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Dark Matter/Dark Energy Do we need it? How Much? Where? What is It? Hans-Walter Rix January 28, 2004 Observing the Big B

Dark Matter/Dark Energy Do we need it? How Much? Where? What is It? Hans-Walter Rix January 28, 2004 Observing the Big Bang and its Aftermath. Historic Overview Evidence for Dark Matter On galactic scales Galaxy clusters Abundance of massive clusters “Large scale structure”

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Dark Matter/Dark Energy Do we need it? How Much? Where? What is It? Hans-Walter Rix January 28, 2004 Observing the Big B

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  1. Dark Matter/Dark EnergyDo we need it?How Much?Where?What is It?Hans-Walter RixJanuary 28, 2004Observing the Big Bang and its Aftermath

  2. Historic Overview • Evidence for Dark Matter • On galactic scales • Galaxy clusters • Abundance of massive clusters • “Large scale structure” • Measuring the expansion history W,L • Ho measurements • Supernovae • (CMB) • The Complementarity of the Approaches • Nature of the dark matter/dark energy

  3. Dark Matter: How it all started • Zwicky: 1933 • Applies virial theorem to individual galaxies in Coma • Applies virial theorem to the ensemble of galaxies •  Mtot >> N x Mgalaxies Zwicky 1933, Helvetica Physica Acta, 6, 110

  4. 1st rotation curve of M31 (1939) • Concluded: must be dim stars in the outer parts of M31 • 1959: Woltjer and Kahn: • M31 is approaching (returning after initial expansion on elongated orbit). • What mass is needed for torbit < tuniverse? M>1.8x1012 Msun !!

  5. Dark Matter Evidence from “Rotation Curves” • E.g. NGC3198: Begeman 1989 • HI (neutral hydrogen)more extended than stars, measure 21cm line  flat rotation curve -- found in all spiral (=gas rich) galaxies! Vc=const M~r or r~r-2

  6. X-ray image from ROSAT of M87 Dark Matter Evidence from X-rays • How much mass does it take to keep hot gas in hydrostatic equilibrium? •  total mass grows M~r also around big ellipticals

  7. Dark Matter From Satellites Portrait of the “Local Group” (Grebel 2001) Prada et al 2003 “Stacked satellite velocities” of 500 spiral galaxies D.M. halos extend to >200kpc

  8. Stellar density contours of Draco from SDSS Odenkirchen et al 2001  Draco is a bound system in equilibrium Dark Matter Evidence Nearby:the Draco dwarf galaxy Sky image of Draco Dsun 70 kpc

  9. Stellar density profile of Draco Giant stars with velocities measured • Estimate *(r) from stellar distribution • Giant stars as kinematic tracers • - need velocity precision of  3 km/s Radial Profile and Kinematics of Draco • Modelling options • Stars only tot(r) = *(r) • Stars + DM: tot(r) = *(r) + DM(r)

  10. Velocity dispersion profile Jeans equation model Anisotropy Enclosed mass Try models with different DM profiles  M (<10‘) well constrained Mass Modelling of Draco Expected (M/L)* ~ 2  Draco is dark matter dominated

  11. Rotation Curves of Spiral Galaxies

  12. Rotation Curves of Spiral Galaxies • Rotation curves show that DM is needed • Total (stars,gas,DM) rotation curve is v~const. for 2-8 Rexp • A so-called non-singular isothermal (s=const.) DM distribution often fits well: • But, is this dark matter profile • Physically motivated? • Physically plausible? • Expectation from cosmological simulations: NFW profile • r~r-1 at small radii and • r~r-3 at large radii

  13. Van Albada et al 1985, ApJ, 295, 305 Navarro 1997 Degeneracies in Fitting Rotation Curves Rotation curves do not contain enough information to: Determine the ratio of star to DM mass Distinguish the radial profile of DM Dark matter at small radii is poorly understood!

