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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
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.
dark matter detection
Dark Matter Detection
  • To detect, generally look for signatures of Earth moving through DM fluid (seasonal).
really cold and collisionless
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…
modified newtonian dynamics mond

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…
bullet cluster dark matter is collisionless
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.
orders of magnitude
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.
early n body merger simulations

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

mergers fuel starbursts transform morphologies
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.
merger trees semi analytic models
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.
ellipticals nature vs nurture
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.
kinematics of merger remnants







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

dry mergers
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!
clusters galaxy harassment
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.
spiral spiral spiral
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

mergers agn
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