Exploring Local Dark Matter with the Space Interferometry Mission

(SIM PlanetQuest)

From Quantum to Cosmos: Fundamental Physics in Space for the Next Decade

Figure courtesy ofB. Gibson (Central Lancashire)

Steven Majewski (Univ.Virginia)

Authors of Recent SIM Local Dark Matter White Paper

Steven Majewski (Univ. Virginia)James Bullock (UC-Irvine)Andreas Burkert (Univ.-Sternwarte Munich)Brad Gibson (Univ. Central Lancashire)Oleg Gnedin (Univ. Mich) Eva Grebel (Astron. Rechens-Institut, Univ. Heidelberg)Puragra Guhathakurta (UC-Santa Cruz)Amina Helmi (Kapteyn Astron. Institute, Groningen)Kathryn Johnston (Columbia Univ.)Pavel Kroupa (Argelander Inst. for Astronomy, Univ. Bonn)Manuel Metz (Argelander Inst. for Astronomy, Univ. Bonn)Ben Moore (Inst. For Theoretical Physics, Univ. Zurich)Richard Patterson (Univ. Virginia)Ed Shaya (Univ. Maryland)Louis Strigari (UC-Irvine)Roeland van der Marel (STScI)

Growth of Structure in a Cold Dark Matter Universe

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Animation by Ben MooreUniversity of Zurich

Numerical simulations make rich variety of predictions about structure and dynamics on galactic to largest scales.• Great success in matching observations on largest scales.• But numerous problems matching data on galaxy scales, e.g.:

– “missing satellites problem”/mass spectrum of subhalos– “central cusps problem”– “angular momentum problems”

Abadi et al. (2003): “Current cosmological simulations have difficulties making anything that looks like a real galaxy.”

Thus a current focus for advancing DM theory is attemptingto resolve problems on small (galaxy) scales.

• So understanding/explaining dynamics of Local Group, Milky Way and satellite system are central to progressin DM theory, hierarchical formation, galaxy evolution.

• Microscopic nature of DM affects the way it clusters around galaxies, thus can be probed by exploration of LG & MW.• Deriving a globally self-consistent MW DM halo model will provide information on the mass range and dissipational properties of the dark matter particle.

• Microscopic nature of DM affects the way it clusters around galaxies, thus can be probed by exploration of LG & MW.• Deriving a globally self-consistent MW DM halo model will provide information on the mass range and dissipational properties of the dark matter particle.

– useful information for experiments that aim to detect DM particle directly on (inside) Earth.

CDMSII (Berkeley)Soudan Mine, MN LUX (Brown Univ.) -- Homestake Mine, SD

XENON (Columbia Univ.)Gran Sasso Massif, Italy

1) Measure shape/orientation/density law/lumpiness of MW potential w/tidal streams (SIM far, Gaia close)2) Measure shape/orientation of galaxy potentials with

hypervelocity stars (SIM) 3) Mapping late infall via orbits of satellites (SIM) 4) Measure ang. momentum dist’n/anisotropy/orbits

of MW stars, clusters (SIM far, Gaia close)5) Measure DM temperature by mapping DM phase

space density (i.e. cusp vs. core) in dSph (SIM) 6) Local Group dynamics (Shaya talk) (SIM)

Astrometric experiments to measure galactic dynamics, structure, local dark matter in SIM/Gaia era

Halo Shape

• CDM predicts DM halos to be trixial, but rounder at larger radii.

Dwarf Galaxy vs. Milky Way-like System

Animation by Kathryn JohnstonColumbia University

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Tidal Tails Are Very Sensitive Galactic Mass Probes

NGC 5907: Modeling the Tidal Disruption

Extragalactic systems: With no RVs and only on-sky projection, left with degeneracies of orbital precession, ellipticity, halo shape, etc … … but should not be problem inside of Milky Way…

Martinez-Delgado et al. (2008)

Tidal Tails Are Very Sensitive Galactic Mass Probes

• Early work on 2-D data (i.e. “great circle” of presumed Sgr carbon stars & 2MASS M giants) suggested Galactic DM halo ~ spherical (Ibata et al. 2001, 2003, Majewski et al. 2003).

Application in the Milky Way

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Majewski et al. 2003, Law et al. 2005

• Helmi (2004) with 3 phase space coordinates(spatial positions + RVs) finds need for prolate halo.

Application in the Milky Way

~50 kpc

Johnston, Law & Majewski (2005) - 4 coords (3 space + RV):• Gives only slightly oblate halo to ~50 kpc (q ~ 0.92 +/- 0.2).

• But some problems with matching leading arm velocities unresolved.

• Strongly rules out prolate (5): Precesses Sgr backwards.

Tidal Tails Are Very Sensitive Galactic Mass Probes

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• Experiment requires 6-D phase space information for stream stars. • Requires SIM-accurate proper motions for faint, distant stars

(e.g., < ~10 as/yr at V ~18 for ~100 kpc giant stars).• Gaia useful for nearby (~10 kpc) streams.

