ 8× 107M!
ρ ∝ r−1
ρ ∝ r−α
Density profiles from Jeans equations: assumptions
10 20 30 40 500.01
0.1
1
10
100
R (arcmin)
Draco
0 10 20 30 400
5
10
15
20
R (arcmin)
1 5 10 500.01
0.1
1
10
R (arcmin)
Carina
0 10 20 300
5
10
15
R (arcmin)
• Masses:
• Conclusion: dSph kinematics are consistent with cored haloes
Density profiles from Jeans equations
3− 8× 107 M! =⇒ M/L = 13− 240 M!/L!
r (kpc)
ρ(M
!pc
−3
)
Ursa Minor
Draco
LeoII
LeoI
Carina
Sextans1/r
10−1 10010−3
10−2
10−1
100
Gilmore et al. (2007)
• Majority of current data consistent with cored dark matter distributions and a mass scale (interior to light) of
• Mean dark matter density
• NFW halo:
Global trend of dSph haloes
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Mateo et al. (1998), Wilkinson et al. (2006), Gilmore et al. (2007)
∼ 3× 107M!
ρ(r < 10 pc) ≈ 60 M! pc−3(2 TeV/c2cm−3)
! 0.1 M! pc−3(5GeV/c2 cm−3)
Size distribution of stellar systems
Gilmore et al. (2007)
The next step - resolving the cusp/core issue with the VLT
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• Degeneracy between velocity anisotropy and inner density profile can be broken using ~500 radial velocities in the inner 0.2 kpc
• Program to apply this to Carina dSph currently underway at VLT
Conclusions
• Dwarf spheroidal galaxies are valuable laboratories for testing the properties of dark matter
• No kinematic evidence that tides have inflated central velocity dispersions of dSphs
• Current kinematic data are consistent with high M/L ratio, cored dark matter distributions - mass scale ( ):
• Mean dark matter density:
• In cusped halo:
• More data being obtained to place more robust constraints on density profiles
∼ 3× 107M!r ! 400 pc
ρ(r < 10 pc) ≈ 60 M! pc−3(2 TeV/c2cm−3)
! 0.1 M! pc−3(5GeV/c2 cm−3)