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

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

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

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

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

Galaxy Mergers

Orders of magnitude 2x10 12 M  galaxies 300 km/s  J (~ SNe, ~binding energy). 2x10 12 M  galaxies 300 km/s  J (~ SNe, ~binding energy). Power (assuming 1Gyr time): W (1 SN) Power (assuming 1Gyr time): W (1 SN) Stellar collisions VERY rare: Near center, ~1000 stars/ly 2  collision prob ~ Stellar collisions VERY rare: Near center, ~1000 stars/ly 2  collision prob ~ 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~10 6, so M ~1. 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~10 6, so M ~1. Hence old stellar population reconfigures, but new stars may be formed via collisional processes. Hence old stellar population reconfigures, but new stars may be formed via collisional processes.

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

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

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

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

Spiral Galaxy Formation

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

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

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

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

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