Animations by C. Moskowitz & K. Johnston (Wesleyan University)

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correct potential incorrect potential

Sloan Digital Sky Survey

From Carl Grillmair, in Unwin et al. (2007)

Now finding many lower surface brightness streams in the Milky Way halo with starcounts and radial velocity surveys.

Needed: Proper Motions at as/yr Level

SIM PlanetQuest: - 4 as/year for V~ 15-20 (giant) stars - For 100s of pre-selected tidal stream targets expect 1% accuracy on

halo flattening and LSR

- Milky Way mass profile from multiple streams.

- Gaia could do only for nearby (few 10 kpc) streams.

Hypervelocity Stars Brown et al. (2005, 2006):

Half-dozen stars w/Galactocentric velocity = 550-720 km/sHills (1998), Yu & Tremaine (2003):

Only known mechanism: ejection from deep potential of SMBH

Gnedin et al. (2006): Modeling HVS SDSS J090745.0+024507

Milky Way: q1/q3 = 0.9, q2/q3 = 0.7 (triaxial, prolate)

Most of halo shape sensitivity in transverse velocity at large r.

Deviation in transversevelocity by non-sphericalpotential

• True distance of HVS cleanly determined from = 100 as yr-1.• Constraints on orientation of triaxial halo with = 20 as yr-1.• Constraints on axial ratios with = 10 as yr-1.• Known HVSs have V = 16-20 (SIM territory)

XGC major axis

If MS70 kpc

If BHB40 kpc

YGCmajor axis

ZGC major axis

ZGC major axis

YGCmajor axis

XGC major axis

HVS SDSS J090745.0+024507

Can the method be generalized?• Existence of an HVS from LMC recently reported

Gualandris et al. (2007), Bonanos (2008)– Deriving full 3-D trajectory would pin down the location of massive black hole in LMC.

• Numerous M31 HVSs expected, including 1000s within virialized halo of MW (Sherwin et al. 2008).

– Tell us about M31 halo?– Mass distribution of Local Group?

• Must have as astrometry at faint mags -- SIM only

The Milky Way Then and Now

CDM models suggest that Milky Way of today:• Is very lumpy should have numerous “subhalos”/satellites.

0.4 billion years old 13.4 billion years old

Courtesy Ben MooreUniversity of Zurich

Mass spectrum of subhalos is a function of DM physics

• Mass spectrum ~ M-1 (Dieman et al. 2008), but cut-off mass function of particle nature of DM.

• If DM = cold (e.g., WIMPS), minimum mass = earth mass, number of subhalos ~ 1013.

• If DM = warm (e.g., sterile ),minimum mass = 108 Msun,number of subhalos ~ < 100.

(Stadel et al., in prep.)

In either case, where are the “missing satellites”?

• Possibly mainly DARK.• Only most massive dozen or so lumps form stars (red lumps above)?• Visible satellites represent only tips of the dark matter icebergs?

Moore et al. (1999), Kaufmann et al. (1993), Klypin et al. (1999)

mass

Num

ber

of s

ubha

los

Measuring Halo (Dark) Lumpinesse.g., Johnston, Spergel & Haydn (2002)

perturbation of circular orbits in halo with 256 lumps

After 1.3 Gyr

After 2.6 Gyr

After 4 Gyr

angulardeviations

velocitydeviations

angulardeviations

velocitydeviations

angulardeviations

velocitydeviations

Sensitivity of test increases with long cold streams…

Grillmair (2006)

… and 6-D data (SIM): • For example, perturbation points in

streams should be identifiable with trace back of stream star orbits.

Rockosi et al. (2002), Odenkirchen et al. (2001,2003)

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Testing Hierarchical Formationand Late Infall

• Infall of DM onto MW leaves fingerprint in the orbitsof satellite galaxies, any accreted globular clusters and halo stars.

• Models point to infall alongfilaments.

(Moore et al. 2001)

z = 10 z = 0

(Moore et al. 2006)

Evolution of luminous subhalos in a MW galaxy:

• Surviving galaxy satellites of today (boxes) were most distant subhalos at z = 10, last to fall into MW.

• Earlier infall came from closer matter, and luminous parts now spread out among the debris (stars and star clusters) of halo.

• In either case, orbital shapes/correlations tell us how infall proceededat corresponding infall epoch.

• Kinematics of late and early infall expected to differ.

Current Milky Way satellites show strong spatial anisotropyand hint at evidence for correlated orbits:

Orbital poles for MW satellites (Palma, Majewski & Johnston 2003)

• Infall in a few groups of DM subhalos?• Break-up of formerly larger satellites?• Formed as “tidal dwarfs”?

