First detection of dark matter How Much Dark Matter is There?

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1 Galaxies AS7007, 2012 Lecture 3: Dark matter in galaxies Outline I What is dark matter? How much dark matter is there? How do we know it exists? Dark matter candidates The Cold Dark Matter (CDM) model Outline II What is Dark Matter? Dark halos and subhalos Problems with CDM Dark matter annihilation Missing baryons Dark Matter Luminous Matter First detection of dark matter How Much Dark Matter is There? Ω Μ = ρ Μ / ρ c Recent measurements: Ω Μ 0.27 Ω Λ 0.73 Ω Lum ~2% (Luminous) ~98% (Dark) Fritz Zwicky(1933): Dark matter in the Coma Cluster 1

2 How Do We Know it Exists? Dynamics of Galaxies I Cosmological Parameters + Inventory of luminous material Dynamics of galaxies Dynamics and gas properties of galaxy clusters Gravitational Lensing Galaxy Stars + Gas + Dust + Supermassive Black Hole + Dark Matter Dynamics of Galaxies II Dynamics of Galaxy Clusters V rot Observed Visible galaxy Balance between kinetic and potential energy Virial theorem: Expected R M vir = v 2 G R G Visible galaxy R Dark matter halo Check outsect in Schneider for details Hot Gas in Galaxy Clusters Gravitational Lensing X-ray gas, T= K High mass required to keep the hot gas from leaving the cluster! If gas in hydrostatic equilibrium Luminosity and temperature profile mass profile 2

3 Gravitational LensingII Intermission: One of theseis not a lens whichone? Baryonic and non-baryonic matter Ω Μ 0.27 Ω baryons 0.04 Mostof the matter(85%) in the Universe shares no resemblanceto the matter we know from everyday life! Particles with 3 quarks, like the proton and neutron A Few Non-baryonic* Dark Matter Candidates Supersymmetric particles Axions Sterile neutrinos Primordial black holes Preon stars Quark nuggets Mirror matter Matter in parallel branes Kaluza-Klein particles * or evading current constraints on the cosmic baryon density What is supersymmetry(susy)? A high-energy extension of the standard model SUSY predicts a symmetry between bosons and fermions: Standard particle SUSY partner fermion(e.g. quark) boson (e.g. squark) boson (e.g. photon) fermion(e.g. photino) Zoo of new particles: selektrons, sneutrinos, gluinos, Higgsinos, gravitinos, axinos... WeaklyInteractingMassive Particles(WIMPs) Interactions through weak force and gravity only dark matter transparent Weak-scale interactions right cosmological density to be dark matter ( The WIMP miracle ) Massive (GeV to TeV scale) No WIMP candidate in standard model of particle physics The canonicalwimp is a SUSY particle(oftena neutralino), but not all WIMP candidates are SUSYs 3

4 WIMPs in your morning coffee Hot and Cold Dark Matter Hot Dark Matter (HDM) Relativistic early on (at decoupling) Ruled out by observations Generic assumptions( 100 GeV WIMPs) Handful of WIMPs in an average-sized coffee cup Cold Dark Matter (CDM) Non-relativistic early on (at decoupling) The standard model for the non-baryonic dark matter Successful in explaining the formation of large scale structure (galaxies, galaxy clusters, voids and filaments) Additional Assumed CDM Properties The Universe according to CDM Collisionless interacts mainlythrough gravity Dissipationless cannot cool by radiating photons Long-lived particles Behaves as perfect fluid on large scales The dark matter halo Voids, halos and filaments Void: low-density region Halo: high-density region Filament: connects the halos Schematic illustration Whatit looks like in actual N-body simulations 4

5 A hierarchy of dark matter halos All galaxyclusters and almostall galaxiesform at the centreof dark matterhalos Halo massrange: Msolar M halo > Msolar: Galaxygroupsand clusters M halo Msolar: Largegalaxies M halo Msolar: Dwarfgalaxies M halo <10 8 Msolar:??? Largely untested part of the CDM paradigm The veryfirst stars are predictedto form in these halos at z>15, butwhereare thesehalos now? A hierarchyof dark matterhalos II Halo massrange: Msolar Lowercutoffdependson detailedpropertiesof the dark matterparticles, couldbe to 10 7 Msolar, depending on the model Mass function shape: Always far more low-mass halos than high-mass ones Low-masshalos assemblefirst, thenmergeto form high-mass ones Second small halo falls intothe bigone The formation of a halo Small halo falls intobigone Disruption begins- big halo grows more massive First small halo completely disrupted Time The Aquariussimulation (Springelet al. 2008) Subhalos The tumultuous life of a subhalo Massive halos are assembled by the accretion of halos of lowermass Manyaccretedhalos get disruptedin the tidal fieldof the halo theyfellinto, butsome temporarily survive in the form of subhalos On average 10% of the massof a halo is in the form of subhalos at the currenttime 5

