1• gas is important mass component of cluster• emission by thermal bremsstrahlung (free-free).• LX~ 1043-1045erg/s (5x1044erg/s for Coma)X-ray emitting gas in clusters• gas is important mass component of cluster• emission by thermal bremsstrahlung (free-free).• LX~ 1043-1045erg/s (5x1044erg/s for Coma)••Hydra A - ChandraHydra A - Optical= hν[CO fig. 27.17]Energy ÆFlux ÆT ~ 107K. Why?• Heated by shocks: infall, radio jets, SNe, etc. • Temp. set by (heating rate) = (cooling rate).• Cooling rate ∫lνdνdepends on nenpT1/2• low density Î high Tamplitudefreq.distr.Had wrong power of T on previous version of this slide.What is Dark Matter?Candidates• Cold dark matter• “cold” means v << c• Leading candidate: Weakly Interacting Massive Particles (WIMPs)• Hot dark matter• “hot” means v ~ c• Leading candidate: neutrinos• Baryonic dark matter• Black dwarfs, black holes, failed stars, etc.• Massive Compact Halo Objects (MACHOs)• General Relativity is wrong• MOND• Other alternate theories of gravityCO pp. 896-898pp. 1232-12332MOdified Newtonian Dynamics (MOND)• Invented as ad-hoc explanation of flat rotation curves for galaxiesSuppose that Fgravfalls off slower than r -2• Originally not relativistically covariant • But there is now a version that does this• Cannot also explain temperature, density structure of galaxy clusters.• Dark matter simultaneously explains:• Flat rotation curves• Gravitational lensing results• Structure formation (coming attraction)We don’t want to trade it in for something that only explains one of these.Existence of Dark Matter: Blue = Total Mass Distribution (deduced from gravitational lensing). Red = Hot Gas Distribution (deduced from X-ray emission). Collision between 2 galaxy clusters. Normal matter (gas) loses energy in collisions. Dark Matter particles do not.movie1 movie23Baryonic Dark Matter• Candidates include black holes, neutron stars, brown dwarfs, cool white dwarfs, etc.• Use gravitational lensing to search for MACHOS• cross section is Einstein radiusθE= (M/Msun)1/2(D/10 kpc)1/2mas• variability timescale t ~ 0.2 (M/Msun)1/2(D/10kpc)1/2(v /200 km/s)-1yr• ==> if entire mass of MW is in Machos, still need to observe 106sources to find one microlensed background source at any given time.• Using LMC, SMC stars as background sources• LMC at 50 kpc, but MW halo goes to 200 kpcOrbiting MACHO crosses our line-of-sight.Gravitational lensing causes brightening.LargeMagellanicCloud• Two major searchesMACHO teamEROS Time (days) ÎFlux ÎMACHO example:Binary star as lensing objectCausticsLight curves4The Result for MACHOsThe Result for MACHOs• MACHO Project: • 5.7 yrs, 11.9 million stars• 13-17 microlensed events• 2-4 expected from known stellar populations•EROS• 3 events towards LMC, 1 towards SMC• Fraction of MW halo in < 1 Msundark objects is < 20-40%.Outof date5Hot & Cold Dark Matter• Dark Matter = matter not coupled to electromagnetic field• unable to condense by dissipation• Hot Dark Matter (HDM) • relativistic for T ~105K• can free-stream out of galaxy-sized matter concentrations. • erases small-scale structures early in life of universe.Î top-down structure formation, starting from large structures with 1013Msun.• Cold Dark Matter (CDM)• slow moving (non-relativistic)• does not erase small concentrations.• preponderance of low-mass structures predicted by inflation survive.Î less massive concentrations form first (bottom up structure formation).Early U. contained more small matter condensations than large ones. Neutrino mass• Hot Dark Matter (HDM) • Prime candidates for HDM are massive neutrinos.• there should be a cosmic background flux of neutrinos similar to CMB.– frozen out at T ~ 1010K• predicted neutrino density = 3nphotons/11• ~100 cm-3at present time• ==> need mν> 50eV/c2 for Ω = 16Measuring the Neutrino MassSuper Kamiokande (Japan)• Large chamber deep underground.• Neutrinos interact (weakly) with water.• 13,000 photocells detect resulting light.• 1998: Found neutrino oscillations• Three types of neutrinos known.• Neutrinos change back and forth between types while in transit.• Can only happen if neutrinos have mass.• But mass is small.• need mν> 50eV/c2for Ω = 1• Mass differences are ∆mν~ 0.1 eV/c2 (+ upper limit on electron neutrino: mν< 2.2 eV/c2)• Still…. mass density of neutrinos ≅ mass density in visible stars.N-body simulations Î CDM• Start with perturbation spectrum at time of decoupling • Follow perturbations into highly non-linear regime.• HDM models become too highly clustered over observed lifetime of galaxiesCDM HDMStandard CDM = SDCM,replaced by ΛCDM model7Cold Dark Matter (CDM)• slow moving• mass power spectrum from inflation only slightly modified by free-streaming• less massive concentrations form first (bottom up).CDM candidates• Axions• zero momentum• very light ==> huge number density needed to make up ΩM• should be detectable within a few years if present.•WIMPs• Weakly Interacting Massive Particles• 50x proton mass• set by the weak interaction cross-section• Leftovers from GUT era• Expansion, cooling of UÎ frozen out of equilibrium reactions• Lots of theories Î lots of candidatesFig. 30.2Grand Unified TheoryCDMMenu of the DayAxionsAxinosGravatinosNeutralinosWimpzillas...Cold Dark Matter in the LabCDM candidates• Axions• zero momentum• very light ==> huge number density needed to make up ΩM•WIMPs• Weakly Interacting Massive Particles• 50x proton mass• set by the weak interaction cross-section• χ neutralino is best candidate• Can be detected through elastic scattering off various target nuclei• measure recoil energy imparted to target• look for seasonal variation due to Earth’s orbital motion• these WIMPS are the MW halo• Massive neutrinos (m ~ 100-1000 mp) already ruled out.• Hope is to identify CDM, then manufacture it in Large Hadron ColliderCDMMenu of the