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Dark and Luminous Matter in Cluster Mergers

Dark and Luminous Matter in Cluster Mergers. Anja von der Linden KIPAC / Stanford. The Standard Model. The Standard Model. Cold Dark Matter (+ Dark Energy) successfully describes many observations: mass of galaxy clusters galaxy rotation curves Cosmic Microwave Background

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Dark and Luminous Matter in Cluster Mergers

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  1. Dark and Luminous Matter in Cluster Mergers Anja von der Linden KIPAC / Stanford SLAC Annual Program Review

  2. The Standard Model SLAC Annual Program Review

  3. The Standard Model • Cold Dark Matter (+ Dark Energy) successfully describes many observations: • mass of galaxy clusters • galaxy rotation curves • Cosmic Microwave Background • Structure Formation SLAC Annual Program Review

  4. astrophysics has motivated Dark Matter • particle nature of DM likely to be disclosed by Particle Physics • What else can we learn about DM from astrophysics? • our laboratory: (merging) Clusters of Galaxies • form at the nodes of the cosmic web • “bottom up”: small structures form first, merge into larger ones • matter content: ~ 85% DM, ~ 13% (hot) gas, ~ 2% stars/galaxies SLAC Annual Program Review

  5. credit: Volker Springel, MPA SLAC Annual Program Review

  6. Observing Cluster Mergers • Strategy: • combine high-quality, multi-wavelength observations • X-rays: • hot intracluster gas • Chandra, XMM • Optical: • cluster galaxies • Total mass distribution via strong and weak lensing • Hubble Space Telescope, Subaru 8m telescope • Team: Steve Allen Glenn Morris Evan Million Marusa Bradac (now UCSB) Anja von der Linden Doug Applegate plus more collaborators at KIPAC, in Santa Barbara, Hawaii, ... SLAC Annual Program Review

  7. The Bullet Cluster X-rayDark Matter SLAC Annual Program Review

  8. The Bullet Cluster • Head-on collision of two clusters • Smaller cluster has cool, dense gas core (“Bullet”) • Shock front allows velocity estimate: ~ 4500 km/s • Dark Matter coincides with galaxies • Both (nearly) collisionless • Gas separated from Dark Matter • Inelastic collision KIPAC papers on the Bullet Cluster: Bradac et al. 2006, Clowe et al. 2006, Randall et al. 2008 SLAC Annual Program Review

  9. The Bullet Cluster SLAC Annual Program Review

  10. The Bullet Cluster • the Bullet Cluster has been labeled as “Direct Proof of Dark Matter” • very hard to explain without (cold, nearly collisionless) Dark Matter • But is it unique? • Are there systematics in the analysis / geometry? • Significant uncertainties from using only a single object? SLAC Annual Program Review

  11. Hot off the press: MACS0025 • selected in X-rays from the MACS sample (MAssive Cluster Survey, Ebeling et al. 2001, 2007) • characteristic merger signatures: • highly elliptical • shifting of X-ray centroid • distant: z=0.586 • observed with Chandra, HST Bradac et al. 2008, ApJ, in press Chandra press release coming up SLAC Annual Program Review

  12. Multiple Images AB: z=2.8 (spectroscopically confirmed) D: z~2.8 (photometric) C: z~1.0 (photometric) SLAC Annual Program Review

  13. Gravitational Lensing:Total mass SLAC Annual Program Review

  14. Mass + gas + light SLAC Annual Program Review

  15. separation of DM and gas z=0.296 1.5 :1 merger one “fuzz-ball”, one bullet separation of DM and gas z=0.586 equal-mass merger two “fuzz-balls” Bullet vs. MACS0025 SLAC Annual Program Review

  16. Dark Matter Cross Section • Dark Matter clumps have passed through each other • -> typically less than one interaction per particle projected mass surface density (from lensing): number of DM particles: • -> Dark Matter self-interaction cross section: SLAC Annual Program Review

  17. Conclusions • The Bullet Cluster and MACS0025 have different initial conditions and different merger dynamics, • yet both are easily explained by the presence of cold, collisionless Dark Matter. • Such cluster mergers are thus strong evidence for the validity of the Standard Model. • Are there more such systems? • Stay tuned... SLAC Annual Program Review

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