6 Intermission: What does this picture have to do with subhalos? Dark halo density profiles I Dark halo Visible galaxy r Famousdark matter-only, N-bodysimulations by Navarro, Frenk& White (1996, 1997) ρ NFW ρs ( r) = ( r / r ) s ( 1+ r / r ) 2 NFW profilenowslightlyoutdated, butstill in active use s ρ r -1 at small r ρ r -3 at larger CDM problem I : The core/cuspissue Predicted by dark matter-only simulations based on CDM(density cusp) Favoured by observations of dark matterdominated galaxies (density core) Possiblesolution: Baryonic processes(supernova explosions, feedback ) may have altered the CDM density profile (Governato et al. 2010, Nature) Density profiles of real galaxies I Singular Isothermal sphere ρ( r0 ) ρ SIS( r) = ( r / r ) 0 2 σ(r) = constant ρ(r) whenr 0 M(<r) whenr Outer truncation required! Works reasonably well for massive galaxies acting as strong gravitational lenses, probably due to baryon-domination in the centre Density profiles of real galaxies II Pseudo-isothermal sphere (cored) CDM problem II: Missing satellites Should not dwarf galaxies form inside the subhalos? ρ PIS ρ0 ( r) = 1+ ( r / r ) c 2 ρ(r) ρ 0 whenr 0 M(<r) whenr Outer truncation necessary! Works reasonably well for dark matter-dominated galaxies (dwarfs and low surface brightness galaxies) Naïve expectation Observed A factor of too few satellite galaxies around the Milky Way! 6

7 CDM problem II: Missing satellites Possible solutions: Vanilla CDM incorrect alternative models (e.g. warm dark matter) produce fewer subhalos Star formation in low-masssubhalos inefficient lots of ultrafaint or completely dark subhalos awaitingdetectionaroundthe Milky Way CDM problem II: Missing satellites Confusing input from gravitational lensing: Lensingrequiresmoresubhalos, or at leasta different halo mass function than predicted by CDM z 2 z 0.5 Vegetti et al. (2012, Nature) Data Model Surface mass density WIMP annihilation WIMPs predicted to annihilate in regions where the CDM density is high Subhalos should glow in gamma-rays Subhalo Fermi Gamma-ray Telescope Mass-to-Light Ratios Mass-to-light: M L M L solar solar Launched in 2008, but still no clear-cut signatures of WIMP annihilation in subhalos Observed luminosity Different choices for M: M tot = Total mass Dynamical mass-to-light ratio M stars = Mass of stars & stellar remnants Stellar mass-to-light ratio 7

8 Mass-to-Light Ratios II What are M/L-ratios good for? The mass-to-light ratio indicates how dark matter-dominated a certain object is Higher M/L More dark-matter dominated Typically: (M/L) stars < 10 (from models) (M/L) tot ~100 for large galaxies (M/L) tot ~ 300 for galaxy clusters (M/L) tot ~ 1000 for ultrafaint dwarf galaxies (M/L) tot > (M/L) stars Dark matter! Mass-to-Light Ratios III Dwarf galaxies Perhaps too steep Galaxy groups Milky Way Galaxy clusters Modelby Van den Bosch et al. (2005) About1/3 of the cosmicbaryonsstill unaccounted for at z=0 Baryonfractionf b below cosmic averagein nearlyall galaxies Some of the missing baryonsmaybe in the warm intergalacticmedium (in between halos) Baryon fractions Baryonic Tully-Fisher McGaugh(2010) Tidal dwarf galaxies TDGsform outof shreddeddisk material Only type of galaxy predicted to be nearly CDM-free ButM/L high Some form of dark matter still present? Dark baryons? 8