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Testing the Equivalence Principle for Dark Matter Using Tidal Streams

Testing the Equivalence Principle for Dark Matter Using Tidal Streams. Michael Kesden, CITA Collaborator: Marc Kamionkowski, Caltech COSMO ‘06 Tahoe City, CA Thursday, September 28, 2006. What is the Dark Matter?.

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Testing the Equivalence Principle for Dark Matter Using Tidal Streams

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  1. Testing the Equivalence Principle for Dark Matter Using Tidal Streams Michael Kesden, CITA Collaborator: Marc Kamionkowski, Caltech COSMO ‘06 Tahoe City, CA Thursday, September 28, 2006

  2. What is the Dark Matter? • Galactic rotation curves, large-scale structure (LSS), galaxy clusters all indicate ΩDM 0.25 • Extensions to the Standard Model offer many possible WIMPs (axions, neutralinos, etc.) • Detection of non-gravitational interactions could help identify DM. What interactions might be observable?

  3. Long-range DM interactions • Perhaps DM interacts with DE • Log(/mPl4)  -120 « 1 •  ~ m … Why now? • Maybe acceleration due to scalar field, just like inflation. Scalar field should couple generically. • String theory includes “dilatons”, light, neutral scalar fields that might interact with DM (Damour et al. 1990, Gubser & Peebles, 2004)

  4. A “5th Force” for Dark Matter? • Long-range DM force interpreted as violation of the equivalence principle (EP), the universality of free fall between stars and DM • Laboratory tests place tight limits on fifth force in visible sector (Su et al., 1994); no such limits for DM • Modeled by Lint = g V = -g2/4r exp{-mr} (Frieman & Gradwohl, 1991) • Force suppressed by a factor 2  g2mPl2/4m2 compared to gravity; how might we detect such a force?

  5. Cosmic Tests for 5th Force • LSS • Attractive DM force enhances structure for (r < m-1) (Gradwohl & Frieman, 1992) • 5th force leads to scale-independent bias (Amendola & Tocchini-Valentini, 2002) • CMB • Models where coupled DE traces DM constrained by WMAP (Amendola & Quercellini, 2003) • Clusters • Baryons preferentially lost during mergers • Is there new test with different systematics, greater sensitivity?

  6. Tidal Disruptions • Galaxies form hierarchically; dwarf galaxies in Local Group continue to merge with Milky Way • Smaller galaxies tidally disrupted by larger hosts at distances R where: rsat > rtid ~ R(msat/2MR)1/3 • Tidal disruption establishes energy scales: • Esat» Etid» Ebin disrupted stars retain similar orbits to satellite; trail/lead with gain/loss in energy

  7. Tidal-stream Asymmetry • Non-uniformity of Galactic gradient leads to natural asymmetry:  • DM force displaces stars from bottom of satellite’s potential well, a new DM-induced asymmetry  • DM asymmetry exceeds natural asymmetry when:

  8. Sagittarius Dwarf Spheroidal • Sgr dwarf is closest satellite at 24 kpc • Stellar stream observed by 2MASS using M-giants with known age, color-magnitude relation • Surface densities, radial velocities, distances well-measured for leading: -100º <  < -30º trailing: 25º <  < 90º (Law, Johnston, & Majewski, 2005) • Stellar densities also measured by SDSS (Belokurov et al., 2006)

  9. Simulations • N-body simulation of satellite galaxy with: • M = 5  108 M, M/L = 40 M/L • Pericenter = 14 kpc, Apocenter = 59 kpc • Initial conditions generated by GALACTICS (Widrow & Dubinski, 2005) • Simulations evolved using GADGET-2 (Springel, 2005)

  10. Stellar Streams of Sgr Dwarf

  11. Tidal Streams Probe “Fifth” Force

  12. Satellite Mass

  13. Satellite Spin

  14. Satellite Orbit

  15. Galactic Model

  16. Stellar density profile

  17. Mass-to-Light Ratio

  18. Leading-to-Trailing Stream Ratios • Attractive force suppresses leading-to-trailing ratio CurveColor Standard black Satellite Mass magenta Satellite Spin red Circular Orbit top blue Planar orbit bottom blue Heavy disk cyan Two profiles green Lower M/L yellow

  19. Conclusions • We don’t know what the DM is. Theory suggests we consider the possibility of a long-range “fifth force”. • Tidally disrupting galaxies ideal test; core DM-dominated but not streams • Attractive DM-force sweeps core ahead. Disrupted stars preferentially gain energy; LTR suppressed. • Tidal streams are a messy probe of new physics, but the signature of a DM force is very distinctive, model-independent. • The Sgr tidal stream is well observed; new tidal streams have been discovered in last few months in SDSS. Future surveys like SIM or Gaia will find even more. • Like dropping stars and DM off Leaning Tower of Pisa!

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