Determining the Nature of Dark Matter with Astrometry

Louie StrigariUC Irvine

Center for Cosmology

Fermilab, 4.16.2007

Collaborators: James Bullock, Juerg Diemand, Manoj Kaplinghat, Michael Kuhlen,Piero Madau, Steve Majewski, Ricardo Munoz

Dark Matter in Cosmology

Standard

WIMP

‘Cold’

Proliferation of candidates

astrophysical implications

The `nature’ of dark matter

not hot

cold vs warm

Is warm interesting?

Non-baryonic darkmatter

Ωh2 = 0.113 ±0.009

Astrophysical Constraints on Dark Matter

Milky Way SatellitesNumber countsDistribution (radial and mass)Structure of Dark Matter Halos

Leo I

Low mass ‘field’ galaxies:rotation curves

High Precision data sets:

What can we learn in the future?

What can we learn now?

CDM: Cosmological Consequences

Hundreds of dark matter-dominatedMilky Way satellite galaxies [Klypin etal, Moore et al 1999]

Dark mini-halos abundance in thecentral regions [Diemand et al 2006]

CDM free streaming: structure downto earth mass scales

Orders of magnitude more darksubhalos than observed satellites ofMW or M31: the ‘missing satellitesproblem’

CDM: Cosmological Consequences

Simon et al. 2005, Kuzio de Naray 2006

cusp

core

Halo density profile scaling as 1/r inthe central regions [Navarro et al 2004,Diemand et al 2004]

Phase-space density, Q = ρ/σ3, isenormous

!

QCDM " 7 #1014mcdm

100GeV

$

% &

'

( )

3 / 2

Msun pc*3(km /s)

*3

Low mass dark matter halos maybe less `cuspy’ than predicted inCDM

Dark matter freezes out with non-negligible velocitiesFree streaming: Reduces the numberof small halos

A standard fix: Warm dark matter

• Narayan et al 2000: mWDM > 750 eV• Viel et al 2005: mWDM > 550 eV• New data and analysis

Seljak et al 2006Viel et al 2006

Abazajian 2006

Cosmological Constraints

Warm dark matter: Cosmological Consequences

Less dense dark matter halosReduced phase space density

Hogan & Dalcanton 2000

!

Q " 5 #10$4m

keV

%

& '

(

) *

4

Msun pc$3(km /s)

$3

[Tremaine-Gunn Bound]

Are the dynamics of dwarfgalaxies set by dark matterphysics?

Some other ‘Cosmological’ Fixes (Crude list)

Inflaton Potential (Kamionkowski & Liddle 1999) Zentner & Bullock 2003

Self-Interacting Dark Matter (Spergel & Steinhardt 2000) Q-balls (Kusenko & Steinhardt 2001) Fuzzy Dark Matter (Hu et al. 2001) Annihilating Dark Matter (Kaplinghat, Knox & Turner 2001)

(See also Beacom, Bell & Mack 2006) Decaying Dark Matter (Sanchez-Salcedo 2003, Cen 2000)

Can we `fix’ small scale structure and still connectdark matter to weak scale physics?

Dark Matter from Early Decays (SuperWIMPs)

!

Q "10#610

#3

$m /mDM

%

& ' '

(

) * *

3

zdecay

1000

%

& '

(

) *

3

Msun pc#3(km /s)

#3

Cembranos et al., Kaplinghat (2005)Is dark matter from decays just aone-parameter family of models?

What if dark matter freezes-out,then decays to a `superweaklyinteracting particle?’

Large velocity at production: 0.1-1c

Free-streaming scale: Q-1/3

Reduced Phase-Space Density

Dark matter from late decays

LS, Kaplinghat, Bullock 2006

Lifetime: 1014 sec.

Lifetime: 1012 sec.

Neutrino WDM

Also, dark matter in UED or SUSYmay be decaying ‘now’ [Cembranos, Feng, LS 2007]

Mass splitting is a freeparameter: what if they are oforder GeV? (Universal ExtraDimensions)

Free-streaming scale nowdepends on the lifetime: Q-1/3 τ-1/3

(Meta-CDM)

MeV Dark Matter

Hooper,Kaplinghat,LS,Zurek

Motivations: 511 keVemission from Galacticbulge

Cutoff scale of about105-106 solar mass

Interesting models givetruncated power spectra.Limits/systematics up fordebate

Census of Milky Way Satellites (Circa 2003)

Name orbital radius (kpc) DiscoveredLMC 50 1519SMC 60 1519Sculptor 80 1937Fornax 138 1938Leo II 205 1950Leo I 250 1950Ursa Minor 66 1954Draco 80 1954Carina 101 1977Sextans 86 1990Sagittarius 5 1994

About a dozen satellites of M31

Possible that up to 3x more exist at these luminosities [e.g. willman et al 2004]

