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Exploring Local Dark Matter with the
Space Interferometry Mission
(SIM PlanetQuest)
Figure courtesy of
B. Gibson (Central Lancashire)
Steven Majewski (Univ.Virginia)
From Quantum to Cosmos:
Fundamental Physics in Space for the Next Decade
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
QuickTime™ and a
YUV420 codec decompressor
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Animation by Ben Moore
University 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 attempting
to resolve problems on small (galaxy) scales.
• So understanding/explaining dynamics of Local Group,
Milky Way and satellite system are central to progress
in 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.
XENON (Columbia Univ.)
Gran Sasso Massif, Italy
CDMSII (Berkeley)
Soudan Mine, MN
LUX (Brown Univ.) -- Homestake Mine, SD
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)
Halo Shape
• CDM predicts DM halos to be trixial,
but rounder at larger radii.
Tidal Tails Are Very Sensitive Galactic Mass Probes
Dwarf Galaxy vs. Milky Way-like System
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Animation by Kathryn Johnston
Columbia University
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…
Application in the Milky Way
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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).
Application in the Milky Way
• Helmi (2004) with 3 phase space coordinates
(spatial positions + RVs) finds need for prolate halo.
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.
Tidal Tails Are Very Sensitive Galactic Mass Probes
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correct potential
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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.
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)
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 QLSR
- 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/s
Hills (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)
Deviation
in transverse
velocity by
non-spherical
potential
Most of halo shape sensitivity in transverse velocity at large r.
• True distance of HVS cleanly determined from s = 100 as yr-1.
• Constraints on orientation of triaxial halo with s = 20 as yr-1.
• Constraints on axial ratios with s = 10 as 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
If BHB
40 kpc
XGC
major axis
ZGC major axis
YGC
major axis
XGC
major axis
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
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.
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”?
Number of subhalos
Moore et al. (1999), Kaufmann et al. (1993), Klypin et al. (1999)
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?
Measuring Halo (Dark) Lumpiness
e.g., Johnston, Spergel & Haydn (2002)
perturbation of circular orbits in halo with 256 lumps
After 1.3 Gyr
angular
deviations
velocity
deviations
After 2.6 Gyr
angular
deviations
velocity
deviations
angular
deviations
After 4 Gyr
velocity
deviations
Sensitivity of test increases with long cold streams…
Grillmair (2006)
Rockosi et al. (2002), Odenkirchen et al. (2001,2003)
… and 6-D data (SIM):
• For example, perturbation points in
streams should be identifiable with
trace back of stream star orbits.
QuickTime™ and a
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Testing Hierarchical Formation
and Late Infall
• Infall of DM onto MW
leaves fingerprint in the orbits
of satellite galaxies, any
accreted globular clusters
and halo stars.
• Models point to infall along
filaments.
(Moore et al. 2001)
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.
Current Milky Way satellites show strong spatial anisotropy
and 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)
CDM predicts halo anisotropy gradient (more radial at larger r)
z = 10
z=0
(Moore et al. 2006)
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.
New Test: Stellar ’s in M.W. dSph’s.
CDM:
High primordial phase space density
WIMPS: e.g., Q  7 1014  mcdm  M pc -3 (km/s)-3
CDM
sun
100GeV 
axions,
neutralinos,
3/2
Cuspy “NFW” profiles
WDM:
Low primordial phase space density
e.g., gravitinos,
light sterile ’s
Cored density profiles
CDM
cusp
WDM
core
Region
Probed
by
dSph
stars
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).
Without SIM
• 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)
Velocity Anisotropy of Stars
Leo I
With SIM
Log-slope of dark matter density profile
Determining the Nature of Dark Matter with SIM
Error in measured slope
• ~100 days of SIM time (~key project) will provide
approximately 200 stars in Draco dSph to V = 19
with 5 km/s transverse velocities (sufficient).
Assuming < 7 km/s errors =
< 20 as at 80 kpc (Draco)
3 km/s
5 km/s
7 km/s
10 km/s
luminosity
function
Strigari, Bullock, Kaplinghat, Kazantzidis, Majewski & Munoz 2008
Local Group Dynamics with SIM (Ed Shaya Talk)
Group: ’s of ~30
galaxies in the Local Group.
• Local
• 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.
Shaya et al.
• Test cosmological
expectations
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)
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.
… or has the problem just been one of accounting??
About a dozen or more recent discoveries:
Ursa Major
Bootes
Can Ven I
Willman et al. 2005 Belokurov et al. 2006 Zucker et al. 2006
Munoz et al. 2006
Where are the “missing satellites”?
number
Moore et al. (1999), Kaufmann et al. (1993), Klypin et al. (1999)
mass
• Are there enough discoveries to “fill the gap”?
• Doesn’t fix shortfall at all masses…
Sagittarius’ Debris Stream Dynamically Cold For ~2 Gyr
Trailing arm data from Ibata et al. (1997), Majewski et al. (2004, 2007)
• If sv 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.
L, orbital longitude (deg)
• Longer Sgr,, initially colder
streams, and/or 6-D data will
yield more definitive results.
Animation by
James Bullock &
Kathryn Johnston (2005)
Hierarchical
Merging Seen on
Galactic Scales
Bullock & Johnston 2005
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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.)
Growing Convergence of Stream Data and Models
Known Milky Way Streams
Bullock & Johnston Model