Testing the Equivalence Principle for Dark Matter Using Tidal Streams

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Testing the Equivalence Principle for Dark Matter Using Tidal Streams

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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

- 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?

- 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)

- 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 = -g2/4r exp{-mr} (Frieman & Gradwohl, 1991)
- Force suppressed by a factor 2 g2mPl2/4m2 compared to gravity; how might we detect such a 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?

- 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

- 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:

- 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)

- 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)

- Attractive force suppresses leading-to-trailing ratio
CurveColor

Standardblack

Satellite Massmagenta

Satellite Spinred

Circular Orbittop blue

Planar orbitbottom blue

Heavy diskcyan

Two profilesgreen

Lower M/Lyellow

- 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!