Some observed properties of Dark Matter:

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Some observed properties of Dark Matter

• Gerry Gilmore
• Institute of Astronomy, Cambridge

Some details are in ApJ 663 948 2007
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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?

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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
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Physical scales of interest correspond to
smallest galaxies
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LCDM cosmology is extremely successful on large
scales But the smallest galaxies are the places
one must see thenature of dark matter, galaxy
formation astrophysics
Inner DM mass density depends on the type(s) of
DM
Dwarf galaxy mass function depends on DM type
Figs Ostriker Steinhardt 2003
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Challenges for small-scale CDMall related to the
structure of the first bound halos

• On large gtMpc 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
  zgt1,
• 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...

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Main Focus Dwarf Spheroidals

• Low luminosity, low surface-brightness satellite
  galaxies, classical L 106L?, ?V gtgt 24 mag/sq?
• Apparently dark-matter dominated
• ? 10km/s, 10 lt M/L lt 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?

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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
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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.

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Derived satellite luminosity function
Koposov et al 07 arXiv0706.2687
Open symbols Volume-corrected satellite LF from
DR5
Filled symbols all Local Group dSph
Coloured curves Semi-analytic theory (Benson et
al 02, red Somerville 02, blue) We ignore BIG
surface- brightness discrepancies REALLY NEED MASS
Grey curve power- law fit to data slope 1.1
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Assume featureless perturbation power-spectrum,
blame problems on astrophysics, or try
astroparticles
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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?

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Walcher et al 2005
Dotted line is virial theorem for stars, no DM
There is a discontinuity in (stellar) phase-space
density between small galaxies and star
clusters. Why?
dSph
? Dark Matter?
Phase space density ( ?/s3) 1/(s2 rh)
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Slightly different perspective
(updated data)
M31 MWG Other
boundary
Nuclear clusters, UCDs, M/L 3
Pure stars
Dark Matter haloes
Tidal tails non-equilibrium?
dSph galaxies
star clusters
GG, Wilkinson, Wyse et al 2007
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Gilmore etal 2007, plus Sharina etal mnras 384,
1544 2008
surface brightness selection bias
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Conclusion

• There is a well-established size bi-modality
• all systems with size lt 30pc are purely stellar
• -16lt Mv lt 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 lt r lt 100pc
• 100pc seems a physical scale imprinted on, or by,
  Dark Matter

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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)
• 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 AA 446 933 for a
  recent study

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Omega Cen Reijns etal AA 445 503
Mass does not follow light
Leo II Koch etal
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Mateo, Walker etal large survey we are joining
datasets. Note different scales information at
small and large r poor.
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From kinematics to dynamics Jeans equation, and
full distribution function modelling

• 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 ß(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 ß
Radially varying ß
Leo II
Core slightly favoured, but not conclusive
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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
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Conclusion two

• 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.

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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
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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?)
(old data)
?Globular star clusters, no DM?
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Do the lowest luminosity gals have lower central
velocity dispersions, and lower masses?
Martin etal MN 380 281Simon Geha ApJ 670 313,
2007
But Strigari, Geha etal, 2008, Nat 454 ? no mass
trend
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Consistency?

• 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 4 x 107M?
• 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.

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Implications for Dark Matter

• Characteristic Density 10GeV/c²/cm³
• If DM is very massive particles, they must be
  extremely dilute (Higgs gt100GeV)
• 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?

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Properties of Dark-Matter dominated dSph
galaxies
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From kinematics to dynamicsanisotropy vs mass
profile degeneracy
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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

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Main sequence luminosity functions of UMi dSph
and of globular clusters are indistinguishable.
? normal stellar M/L
HST star counts
Wyse et al 2002
Massive-star IMF constrained by
elemental abundances also normal
M92
?
M15
?
0.3M?
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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.
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Kormendy Freeman 04
Disk galaxy scaling relations are
well- established Do the dSph (green)
fit? Yes, KF
No, they tend to lie OFF the disk galaxy
extrapolations.. Limits instead
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Can feedback modify CDM structure?

• 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?