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Testing the Equivalence Principle for Dark Matter Using Tidal StreamsPowerPoint Presentation

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

What is the Dark Matter? Tidal Streams

- 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?

Long-range DM interactions Tidal Streams

- 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)

A “5th Force” for Dark Matter? Tidal Streams

- 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?

Cosmic Tests for 5th Force Tidal Streams

- 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?

Tidal Disruptions Tidal Streams

- 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

Tidal-stream Asymmetry Tidal Streams

- 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:

Sagittarius Dwarf Spheroidal Tidal Streams

- 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)

Simulations Tidal Streams

- 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)

Stellar Streams of Sgr Dwarf Tidal Streams

Tidal Streams Probe “Fifth” Force Tidal Streams

Satellite Mass Tidal Streams

Satellite Spin Tidal Streams

Satellite Orbit Tidal Streams

Galactic Model Tidal Streams

Stellar density profile Tidal Streams

Mass-to-Light Ratio Tidal Streams

Leading-to-Trailing Stream Ratios Tidal Streams

- 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

Conclusions Tidal Streams

- 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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