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Some observed properties of
Dark Matter:
Gerry Gilmore
Institute of Astronomy, Cambridge
Some details are in ApJ 663 948 2007
Particle-astrophysics joint challenges
•MSSM has 120+ free parameters…
• neutrino masses and mixing
• baryogenesis
– matter anti-matter asymmetry
• dark matter
• dark energy
Scale invariant power spectrum implies a UV
divergence
 a physical cut-off at some scale:
Is that scale astrophysically relevant?
Can it be deconvolved from feedback?
Cosmological simulations are extremely impressive, but are limited:
they are based on an extrapolation from observation, not on
fundamental principles. This is good!
Comparing simulations to data is testing the physics which limits the
extrapolation
slide from Dominic Schwarz
Physical scales of interest correspond to smallest galaxies
LCDM cosmology is extremely successful on large scales
But: the smallest galaxies are the places one must see the
nature of dark matter, & galaxy formation astrophysics
Dwarf galaxy mass function
depends on DM type
Inner DM mass density
depends on the type(s) of DM
Figs: Ostriker & Steinhardt 2003
Challenges for small-scale CDM
all related to the structure of the first bound halos
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On large [>Mpc] scales LCDM is an astonishingly good
description of data, but n~-1 (and maybe w=-1….) so not
much astro-physics beyond boundary conditions
On galaxy scales there is an opportunity to learn some
physics: everything should happen late. But.
0: big old galaxies, big old disks, SFR peaks z>1,
1: the MWG has a thick disk, just one of them, and it is
old. This seems common.
2: massive old pure-thin-disk galaxies exist. Should they?
3: Sgr in the MWG proves late minor merging happens…
4 the substructure challenge – where are the bodies?
5 the feedback challenge: what is it
6 the early enrichment challenge: what did it? When?
Simulations do not reproduce real galaxies...
Main Focus: Dwarf Spheroidals
 Low luminosity, low surface-brightness satellite
galaxies, ‘classical’ L ~ 106L, V >> 24 mag/sq
Apparently dark-matter dominated
 ~ 10km/s, 10 < M/L < 100
 Metal-poor, all contain very old stars; but
~
intermediate-age stars dominate
Most common galaxy [at least nearby]
Are they the first halos, as CDM requires?
How many are there?
23 known, 15 recent discoveries, 13 by IoA group
One we can answer! How many were there?
The satellite number problem: not just MWG
LMC should have many satellites – easy to see in HI – but not seen.
predictions running out just where the data are today?
Standard CDM galaxy formation seems to have
only two robust predictions:
there should be many small halos
the small-scale mass profiles should be cusped
and both these are disputed….
Ishiyama, Fukushige, Makino 0708.1987. dashed line from Moore etal
CDM predicts many more satellite haloes than
observed galaxies, at all masses (Moore et al 1999)
Luminosity Function OK? if suppress star formation
in many? (e.g. Bullock et al 02.) Mass function critical.
Derived satellite luminosity function
Koposov et al 07 arXiv:0706.2687
Open symbols:
Volume-corrected
satellite LF from DR5
Filled symbols:
‘all Local Group dSph’
Grey curve: powerlaw ‘fit’ to data: slope 1.1
Coloured curves:
Semi-analytic theory
(Benson et al 02, red
Somerville 02, blue)
We ignore BIG surfacebrightness discrepancies
REALLY NEED MASS
Assume featureless perturbation powerspectrum, blame problems on
astrophysics, or try astroparticles
Mass measurements: the context
• Why are there no CDM-dominated substructures inside
low-mass gas-rich galaxies? They are predicted to be
very common, long-lived, and are ideal gas traps.
• Dark halos are `predicted’ down to sub-earth
masses; but…
• Neither the local disk, nor star clusters, nor
spiral arms, nor Giant Molecular (gas) Clouds,
nor the solar system, have associated DM:
Given the absence of a very local enhancement,
what is the smallest scale on which DM is
concentrated? How can sub-halos in dSph
galaxies with star formation over 10Gyr avoid
collecting any baryons?
Dotted line is virial theorem
for stars, no DM
There is a discontinuity
in (stellar) phase-space
density between small
galaxies and star
clusters.
Walcher et al 2005
dSph
Why?
 Dark Matter?
Phase space density (~ ρ/σ3) ~ 1/(σ2 rh)
Slightly different perspective…
(updated data)
M31; MWG; Other
Pure stars
boundary
Nuclear clusters,
UCDs, M/L ~ 3
Dark Matter
haloes
Tidal tails:
non-equilibrium?
star clusters
GG, Wilkinson, Wyse et al 2007
dSph galaxies
Gilmore etal 2007, plus Sharina etal mnras 384, 1544 2008
surface
brightness
selection bias
Conclusion :
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There is a well-established size bi-modality:
 all
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systems with size < 30pc are purely stellar
−16< Mv < 0 (!!) M/L ~ 3;
 all systems with size greater than ~100pc
have a dark-matter halo, and self-enrich
 There are no known (virial equilibrium)
systems with half-light radius 30 < r < 100pc
 100pc seems a physical scale imprinted on, or
by, Dark Matter
Mass measurements: ancient history
• The ``standard’’ value for local DM at the Sun is
0.3GeV/c²/cm³, all in a `halo’ component
• (cf pdg.lbl.gov: Eidelman etal 2004)
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the original work, and origin of this value, is the first
analysis to include a full 3-D gravitational potential,
parametric modelling, and a direct determination of
both the relevant density scale length and kinematic
(pressure) gradients from data, allowing full DF
modelling:
Kuijken & Gilmore 1989 (MN 239 571, 605, 651), 1991
(ApJ 367 L9);
Gilmore, Wyse & Kuijken (ARAA 27 555 1989):
cf Bienayme etal 2006 A&A 446 933 for a recent study
Omega Cen: Reijns etal A&A 445 503
Mass does not follow light
Leo II: Koch etal
Mateo, Walker etal large survey: we are joining datasets.
