Testing the Equivalence Testing the Equivalence Principle for Dark Matter Principle for Dark Matter

Using Tidal StreamsUsing Tidal Streams

Michael Kesden, CITAMichael Kesden, CITA

Collaborator: Marc Kamionkowski, Collaborator: Marc Kamionkowski, CaltechCaltech

COSMO ‘06COSMO ‘06

Tahoe City, CATahoe City, CA

Thursday, September 28, 2006Thursday, September 28, 2006

What is the Dark Matter?What is the Dark Matter?

Galactic rotation curves, large-scale Galactic rotation curves, large-scale structure (LSS), galaxy clusters all indicate structure (LSS), galaxy clusters all indicate ΩΩDMDM 0.25 0.25

Extensions to the Standard Model offer Extensions to the Standard Model offer many possible WIMPs (axions, many possible WIMPs (axions, neutralinos, etc.)neutralinos, etc.)

Detection of non-gravitational interactions Detection of non-gravitational interactions could help identify DM. What interactions could help identify DM. What interactions might be observable?might be observable?

Long-range DM interactionsLong-range DM interactions

• Perhaps DM interacts with DEPerhaps DM interacts with DE• Log(Log(/m/mPlPl

44) ) -120 « 1 -120 « 1

• ~ ~ mm … Why now? … Why now?

• Maybe acceleration due to scalar field, just like Maybe acceleration due to scalar field, just like inflation. Scalar field should couple inflation. Scalar field should couple generically.generically.

• String theory includes “dilatons”, light, neutral String theory includes “dilatons”, light, neutral scalar fields that might interact with DM (Damour scalar fields that might interact with DM (Damour et al.et al. 1990, Gubser & Peebles, 2004) 1990, Gubser & Peebles, 2004)

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

• Long-range DM force interpreted as violation of Long-range DM force interpreted as violation of the equivalence principle (EP), the universality the equivalence principle (EP), the universality of free fall between stars and DMof free fall between stars and DM

• Laboratory tests place tight limits on fifth force in Laboratory tests place tight limits on fifth force in visible sector (Su visible sector (Su et alet al., 1994); no such limits for ., 1994); no such limits for DMDM

• Modeled by LModeled by Lintint = g = g V V = -g = -g22/4/4r exp-mr exp-mr r (Frieman & Gradwohl, 1991)(Frieman & Gradwohl, 1991)

• Force suppressed by a factor Force suppressed by a factor 22 g g22mmPlPl22/4/4mm

22 compared to gravity; how might we detect such compared to gravity; how might we detect such a force?a force?

Cosmic Tests for 5th ForceCosmic Tests for 5th Force

• LSSLSS• Attractive DM force enhances structure for (r < mAttractive DM force enhances structure for (r < m

-1-1) ) (Gradwohl & Frieman, 1992)(Gradwohl & Frieman, 1992)

• 5th force leads to scale-independent bias (Amendola & 5th force leads to scale-independent bias (Amendola & Tocchini-Valentini, 2002)Tocchini-Valentini, 2002)

• CMBCMB• Models where coupled DE traces DM constrained by Models where coupled DE traces DM constrained by

WMAP (Amendola & Quercellini, 2003)WMAP (Amendola & Quercellini, 2003)

• ClustersClusters• Baryons preferentially lost during mergersBaryons preferentially lost during mergers

• Is there new test with different systematics, greater Is there new test with different systematics, greater sensitivity?sensitivity?

Tidal DisruptionsTidal Disruptions

• Galaxies form Galaxies form hierarchicallyhierarchically; dwarf galaxies in Local Group ; dwarf galaxies in Local Group continue to merge with Milky Waycontinue to merge with Milky Way

• Smaller galaxies tidally disrupted by larger hosts at distances R Smaller galaxies tidally disrupted by larger hosts at distances R where:where:

rrsatsat > r > rtidtid ~ R(m ~ R(msatsat/2M/2MRR))1/3 1/3

• Tidal disruption establishes energy scales:Tidal disruption establishes energy scales:

• EEsatsat » E » Etidtid » E » Ebinbin disrupted stars retain similar orbits to satellite; disrupted stars retain similar orbits to satellite; trail/lead with gain/loss in energytrail/lead with gain/loss in energy