  14. Satellites to the Milky Way  tracers of the mass in the halo SDSS sample: isolated MW-like galaxies 0.5 satellites per galaxy x 1000 galaxies  Synthetic galaxy with 500 satellites unbound systems Dark Matter in Galaxy HalosPrada et al 2003 • MW-like galaxies are at the center of dark matter halos that extend to >200 kpc • DM density profile in the outer parts r~r-3 • identify satellite candidates • make a conservative rejection of unbound systems • calculate resulting velocity dispersion of satellites • compare to cosmological halo formation models  good match

  15. Projected Mass Overdensity Projected Radius Galaxy-Galaxy Lensing • As clusters, individual galaxies distort background images, too. • Yet, these distortions are much smaller • Co-add signal from many equivalent (?) galaxies • Galaxy-galaxy lensing signals show that galaxy halos extend far (>200 kpc) Strong galaxy-galaxy lensing

  16. Dark Matter in Galaxy Clusters • Orbital motions of the galaxies • X-ray gas • Gravitational lensing

  17. T = 106 K  X-ray emission Dark Matter in Galaxy Clusters • In galaxy clusters the masses can be measured three ways • Galaxy clusters contain hot gas ( bound by dark matter?) • Galaxy velocity dispersion • Gravitational lensing

  18. X-Ray Gas in Hydrostatic Exquilibrium Mstars~Mgas~3x1013MSun Mtot,cluster(Rvirial)~1015MSun

  19. Mass Census in Clusters(White et al 1993) • Stars + Hot Gas  baryonic mass • Dynamics, etc..  total mass • Within the “Abell radius” there was not enough time to concentrate baryons  Mbary/Mtotal should be cosmic average • Observed Mbary/Mtotal~1/8 e.g. in Coma cluster • From nucleosynthesis: Wbary~0.035 (H0=70) • Wtotal ~ 0.3

  20. W=1,L=0 z=2 Density profile of low-mass halo z=1 Density profile of high-mass halo z=0 What to Expect for Dark Matter Halos?(e.g. Navarro, Frenk and White 1997) • Start with “cosmological simulations” • Isolate, and re-simulate at higher resolutions sub-regions that will lead to a halo.

  21. Universal Dark Matter Halo Profiles(e.g. Navarro, Frenk and White 1997) • For all simulations with collisionless, cold, dark matter, regardless of • Initial fluctuation spectrum • Mass of collapsed object • “Cosmology”, i.e. W,L one finds the functional form In simul: d and mass are correlated D.M. halos do not have flat rotation curve! dc Log(vc) Log(Masshalo) D.M. Halos are 1-D Sequence?!

  22. Standard Candles Approach: • Select objects whose intrinsic luminosity can be estimated, either from physical first principles, from empirical calibrations of nearby examples or can be inferred from another distance-independent observable. • Instrinsic luminosity + apparent flux  distance (modulus) Examples: • Cepheids: luminosity estimate from their pulsation period • Spiral Galaxies: luminosity estimate from their disk rotation curve • Type-Ia Supernovae (SNIa): luminosity estimate from their light-curve shape

  23. lightcurves Cepheid Distances • E.g. HST Key-Project to measure Hubble constant, H0 (Freedman, Kennicutt,Mould, et al.) • Compare Cepheid brightness in M81 to LMC and local Milky Way Cepheids  DM81=3.63+-0.34 • This way we can measure distances to Galaxies with where Supernovae exploded. Note:for nearby (<50Mpc) galaxies distance and redshift are correlated with considerable scatter  Measuring H0 is not easy

  24. Perlmutter etal 2002  Supernova Type Ia Distances • SN Ia: white dwarf stars near the Chandrasekhar mass limit (1.4 Msun), where Carbon and Oxygen burn explosively. • Most luminous variety of Supernova. Can be seen to z>1!

  25. SN Ia as Pseudo-Standard Candles Phillips, Hamuy, Ries, Kirshner and others ~1996 • Intrinsically bright SN Ia decline slower • SN Ia: H0=67+-5 km/s/Mpc (Current estimate (all methods): H0=70+-5) Note: • still needs Cepheid calibration • Galaxy velocities differ from the local mean by ~300 km/s  systematic uncertainty in H0 with correction

  26. Distant Supernovae • The distance modulus M-m to a certain redshift z depends on the expansion history, not just the current expansion rate. • Type-Ia Supernovae can be seen to great distances: z>1 probes of the expansion history. • 1998: expansion of the Universe is accelerating (!?) • Riess etal 1998, AJ, 116, 1009 • Perlmutter et al. 1999, ApJ, 517, 565

