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Dark Matter Candidates: Particles

This document delves into various dark matter candidates, including Weakly Interacting Massive Particles (WIMPs) and neutralinos, which relate to supersymmetry. It discusses axions, introduced to explain CP violation and their mass limits. Key detection methods focus on the signatures of Earth moving through dark matter. The parameters of cold dark matter (CDM) are evaluated, along with issues like cuspiness and substructure. The document also highlights alternative models such as Modified Newtonian Dynamics (MOND), and the effects of galaxy mergers on dark matter and galaxy morphology.

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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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