To derive transverse velocities good to 10 km/s requires:• ~ 10 as for satellites at ~250 kpc (Leo I, II, CanVen)

for V ~ 19.5 giant stars (SIM only) • ~ 20 as for satellites at ~100 kpc (UMi, Dra, Car, …)

for V ~ 17.5 giant stars (SIM or Gaia many star average)• Gaia cannot play this game for many of the

newfound ultra-low luminosity dSphs (even close ones)because there are few/no member stars bright enough:

(Belokurov et al. 2007)

z = 10 z = 0

(Moore et al. 2006)

CDM predicts halo anisotropy gradient (more radial at larger r)

To test, need in situ measures of halo star orbital anisotropy:•Similar proper motion requirements as for dSphs, but single stars.•Gaia relegated only to inner halo here.•Few 100 stars to 5 km/s (compared to RV dispersion ~100 km/s)

Determining the Nature of Dark Matter with SIM

• CDM: potentially ruinous difficulties on small scales:

• Missing satellites problem• Angular momentum/too small disks problem

• Cusps predicted, but rotation curves prefer cored profiles, and luminous matter profiles are cored.

CDM:

€

QCDM ≈ 7 ×1014 mcdm

100GeV

⎛

⎝ ⎜

⎞

⎠ ⎟3 / 2

Msun pc−3(km /s)−3

WDM:

High primordial phase space density

Low primordial phase space density

Cuspy “NFW” profiles

Cored density profiles

CDMcusp

WDM core

WIMPS: e.g., axions, neutralinos,

e.g., gravitinos, light sterile ’s

RegionProbed

bydSphstars

New Test: Stellar ’s in M.W. dSph’s.

Log-slope of dark matter density profile

Velo

city

An

isotr

op

y of

Sta

rs

• Future: 200 proper motions at ~5 km/s with SIM will break this degeneracy.

Measure log-slope of DM density profile at stellar radius to 0.2.

Discriminate between viable WDM and CDM at the ~3 sigma level.

Strigari et al. (2007, 2008)

• MW dSphs ideal for testing nature of Dark Matter.

• But currently: Radial velocity studies have strong degeneracy between DM density slope and stellar velocity anisotropy.

• Even with 1000’s of RVs, can’t distinguish cored from cusp halos (WDM vs CDM).

With SIM

Without SIM

Leo I

Determining the Nature of Dark Matter with SIM

Strigari, Bullock, Kaplinghat, Kazantzidis, Majewski & Munoz 2008

• ~100 days of SIM time (~key project) will provide approximately 200 stars in Draco dSph to V = 19with 5 km/s transverse velocities (sufficient).

Err

or in

mea

sure

d sl

ope

Determining the Nature of Dark Matter with SIM

Assuming < 7 km/s errors =< 20 as at 80 kpc (Draco)

luminosity function

3 km/s5 km/s7 km/s10 km/s

Local Group Dynamics with SIM (Ed Shaya Talk)

• Local Group: ’s of ~30 galaxies in the Local Group.

• Constrain LG matter distribution

• Proper motions key to constraining mass on ~ 5 Mpc scale.

• Positions/orbits of galaxies back in time, masses of individual galaxies.

• Test cosmological expectations

Shaya et al.

Growth of Structure in a Cold Dark Matter Universe

CDM Galaxy Merger Tree (Wechsler et al. 2002)

• Since Searle & Zinn (1978) notion of accretion, including “late infall”, a central question of Milky Way (MW) formation studies.

• Merging also a key element of galaxy formation models with CDM.

time

Gnedin et al. (2006): Modeling HVS SDSS J090745.0+024507

Milky Way: q1/q3 = 0.9, q2/q3 = 0.7 (triaxial, prolate)

Most of halo shape sensitivity in transverse velocity at large r.

BootesBelokurov et al. 2006

Munoz et al. 2006

Can Ven I

Zucker et al. 2006Ursa Major

Willman et al. 2005

… or has the problem just been one of accounting??

About a dozen or more recent discoveries:

Where are the “missing satellites”?

• Are there enough discoveries to “fill the gap”?• Doesn’t fix shortfall at all masses…

Moore et al. (1999), Kaufmann et al. (1993), Klypin et al. (1999)

mass

num

ber

Sagittarius’ Debris Stream Dynamically Cold For ~2 Gyr

• If v all from scattering, Sgr tail hotter than expected for smooth halo…

• … however, consistent with influence of just one LMC-like lump.

• Cannot yet rule out some “lucky” lumpier halos.

• But note, some dispersion is intrinsic to Sgr.

• Longer Sgr,, initially colder streams, and/or 6-D data will yield more definitive results.

Trailing arm data from Ibata et al. (1997), Majewski et al. (2004, 2007)

, orbital longitude (deg)

Animation by James Bullock & Kathryn Johnston (2005)

Streams shown to = 38 mag/arcsec2.

Today ~1 stream with < 30 mag/arcsec2

should be visible per MW-like galaxy.

(Johnston et al., in prep.)

HierarchicalMerging Seen on Galactic ScalesBullock & Johnston 2005

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Known Milky Way Streams Bullock & Johnston Model

Growing Convergence of Stream Data and Models