Note different scales: information at small and large r poor.
From kinematics to dynamics: Jeans equation,
and full distribution function modelling
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Relates spatial distribution of stars and their velocity dispersion
tensor to underlying mass profile
Either (i) determine mass profile from projected dispersion
profile, with assumed isotropy, and smooth functional fit to
the light profile
 Or (ii) assume a parameterised mass model M(r) and
velocity dispersion anisotropy β(r) and fit dispersion profile
to find best forms of these (for fixed light profile)
We use distribution function modelling, as opposed to velocity
moments: need large data sets. DF and Jeans’ models agree
Show Jeans’ results here for most objective comparison.
King models are not appropriate for dSph
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Core properties: adding anisotropy
Koch et al 07
AJ 134 566 ‘07
Fixed β
Radially varying β
Leo II
Core slightly favoured, but not conclusive
Derived mass density profiles:
Jeans’ equation with
assumed isotropic
velocity dispersion:
all consistent with
cores.
CDM predicts slope of
-1.3 at 1% of virial radius
and asymptotes to -1
(Diemand et al. 04)
NB these Jeans’ models are to provide the most objective
sample comparison – DF fitted models agree with these
Conclusion two:
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High-quality kinematic data exist
mass analyses  prefers cored mass profiles
Mass-anisotropy degeneracy allows cusps
Substructure, dynamical friction  prefers cores
Equilibrium assumption is valid inside optical radius
More sophisticated DF analyses agree well
Cores always preferred, but not always required
Central densities always similar and low
Consistent results from available DF analyses
Now: extend analysis to lower luminosity systems
[difficult due to small number of stars];
Integrate mass profile to enclosed mass.
Constant mass scale of dSph?
Based on central velocity dispersions only;
line corresponds to dark halo mass of 107M
Mateo 98
ARAA
dSph filled
symbols
2007: extension of dynamic range [UMa, Boo, AndIX],
new kinematic studies:
Mateo plot improves.
Mass enclosed within stellar extent ~ 4 x 107M
Now a factor of 300+ in
luminosity, 1000+ in M/L
Scl – Walker etal
If NFW assumed, virial masses
are 100x larger, Draco is the
most massive Satellite (8.109M)
Globular star clusters, no DM
(old data)
Do the lowest luminosity gals have lower central velocity
dispersions, and lower masses?
Martin etal MN 380 281;Simon & Geha ApJ 670 313, 2007
But: Strigari, Geha etal, 2008, Nat 454  no mass trend
Consistency?
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A minimum half-light size for galaxies, ~100pc
Cored mass profiles, with similar mean mass densities ~0.1M/pc3,
~5GeV/cc
An apparent characteristic (minimum) mass dark halo at 600pc, mass
7
~4 x 10 M
A characteristic mass profile, convolved with a
characteristic normalisation, must imply a
characteristic mass  internal consistency
The information in the Mateo plot is the same as in the size
and mass profile relations
This perhaps continues to the lowest luminosity systems: DM
mass and (baryon) luminosity become uncoupled.
Implications for Dark Matter:
 Characteristic Density ~10GeV/c²/cm³
If DM is very massive particles, they must be
extremely dilute (Higgs >100GeV)
Characteristic Scale above 100pc, several 107M
 power-spectrum scale break?
 This would (perhaps!) naturally solve the
substructure and cusp problems
 Number counts low relative to CDM
 lots of similar challenges on galaxy scales
Need to consider a variety of DM candidates?
Properties of Dark-Matter
dominated dSph galaxies:
From kinematics to dynamics:
anisotropy vs mass profile degeneracy
Implications for/from Astrophysics:
Can one plausibly build a dSph as
observed without disturbing the DM?
 Star formation histories and IMF are easily
determined  survival history, energy input…
Chemical element distributions define gas flows,
accretion/wind rates,
 debris from destruction makes part of the field stellar
halo: well-studied, must also be understood
 `Feedback’ processes are not free parameters
Main sequence luminosity functions of UMi dSph
and of globular clusters are indistinguishable.
 normal stellar M/L
HST star counts
Wyse et al 2002
M92 
M15 
0.3M
Massive-star
IMF constrained
by elemental
abundances –
also normal
Issues: quoted masses assume mass follows light
the two studies disagree by 2sigma for 2 of 3 gals in common.
quoted dispersions not resolved by the RV errors
The available data cannot measure low masses.
Better data are needed….
Kormendy & Freeman 04
Disk galaxy scaling
relations are wellestablished
Do the dSph
(green) fit?
Yes, KF
No, they tend to lie
OFF
the disk galaxy
extrapolations..
Limits instead
Can feedback modify CDM structure?
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Maschenko etal Sci 319 174 2008; Mayer etal Nat 445 738 2007;
Read et al 2006 MN 367 387, MN 366 429, 2005 MN 356 107
More extreme conditions are needed to modify the mass
than are deduced from the stellar populations
DM halos certainly
respond to tides and
mass-loss, but secularly
If history leaves
similar mass profiles,
can it be dominant?