Tidal-stream AsymmetryTidal-stream Asymmetry

• Non-uniformity of Galactic gradient leads to natural asymmetry:Non-uniformity of Galactic gradient leads to natural asymmetry:

• DM force displaces stars from bottom of satellite’s potential well, a DM force displaces stars from bottom of satellite’s potential well, a newnew DM-induced asymmetry DM-induced asymmetry

• DM asymmetry exceeds natural asymmetry when:DM asymmetry exceeds natural asymmetry when:

Sagittarius Dwarf SpheroidalSagittarius Dwarf Spheroidal

• Sgr dwarf is closest satellite at 24 kpcSgr dwarf is closest satellite at 24 kpc• Stellar stream observed by 2MASS using M-Stellar stream observed by 2MASS using M-

giants with known age, color-magnitude relationgiants with known age, color-magnitude relation• Surface densities, radial velocities, distances Surface densities, radial velocities, distances

well-measured forwell-measured forleading: -100º < leading: -100º < < -30º < -30º

trailing: 25º < trailing: 25º < < 90º < 90º(Law, Johnston, & Majewski, 2005)(Law, Johnston, & Majewski, 2005)

• Stellar densities also measured by SDSS Stellar densities also measured by SDSS (Belokurov (Belokurov et alet al., 2006)., 2006)

SimulationsSimulations

• N-body simulation of satellite galaxy with:N-body simulation of satellite galaxy with:• M = 5 M = 5 10 1088 M M,, M/L = 40 M M/L = 40 M/L/L

• Pericenter = 14 kpc, Apocenter = 59 kpcPericenter = 14 kpc, Apocenter = 59 kpc

• Initial conditions generated by Initial conditions generated by GALACTICS (Widrow & Dubinski, 2005)GALACTICS (Widrow & Dubinski, 2005)

• Simulations evolved using GADGET-2 Simulations evolved using GADGET-2 (Springel, 2005)(Springel, 2005)

Stellar Streams of Sgr DwarfStellar Streams of Sgr Dwarf

Tidal Streams Probe “Fifth” Tidal Streams Probe “Fifth” ForceForce

Satellite MassSatellite Mass

Satellite SpinSatellite Spin

Satellite OrbitSatellite Orbit

Galactic ModelGalactic Model

Stellar density profileStellar density profile

Mass-to-Light RatioMass-to-Light Ratio

Leading-to-Trailing Stream Leading-to-Trailing Stream RatiosRatios

• Attractive force Attractive force suppresses leading-suppresses leading-to-trailing ratioto-trailing ratio

CurveCurve ColorColorStandardStandard blackblackSatellite MassSatellite Mass magentamagentaSatellite SpinSatellite Spin redredCircular OrbitCircular Orbit top bluetop bluePlanar orbitPlanar orbit bottom bottom

blueblueHeavy diskHeavy disk cyancyanTwo profilesTwo profiles greengreenLower M/LLower M/L yellowyellow

ConclusionsConclusions

• We don’t know what the DM is. Theory suggests we We don’t know what the DM is. Theory suggests we consider the possibility of a long-range “fifth force”.consider the possibility of a long-range “fifth force”.

• Tidally disrupting galaxies ideal test; core DM-dominated Tidally disrupting galaxies ideal test; core DM-dominated but not streamsbut not streams

• Attractive DM-force sweeps core ahead. Disrupted stars Attractive DM-force sweeps core ahead. Disrupted stars preferentially gain energy; LTR suppressed.preferentially gain energy; LTR suppressed.

• Tidal streams are a messy probe of new physics, but the Tidal streams are a messy probe of new physics, but the signature of a DM force is very distinctive, model-signature of a DM force is very distinctive, model-independent.independent.

• The Sgr tidal stream is well observed; new tidal streams The Sgr tidal stream is well observed; new tidal streams have been discovered in last few months in SDSS. Future have been discovered in last few months in SDSS. Future surveys like SIM or Gaia will find even more.surveys like SIM or Gaia will find even more.

• Like dropping stars and DM off Leaning Tower of Pisa!Like dropping stars and DM off Leaning Tower of Pisa!