  27. H0 dL Literature Compilation Nesseris et al 2004 (astro/ph-0401556) High-z SNIa (2004) W=0,L=0 Remember? W=0.2,L=0.8 W=1,L=0

  28. Estimated virial mass X-ray luminosity Estimating WM from the Abundance of the Most Massive Colapsed Objects at Different Redshifts +X-ray luminosity function = f(z)  Wm~0.3 through comparison with cosmological simulations

  29. Other Lines of Evidence For Dark Matter • Gravitational lensing (MSB’s lecture)  dark matter clumping on largest scales • The Cosmic Microwave Background and the curvature of space (Rachel, Friday)  WM~0.27 • The growth of small fluctuations to strong fluctuations (next lecture)

  30. Matter/Energy Content 1st Synopsis • Establish need for DM in various environments • Mean DM density from • Baryonic/total mass in clusters • Abunance of clusters (=growth rate of most massive objects) • Overall expansion history • Angular diameter distance (CMB) • Luminosity distance (SN Ia)

  31. Alternatives to Dark Matter • MOND: Modified Newtonian Dynamics (Milgrom 1980s-) • Ansatz: • for accelerations a less than a0, gravity behaves as a(a/a0) = GM/r2 • as a(r) ~ 1/r of a < a0: flat rotation curves Note: • a < a0 untested in the lab • single value of a0 works for all rotation curves • But: • No relativistic version of MOND • MOND has trouble explaining DM in cluster and far out in halos

  32. Nature of the Dark Matter • Non-baryonic, to reconcile M ~0.27 with primordial nucleosynthesis Wb~0.018 and large-scale structure growth • Cold: must not escape from potential wells • (Cold) Dark Matter Candidates: • Black holes • Low-mass objects (“MACHO”s, free-floating planets) • Elementary particles Massive Black Holes as Dark Matter Candidates • (one) plausible mass range: ~106 Msun (Lacey and Ostriker, 1985) • But, such massive black holes cannot be the dark matter in dwarf galaxies (Rix and Lake, 1993). • E.g. c.a. 80 BH’s in Draco, they would disrupt the galaxies!

  33. MACHO’s: Massive Compact Halo Objects • Potential mass range: 0.08 MSun (stellar limit) to MEarth Observational test: gravitational microlensing • (MACHO and OGLE) experiments. Idea: • if all the dark matter in the Milky Way’s halo was MACHOS • there is a 10-6 chance that a star (e.g. in the Magellanic Cloud) has a MACHO exactly along the line of sight • focussing  brightening of the stars’ image • as stars move  dime dependent light curve. Implementation: monitor 106 stars

  34. Lensing Lightcurve Large Magellanic Cloud Micro-Lensing Cartoon Microlensing Searches

  35. MACHO Mass Halo Mass Fraction in MACHOs Are MACHOs the Dark Matter? • MACHO’s make up (at most) 15% of the Milky Ways halo mass • Inferred mass range: 0.4MSun Why would they be invisible? • MACHOs are an enigma, but certainly not the solution to the dark matter problem Alternative: lensing by ordinary stars in the LMC or MW

  36. WIMPS as Dark Matter Candidates • “cold” Dark Matter: must become non-relativistic already at T >> 104K  clumping • supersymmetric theories (SUSY) can naturally create particle (pairs with their SUSY partner) - lightest SUSY particle stable: neutralino, gravitino, higgsino, etc. • axions: hypothesized, very light particle; may arise in quantum chromodynamics  WIMPS are a plausible, but not firm, consequence of several theories in particle physics

  37. Towards detecting WIMPS • WIMPS: may have exceedingly rare elastic scattering events with crystals and one may measure the recoil. • However: many other particles/processes interact with crystals  high false detection rate. • Reduce background  deep tunnels (e.g. Gran Sasso) • Search for seasonal signature

  38. Other experiments seem to rule out DAMA A first detection? … Or not The DAMA experiment in the Gran Sasso claimed to have found a seasonal variation ? 50 GeV particles PROBLEM: cross-section could be 1000 times smaller than current limits

  39. Summary • W and L can now be well determined • only through a variety of approaches • Mass census, cluster abundance, luminosity distance/angular diameter distance • Need H0 from local measurements • Too early to discriminate L from alternative models • Hypothesis of universal D.M. explains things on man scales • Nature of D.M. has been limited, but is not known

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