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Some observed properties of Dark Matter:

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

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Some observed properties of Dark Matter:

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  1. Some observed properties of Dark Matter: Gerry Gilmore Institute of Astronomy, Cambridge Some details are in ApJ 663 948 2007

  2. 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?

  3. 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

  4. Physical scales of interest correspond to smallest galaxies

  5. 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

  6. Challenges for small-scale CDMall related to the structure of the first bound halos • 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...

  7. 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? ~

  8. 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

  9. 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.

  10. 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’ 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

  11. Assume featureless perturbation power-spectrum, blame problems on astrophysics, or try astroparticles

  12. 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?

  13. 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 spacedensity (~ ρ/σ3) ~ 1/(σ2 rh)

  14. 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

  15. Gilmore etal 2007, plus Sharina etal mnras 384, 1544 2008 surface brightness selection bias

  16. Conclusion : • There is a well-established size bi-modality: • all 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

  17. 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 A&A 446 933 for a recent study

  18. Omega Cen: Reijns etal A&A 445 503 Mass does not follow light Leo II: Koch etal

  19. Mateo, Walker etal large survey: we are joining datasets. Note different scales: information at small and large r poor.

  20. 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 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

  21. 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

  22. 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

  23. 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.

  24. 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

  25. 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

  26. 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

  27. 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.

  28. 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?

  29. Properties of Dark-Matter dominated dSph galaxies:

  30. From kinematics to dynamics:anisotropy vs mass profile degeneracy

  31. 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

  32. 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

  33. 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….

  34. 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

  35. 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?

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