Census of Milky Way Satellites (Circa 2007)

Name orbital radius (kpc) DiscoveredLMC 50 1519SMC 60 1519Sculptor 80 1937Fornax 138 1938Leo II 205 1950Leo I 250 1950Ursa Minor 66 1954Draco 80 1954Carina 101 1977Sextans 86 1990Sagittarius 5 1994Canis Major 80 2003Ursa Major I 100 2005Willman I 40 2005Bootes 62 2006Canes Venatici I 220 2006Canes Venatici II 150 2006Coma 40 2006Leo IV 160 2006Hercules 140 2006Leo T 420 2007

Belokurov et al. 2006

Dwarf Spheroidal Kinematics

No rotation, dynamically supported by velocity dispersion

Information on DM halo from line of sight velocities

Not subject to the same systematics as rotation curves

Walker et al 2006

The parameter space

At least 5 parameters

Jeans equations:

• Cusps remain cusps even accounting for tidal interactions[Kazantzidis 2004, Dehnen 2005]

• Is there no dark matter in dwarf galaxies? [Kroupa et al. 2005]

Log-slope

Observational Inputs

• We take as inputsthe density of stars

• Errors due todistance to galaxiesnot important

Strigari et al. 2006Cannot distinguish cores from cusps

Fornax interesting because of the population of globular clusters[Goerdt et al 2006]

Line of sight velocity dispersion

Fornax

What can we learn from dwarfs?

Truth = core Truth = cusp

Velo

city

Aniso

trop

y

R

ϕ

Transverse velocities of stars

•Require accuracy onstellar transversevelocities of 5 km/s

•At < 100 kpc, thiscorresponds to accuracy10 micro-arcseconds/yr

Astrometry 101

SIM PlanetQuest

• Astronomy = “star naming”• Astrometry = “star measuring”• SIM uses interferometers in space to

measure angles between celestial objectswith incredible accuracy

Adap

ted

from

: ht

tp:/

/pla

netq

uest

.jpl.n

asa.

gov/

SIM

/sim

_ind

ex.c

fm

Bessel detected it in 1838 (< 0.5 arcsec).Nearest star (Proxima Cen) 0.77 arcsec

Reflex Motion of Sunfrom 100pc (axes 100µas)

ParallacticDisplacementof GalacticCenter

Apparent GravitationalDisplacement of aDistant Star due toJupiter 1 degree away

SIM PlanetQuest (Space Interferometry Mission)

SIM PositionalError Circle

(4µas)

.HipparcosPositionalError Circle(0.64 mas)

HST Positional ErrorCircle (~1.5 mas)

Previous Considerations

• Wilkinson et al 2000 use atwo-parameter model for theDM density profile

• They determine that theinner slope is well-constrained

• However, their model is notgeneral enough. The innerslope is not well-constrained,even with proper motions

Constraints with SIM

LS, Bullock, Kaplinghat ApJL 2007

Breaking the degeneracy

Optimizing observations

Goal: SIM key projectwould entail 1000 hrs ofobserving time

200 stars frommultiple dSphs

What can we learn now?

Diemand, Kuhlen, Madau 2007

[Semi-analytic models of,e.g. Bullock et al 2000,Kravtsov et al 2004,Moore et al 2006,Gnedin & Kravtsov 2006]

-Stoehr et al 2002suggest all of the MWsatellites reside in themost massive subhalos

Constraints on galaxy masses

Redefining the Missing Satellites Problem

LS, Bullock, Kaplinghat, Diemand, Kuhlen, Madau 2007

-MW satellitepopulation does notreside in the mostmassive CDM halos

LS, Bullock, Kaplinghat, Diemand, Kuhlen, Madau 2007

MW satellites could beeither:

-Earliest forming darkmatter halos

-Largest halos beforeaccrection

[See, e.g. semi-analyticmodels of Bullock et al2000, Kravtsov et al2004, Moore et al 2006,Gnedin & Kravtsov 2006]

Redefining the Missing Satellites Problem

Further Applications: Dark Matter Annihilations

LS, Koushiappas, Bullock, Kaplinghat 2007

With substructure, fluxes may be ‘boosted’

LS, Koushiappas, Bullock, Kaplinghat 2007

Conclusions and Outlook

• Proliferation of interesting dark matter models to constrainwith galaxy dynamics. Escape the tyranny of CDM!

• Dwarf galaxies provide a unique test of dark matter• At present, can’t distinguish between cores and cusps. This

will change with astrometric measurements.• Present data strongly constrains mass of galaxies within

about 600 kpc. This can be used to rule out the hypothesisthat the present MW satellites reside in the most massivesubhalos

• New constraints for dark matter annihilations.

Louie Strigari UC Irvine