Exploring Local Dark Matter with the Space Interferometry Mission (SIM PlanetQuest)

1. Exploring Local Dark Matter with the Space Interferometry Mission (SIM PlanetQuest) Figure courtesy ofB. Gibson (Central Lancashire) Steven Majewski (Univ.Virginia) From Quantum to Cosmos: Fundamental Physics in Space for the Next Decade

2. 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)

3. Growth of Structure in a Cold Dark Matter Universe Animation by Ben Moore University of Zurich

4. 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.

5. 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.

6. XENON (Columbia Univ.)Gran Sasso Massif, Italy 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

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

8. Halo Shape Model expectations: more oblate (q ~ 0.6) more typical • CDM predicts DM halos to be trixial, but rounder at larger radii.

9. Tidal Tails Are Very Sensitive Galactic Mass Probes Dwarf Galaxy vs. Milky Way-like System Animation by Kathryn Johnston Columbia University

10. Tidal Tails Are Very Sensitive Galactic Mass Probes NGC 5907: Modeling the Tidal Disruption Martinez-Delgado et al. (2008) 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…

11. Application in the Milky Way Majewski et al. 2003, Law et al. 2005 • 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).

12. Application in the Milky Way • Helmi (2004) with 3 phase space coordinates(spatial positions + RVs) finds need for prolate halo.

13. Johnston, Law & Majewski (2005) - 4 coords (3 space + RV): • Gives only slightly oblate halo to ~50 kpc (q ~ 0.92 +/- 0.2). • Strongly rules out prolate (5s): Precesses Sgr backwards. ~50 kpc • But some problems with matching leading arm velocities unresolved.

14. Tidal Tails Are Very Sensitive Galactic Mass Probes correct potential incorrect potential • 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)

15. Now finding many lower surface brightness streams in the Milky Way halo with starcounts and radial velocity surveys. Sloan Digital Sky Survey From Carl Grillmair, in Unwin et al. (2007)

16. Needed: Proper Motions at mas/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 andQLSR - Milky Way mass profile from multiple streams. • Gaia could do only for nearby (few 10 kpc) streams. MILKY WAY MASS PROFILE FROM MULTIPLE STREAMS.

17. Hypervelocity Stars Brown et al. (2005, 2006): Half-dozen stars w/Galactocentric velocity = 550-720 km/s Hills (1998), Yu & Tremaine (2003): Only known mechanism: ejection from deep potential of SMBH

18. Gnedin et al. (2006): Modeling HVS SDSS J090745.0+024507 Milky Way: q1/q3 = 0.9, q2/q3 = 0.7 (triaxial, prolate) Deviation in transversevelocity by non-sphericalpotential Most of halo shape sensitivity in transverse velocity at large r.

19. True distance of HVS cleanly determined from sm = 100 mas yr-1. • Constraints on orientation of triaxial halo with sm = 20 mas yr-1. • Constraints on axial ratios with sm = 10 mas yr-1. • Known HVSs have V = 16-20 (SIM territory) HVS SDSS J090745.0+024507 If MS 70 kpc ZGC major axis YGC major axis XGC major axis If BHB 40 kpc ZGC major axis XGC major axis YGC major axis

20. 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

21. The Milky Way Then and Now 0.4 billion years old 13.4 billion years old Courtesy Ben Moore University of Zurich • CDM models suggest that Milky Way of today: • Is very lumpy - should have numerous “subhalos”/satellites.

22. 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.)

23. Moore et al. (1999), Kaufmann et al. (1993), Klypin et al. (1999) In either case, where are the “missing satellites”? Number of subhalos mass • 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?

24. e.g., Johnston, Spergel & Haydn (2002) perturbation of circular orbits in halo with 256 lumps Measuring Halo (Dark) Lumpiness angular deviations After 1.3 Gyr velocity deviations angular deviations After 2.6 Gyr velocity deviations angular deviations After 4 Gyr velocity deviations

25. Sensitivity of test increases with long cold streams… Rockosi et al. (2002), Odenkirchen et al. (2001,2003) Grillmair (2006) • … and 6-D data (SIM): • For example, perturbation points in streams should be identifiable with trace back of stream star orbits.

26. 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)

27. Evolution of luminous subhalos in a MW galaxy: z = 10 z = 0 (Moore et al. 2006) • 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 proceeded at corresponding infall epoch. • Kinematics of late and early infall expected to differ.

28. 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”?

29. 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)

30. CDM predicts halo anisotropy gradient (more radial at larger r) z = 0 z = 10 (Moore et al. 2006) • To test, need in situ measures of halo star orbital anisotropy: • Similar proper motion requirements as for dSphs, butsingle stars. • Gaia relegated only to inner halo here. • Few 100 stars to 5 km/s (compared to RV dispersion ~100 km/s)

31. CDM cusp WDM core 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. New Test: Stellar m’s in M.W. dSph’s. CDM: High primordial phase space density WIMPS: e.g., axions, neutralinos, Cuspy “NFW” profiles Region Probed by dSph stars WDM: Low primordial phase space density e.g., gravitinos, light sterile ’s Cored density profiles

32. Determining the Nature of Dark Matter with SIM • 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). Leo I WithoutSIM • 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. With SIM Velocity Anisotropy of Stars Strigari et al. (2007, 2008) Log-slope of dark matter density profile

33. Determining the Nature of Dark Matter with SIM • ~100 days of SIM time (~key project) will provide approximately 200 stars in Draco dSph to V = 19with 5 km/s transverse velocities (sufficient). Assuming < 7 km/s errors = < 20 mas at 80 kpc (Draco) 3 km/s 5 km/s 7 km/s 10 km/s Error in measured slope luminosity function Strigari, Bullock, Kaplinghat, Kazantzidis, Majewski & Munoz 2008

34. Local Group Dynamics with SIM (Ed Shaya Talk) • Local Group:m’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.

37. Growth of Structure in a Cold Dark Matter Universe • 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 CDM Galaxy Merger Tree (Wechsler et al. 2002)

38. 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.

39. … or has the problem just been one of accounting?? About a dozen or more recent discoveries: Can Ven I Bootes Ursa Major Belokurov et al. 2006 Zucker et al. 2006 Willman et al. 2005 Munoz et al. 2006

40. Moore et al. (1999), Kaufmann et al. (1993), Klypin et al. (1999) Where are the “missing satellites”? number mass • Are there enough discoveries to “fill the gap”? • Doesn’t fix shortfall at all masses…

41. Sagittarius’ Debris Stream Dynamically Cold For ~2 Gyr Trailing arm data fromIbata et al. (1997), Majewski et al. (2004, 2007) • If svallfrom 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. L, orbital longitude (deg) rlim

42. Animation by James Bullock & Kathryn Johnston (2005) Hierarchical Merging Seen on Galactic Scales Bullock & 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.)

43. Growing Convergence of Stream Data and Models Bullock & Johnston Model Known Milky Way Streams