Testing the Equivalence Principle for Dark Matter Using Tidal Streams

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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

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What is the Dark Matter?

• Galactic rotation curves, large-scale structure
  (LSS), galaxy clusters all indicate ?DM ? 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?

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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)

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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?

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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?

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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

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Tidal-stream Asymmetry

• Non-uniformity of Galactic gradient leads to
  natural asymmetry
  
• ?
• DM force displaces stars from bottom of
  satellites potential well, a new DM-induced
  asymmetry
  
• ?
• DM asymmetry exceeds natural asymmetry when

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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
• trailing 25º
• (Law, Johnston, Majewski, 2005)
• Stellar densities also measured by SDSS
  (Belokurov et al., 2006)
  

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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)

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Stellar Streams of Sgr Dwarf
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Tidal Streams Probe Fifth Force
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Satellite Mass
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Satellite Spin
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Satellite Orbit
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Galactic Model
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Stellar density profile
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Mass-to-Light Ratio
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Leading-to-Trailing Stream Ratios

• Attractive force suppresses leading-to-trailing
  ratio
  
• Curve Color
• 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

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Conclusions

• We dont 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!