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Dark Matter Candidates: Particles

Dark Matter Candidates: Particles. WIMPs, particularly LSPs : mass >GeV . Neutralinos : New parity associated with supersymmetry (a way for fermions  bosons). Axions : Invented to explain why weak force violates CP, but strong force does not.

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Dark Matter Candidates: Particles

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  1. Dark Matter Candidates: Particles • WIMPs, particularly LSPs: mass >GeV. • Neutralinos: New parity associated with supersymmetry (a way for fermionsbosons). • Axions: Invented to explain why weak force violates CP, but strong force does not. • 10-6<max<10-3eV: Upper limit from SN1987A cooling; lower from BBN. • Currently neutralinos and axions are best candidates for dark matter; neither has been detected or is predicted in Standard Model.

  2. Dark Matter Detection • To detect, generally look for signatures of Earth moving through DM fluid (seasonal).

  3. Really Cold and Collisionless? • 2 problems with CDM halos: Too cuspy, too much substructure. • Dark matter not cold? • Self-interacting (Spergel & Stein-hardt): Must avoid core collapse! • Fuzzy: 10-22eV Bose condensate. • Decaying: ~½ of DM decays into relativistic particles. • Disappearing: Goes into 5th dimension via brane. • Fluid: Scalar field with quartic potential yields “pressure”. • Probably forgotten some…

  4. Aguirre et al 2001 Modified Newtonian Dynamics (MOND) • MOND proposes that on large scales, F=(GMa0)1/2/r. • Can fit RCs of galaxies extremely well. • Can almost fit CMB: 3rd peak is key. • Runs into trouble in clusters and Ly-a forest. • MOND+baryonic DM? Hmm…

  5. Bullet Cluster: Dark Matter is Collisionless Clowe et al 2006 • Interacting cluster lensing+X-rays shows that mass doesn’t trace baryons. • Exactly as predicted by CDM: Dark matter passes thru, gas is shocked. • Difficult with baryonic DM because high velocities would destroy cold, unseen baryons.

  6. Galaxy Mergers

  7. Orders of magnitude • 2x1012 M galaxies colliding @ 300 km/s  1053 J (~108-9 SNe, ~binding energy). • Power (assuming 1Gyr time): 1037 W (1 SN) • Stellar collisions VERY rare: Near center, ~1000 stars/ly2 collision prob ~ 10-11. • OTOH, ISM filling fraction is high, so molecular cloud collisions common, and highly supersonic (T~100K, v~300 km/s  M~300). Coronal gas has T~106, so M~1. • Hence old stellar population reconfigures, but new stars may be formed via collisional processes.

  8. Toomre & Toomre 1972 Early N-body merger simulations • Holmberg 1941: 74 light bulbs and a lot of patience. • Toomre & Toomre 1972: Mergers cause tidal features. • Barnes & Hernquist 1991: Remnants look like ellipticals, with kinematic features. Holmberg 1941

  9. Mergers fuel starbursts & transform morphologies • Mihos & Hernquist 1996: Included SF (Schmidt Law) in hydro sims. • Gas gets driven into central regions owing to dynamical instabilities, fuels starburst. • Remnant looks something like an elliptical.

  10. Merger Trees & Semi-analytic models Wechsler et al 2001 • CDM is a “bottom-up” structure formation model. • Dark matter has no known pressure; it collapses immediately into small units (size unknown). • Units merge thru gravitational instability. • Semi-analytic models (SAMs): Merger tree + MMW disks + heuristic algorithm for how mergers affect galaxies.

  11. Ellipticals: Nature vs. Nurture • Can ellipticals form mostly from low-spin halos? • No! Not enough. • But not totally clear that mergers alone can explain it either… • In simulations, gas reaccretes, E’s  S’s! • Not only must merge spirals, but also prevent reaccretion.

  12. Spiral Galaxy Formation

  13. boxy disky Rotation- supported Pressure- supported Kinematics of merger remnants • Can mergers reproduce E isophotes? • Large E’s boxy, small E’s disky (Davies et al 83). • Naab etal: Put in merger tree, try to reproduce fraction of anisotropic (non-rot) E’s. • Spiral-spiral mergers alone can’t do it! • Need E-E/E-S mergers… • also needs gas supply shut-off above some M*. Naab, Kochfar, Burkert 06

  14. Dry Mergers • If halos merge late, but stars are old dry mergers! • Do dry mergers preserve tight E properties? • Fundamental plane: Rsa I-b. • Red sequence: Tilted!

  15. Clusters & Galaxy Harassment Moore, Katz, Lake 1997 • In clusters, scl»sgal Direct collisions rare! • But morphologies still altered due to harassment: Tidal disturbance from close passage. • Can help explain why clusters have ~no spirals.

  16. Spiral + Spiral = Spiral • If initial systems is gas rich enough, then gas flung to large radii can reaccrete into a spiral. • So gas fraction is another parameter in morphological transformations. • To produce late-type galaxies today, need to prevent growth of bulge  AGN? Robertson et al 2006

  17. Mergers  AGN? • diMatteo, Springel, Hernquist: Assume some fraction of inflow at resolution limit (~100 pc) reaches central BH. • Add feedback energy, grow BH. • Significantly suppresses post-merger SF. • Get red sequence, MBH-s relation, etc. • Realistic? Springel, di Matteo, Hernquist 2005

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