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Tales of Dark Matter on Small Scales

Western University Friday, April 13, 2012. Tales of Dark Matter on Small Scales. Niayesh Afshordi. “Friday the 13th is a date considered to be bad luck in Western superstition .” -Wikipedia. Introduction: Cold Dark Matter (CDM) Why phase space of CDM haloes is hierarchical

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Tales of Dark Matter on Small Scales

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  1. Western University Friday, April 13, 2012 Tales of Dark Matter on Small Scales Niayesh Afshordi

  2. “Friday the 13this a date considered to be bad luck inWesternsuperstition.” -Wikipedia.

  3. Introduction: Cold Dark Matter (CDM) Why phase space of CDM haloes is hierarchical Too big to Fail? A Tale of Small Scale Bound Structures & CDM detection Future Prospects for dark matter astronomy Outline

  4. Roya Mohayaee (IAP, Paris) Ed Bertschinger (MIT) Kathryn Zurek (U-Michigan) Louis Strigari (Stanford) ShantBaghram (U-Waterloo/PI) NA, Mohayaee, Bertschinger:Phys.Rev.D79:083526,2009 NA, Mohayaee, Bertschinger: Phys.Rev.D81:101301, 2010 Baghram, NA, Zurek:Phys.Rev.D84:043511,2011 Baghram, NA, Strigari: in preparation Collaborators

  5. Introduction: Cold Dark Matter (CDM) Why phase space of CDM haloes is hierarchical Too big to Fail? A Tale of Small Scale Bound Structures & CDM detection Future Prospects for dark matter astronomy Outline

  6. Bullet Cluster Baryons (X-ray)Matter (lensing) Dark Matter is Collisionless

  7. Dark Matter is Cold Zaldarriaga & Tegmark 2002

  8. Dark Matter can annihilate to • photons (Fermi) • electrons/positrons (PAMELA/Fermi/WMAP…Planck) • Neutrinos (IceCube) Indirect Detection Springel, et al. 08

  9. Solar system is moving towards the Cygnus constellation in the Milky Way, at 220 km/s  We should see a DM wind coming from Cygnus, which distinguishes it from any other background Directional DM detection 250 km/s Sciolla et al 2008

  10. Gravitational evidence for DM comes from potential: d3xρ(x)/|x-x’| • Direct detection: ρ(x) at solar system • Indirect detection (annihilation): d3xρ(x)2 WIMP detection is much more sensitive to DM sub-structure: Boost = <ρ2>/<ρ>2-1 • Boostto annihilation signal (indirect detection) (e.g., Taylor & Silk 2003) • Variance for direct detection (e.g., Stiff, Widrow, & Frieman 2001) WIMPs: from gravity to detection

  11. Introduction: Cold Dark Matter (CDM) Why phase space of CDM haloes is hierarchical Too big to Fail? A Tale of Small Scale Bound Structures & CDM detection Future Prospects for dark matter astronomy Outline

  12. CDM is really cold • vCDM~10-11c • Phase space density remains constant in lieu of collisions (Liouville’sTheorem): • At CDM freeze out: • Within our Galaxy: • Most of the phase space is empty! • Structures on all scales Hierarchy in the Phase space !!??

  13. Hierarchical Micro-Structure of the Phase Space time

  14. Bound sub-haloes (cluster in real/phase space) Tidal debris (cluster in initial conditions) Fundamental discreteness  analogous to galaxy shot noise (NA, Mohayaee, Bertschinger 2009; Vogelsberger & White 2011) Three types of phase structures Hayashi et al. 2003 Kuhlen, Diemand, et al. NGC 3923

  15. Large Scale Structure Projected Correlation Function SDSS Masjedi, et al. Correlation Functions quantify Hierarchy Phase Space Correlation Function unbound structure bound substructure

  16. Introduction: Cold Dark Matter (CDM) Why phase space of CDM haloes is hierarchical Too big to Fail? A Tale of Small Scale Bound Structures & CDM detection Future Prospects for dark matter astronomy Outline

  17. Subhalos in are dynamically can not host the most luminous dwarf satellites of Milky Way: (Too big to fail?) (Michael Boylan-Kolchin, James S. Bullock, ManojKaplinghat, arXiv :1103.0007) Aquarius simulation: high resolution simulation of 6 Milky- way type halo Too Big to Fail Problem? Baghram, NA, &Strigari, in prep. Courtesy of ShantBaghram: http://pirsa.org/12040110/

  18. *Binning in the Energy and Angular momentum *Non- smooth phase-space density • Now we can rewrite the observable density of stars, by plugging the phase space density function definition as : • Marginalize over fab’s Tricks of the Trade! L E Number of binning = number of free parameters

  19. Introduction: Cold Dark Matter (CDM) Why phase space of CDM haloes is hierarchical Too big to Fail? A Tale of Small Scale Bound Structures & CDM detection Future Prospects for dark matter astronomy Outline

  20. Small sub-haloes become resilient to tidal stripping Stable clustering hypothesis: # of pairs at small physical separation remains constant (Davis & Peebles 77) We extend this to the phase space Unlike the halo model, captures the full hierarchy: sub-haloes, sub-sub-haloes, etc. (also much fewer parameters) Bound Substructures

  21. Stable clustering in phase space can be used to describe bound subn-halo hierarchy sub-haloes and stable clustering 0.03 Boost »106 (½crit,0/½) Standard deviation of linear overdensity on mass scale M Kuhlen, Diemand, et al. NA, et al. 2010

  22. DM annihilation profile:stable clustering vs. simulations Msub > 105M Msub > 106M Msub > 107M Msub > 108M smooth halo M(<r)/M200 Annihilation Luminosity (<r) Bound substructure +tidal cut-off Springel, et al. 08

  23. Massaging the tidal stripping prescription …

  24. Temporal auto-correlation of DM detection in several years… sub-haloes in Direct Detection!

  25. From bound subhaloes: Boost = O(1) = density variance @ solar radius Local DM wind may NOT come from the direction of Cygnus! Phase space correlation will be probed by directional DM detection Dark Matter Astronomy? unbound structure bound substructure Hansen et al. 2005, Vogelsberger et al. 2009, Kuhlen et al. 2010

  26. Dark Matter subhaloesmay even impact Pulsar Timing Hertzberg, Siegel, Fry 2007

  27. Dark Matter subhaloesmay even impact Pulsar Timing log(Pulsar residual power spectrum) log(Pulsar residual power spectrum) Doppler Effect Shapiro Delay Baghram, NA, & Zurek, 2011

  28. CDM has lots of structure on small scales: • Boost in DM annihilation • Time dependence in direct detection signal • Rich structure for directional DM detection • Potentially detectable Pulsar Timing residuals • Correlation Function in Phase Space is a Powerful tool ( stable clustering) • Measuring Dark Matter masses from stellar kinematics is a tricky business Lessons

  29. What about baryons? • They can enhance CDM structures through adiabatic contraction, or destroy them via gravitational collisions/heating • Real haloes are almost certainly more complicated! • Why not just extrapolate simulations? • Simulations are NOT scale-free  tidal limit • Neither is CDM over 10 orders of magnitude! Two Cautionary Notes…

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