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?.

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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


Tahoe City, CA

Thursday, September 28, 2006

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?

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)

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?

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?

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

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:

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)


  • 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 Probe “Fifth” Force

Satellite Mass

Satellite Spin

Satellite Orbit

Galactic Model

Stellar density profile

Mass-to-Light Ratio

Leading-to-Trailing Stream Ratios

  • Attractive force suppresses leading-to-trailing ratio



    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!

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