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Gdansk Jul 02 2005
THE DARK MATTER PROBLEM
Konrad Kuijken
Leiden Observatory
Gdansk Jul 02 2005
Overview
• Evidence for dark matter– Cosmic Microwave Background Radiation– The Milky Way– Galaxy dynamics– Gravitational lensing
• Alternatives
• What is it?
• Prospects
Gdansk Jul 02 2005
CMB
• Last scattering surface at z~1100– Inhomogeneities at 1:105
level– Power spectrum powerful
probe of cosmology
Gdansk Jul 02 2005
CMB
• Early fluctuations in density– Grow gently at first– Start to oscillate when
enter horizon– Photons escape at last
scattering when H atoms form and free electrons disappear (T~3000K).
– Tnow / Tlast scatt defines redshift of CMB
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Wavelength
Tim
e
horizon
Gdansk Jul 02 2005
CMB
• Early fluctuations in density– Grow gently at first– Start to oscillate when
enter horizon– Photons escape at last
scattering when H atoms form and free electrons disappear (T~3000K).
Peak 2
Peak 3
Peak 1
More baryons
PotentialDensity photons + plasma
Higher overdensities (same pressure, more inertia)
x
Gdansk Jul 02 2005
CMB
• Early fluctuations in density– Grow gently at first– Start to oscillate when
enter horizon– Photons escape at last
scattering when H atoms form and free electrons disappear (T~3000K).
Ho
rizo
n c
ross
ing
Las
t sc
atte
rin
g
Peak 1
Peak 2
Peak 3
Time
More baryons
Gdansk Jul 02 2005
CMB
• Spectrum of fluctuations in the CMB (WMAP)– baryon/photon ratio
enhances peaks 1,3,5,…– Strong measurement of
baryon density
– Consistent with Big Bang Nucleosynthesis
(Wayne Hu)
Gdansk Jul 02 2005
CMB• Constraints on dark
matter content: measurement of matter/radiation equality– Radiation: a-4
– Matter: a-3
– Crossover near z~3000 (before last scattering!)
– Changes horizon crossing times for different fluctuation wavelengths
– Moves peaks in CMB angular spectrum!
– Higher (early) peaks move more than 1st (last) peak.
• 1st peak mostly constrains curvature
Horizon crossing
Las
t sc
atte
rin
g
Peak 1
Peak 2
Peak 3
Time
Gdansk Jul 02 2005
CMB
• Parameter constraints on matter content from CMB
– Universe close to flat
– Assume exactly flat strong constraint on m
– Otherwise strong degeneracy between m, (and H0)
Spergel et al. 2003
Gdansk Jul 02 2005
Structure formation
• Gravitational instability causes large-scale structure– Without dark matter, get insufficient structure growth– Foam-like LSS follows out of CDM
Gdansk Jul 02 2005
Structure formation
• Gravitational instability causes large-scale structure– Without dark matter, get insufficient structure growth
– Foam-like LSS follows out of CDM
– Good agreement with observations down to few-Mpc scales
• Combined constraints from CMB (initial conditions) + present-day LSS (in galaxies!) give best constraints on total (cold dark) matter density
m h2.
• Result: 23% dark matter, 4% baryons, 73% dark energy
QuickTime™ and a decompressor
are needed to see this picture.
Gdansk Jul 02 2005
Galaxy dynamics
• General evidence for stronger gravitational fields around galaxies than can be explained– by plausible stellar population M/L ratios– by the shape of the light distribution
• Galaxies are not WYSIWYG – But bathed in extended mass distributions -- dark halos
Gdansk Jul 02 2005
• Tricky:– Radial velocities see no solid-body rotation, need distances– Proper motions are local, require absolute frame
• HI rotation curve:
• Proper motions:– (A-B)=220km/s / 8kpc (Sgr A*)– (A-B)=216km/s / 8kpc (HIPP)
The Rotation Curve of the Milky WayThe Rotation curve is roughly flat out to 20kpc. No Keplerian fall-off.
But rotation curves in other galaxies are much better measured
Gdansk Jul 02 2005
Vertical kinematics
• Unique 3-D measurements of the potential – Solar neighbourhood:
• Vertical kinematics (Oort problem) – Distribution fn.: f(z,vz)=f(Ez)=f((z)+vz
2/2)
– Read off f from velocities at low z (where =0)– Vary to reproduce density at high z
z
Vz E=const.Local disk mass consistent with stars and gas observed
(Siebert et al 2003; Kuijken & Gilmore 1989,1991)
Gdansk Jul 02 2005
How much mass resides in the disk?
• Simple model: Mestel disk• Flat rotation curve• Predicts at sun
• Measurements of total mass density:
Σ( )R V GRcirc= 2 2π
185 2M pcsun
dA/dF Bienayme 2000
dA/dF Holmberg & Flynn 2001
dK Kuijken 1991, Siebert & al. 2003
gK Flynn & Fuchs 1994
Census
Σ1 1 70 5. kpc = ±
0 0 076 0 015= ±. .
0 010 0 01= ±. .
Σ = ±52 13Σ = ±49 9
0 010 0 02= ±. .
Gdansk Jul 02 2005
Flattening of the Halo
• Local potential ~ E4 (disk+halo)
• Flaring of HI layer: halo axis ratio ~0.8– At large radii vertical confining gravity mostly halo
Depends strongly on adopted Galactic
constants!
(Olling & Merrifield)
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Rotation curves of spirals
• Rotation curves: ‘extra gravity’ in outskirts of galaxies
• Extra gravity: extra massExtra gravity: extra mass
halo
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PNe and dark matter around elliptical galaxies
• PN.S project (PI N. Douglas)
– Slitless spectroscopy through narrow-band 5007 filter: find emission-line objects
– Simultaneous counterdispersed images: deduce position and velocity at once.
– Programme to study nearby elliptical galaxies
• Advantage of PNe: – probe large radii (integrated light too faint for
spectroscopy)– Represent old stellar population (?)
Gdansk Jul 02 2005
PN.S optical design: slitless spectroscopy through narrow-band filter
Shutter
Focal plane calibration mask
O[III] filter (tiltable = tuneable)
gratings
Gdansk Jul 02 2005
undispersed field[O III] filter, slitless,dispersed 0°
[O III] filter, slitless,dispersed 180°
PN
star
positions & velocities in one go!
PNe with counter-dispersed imaging
Gdansk Jul 02 2005
reconstructed field;velocity = ½ separation
[O III] filter, slitlessdispersed 0°
[O III] filter, slitless,dispersed 180°
positions & velocities in one go!
PNe with counter-dispersed imaging
Gdansk Jul 02 2005
WHT+PN.S:
March 2002
3 hrs :
197 PN velocities
to 7 Reff ,
v = 20 km/s
E1 , MB = -20.0
D = 11 Mpc
PNe in NGC 3379
Gdansk Jul 02 2005
isotropic constant-M/L Hernquist model
long-slit data(Statler & Smecker-Hane
1999)
29 PNe Ciardullo et al. (1993)
NGC 3379: Dispersion profile
197 PNe from PN.S197 PNe from PN.S
Gdansk Jul 02 2005
NGC 821,
NGC 3379,
NGC 4494:
PN p(R)
declining with R
NGC 821,
NGC 3379,
NGC 4494,
NGC 4697:
PN p(R)
declining with R
Combined dispersion profiles
isotropic constant-M/L Hernquist model
Gdansk Jul 02 2005
Interpreting the Kinematics:Orbital anisotropy
• Radial orbits• at large R, most of
the motion in plane of sky
• Low velocity dispersion cf circular speed
• Peaked velocity distributions
• Tangential orbits• at large R, much of
the motion in line of sight
• High velocity dispersion cf circular speed
• Flat velocity distributions
which?
Gdansk Jul 02 2005
Velocity distribution shaperelates to orbit anisotropy
Van der Marel & Franx 1993
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NGC 3379: orbit models
PN velocities
LOSVDs shown
in radial bins:
• data• simulated from
data• model
• ~isotropic orbits
Gdansk Jul 02 2005
NGC 3379: orbit models
• best fit
• permitted
• excluded
Circular velocity
profile:
Gdansk Jul 02 2005
Results:• constant M/L ruled out at 1 • flat rotation curve ruled out at 6
NGC 3379: orbit models
• cumulative M/L at 5 Reff : = 6 - 9• cf. models of stellar pop M/L: = 4 - 9
(Gerhard et al. 2001, after Maraston 1998)
• at virial radius: non-baryonic fraction = 48 - 86% cf. cosmological fraction = 85 - 86%
(Spergel et al. 2003)
dark matter at large radius?
Gdansk Jul 02 2005
Caution
• Orbital anisotropy hard to measure– Need 100s of velocities or accurate spectra
• Assumed spherical symmetry– What if we see a face-on disk or triaxial galaxy?
• PNe trace overall stellar population?– If colder component, density more concentrated– Underestimate mass if don’t correct density
Gdansk Jul 02 2005
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Caution
• Dekel et al. (2005) disk merger simulations – Make ‘young’ stars
during simulation– Colder, tighter
component– Trace PNe?
dark halo
stars
Enc
lose
d m
ass
r/Reff
Gdansk Jul 02 2005
Caution• Orbital anisotropy hard to measure
– Need 100s of velocities or accurate spectra
• Assumed spherical symmetry– What if we see a face-on disk or triaxial galaxy?
• PNe trace overall stellar population?– If colder component, density more concentrated– Underestimate mass if don’t correct density
• Are the dynamics in equilibrium?
Gdansk Jul 02 2005
Outer envelope of M87 (Weil et al. 1997)
30’ (135kpc)30’ (135kpc)
• Flattened outer envelopeFlattened outer envelope
• Asymmetric Asymmetric unrelaxed unrelaxed
Gdansk Jul 02 2005
Dynamical consequences of dark matter in galaxies
• Static: – rotation curves, dispersion profiles
• Dynamics: – Disk stability (Ostriker & Peebles 1970)
– Angular momentum exchange with bars, warps (Athanassoula 2003, Kuijken&Dubinski 1995)
– Mergers:• Dynamical friction (energy loss to dark halo)
– e.g., LMC or Sgr orbit
Gdansk Jul 02 2005
Gravitational lensing• No dynamical equilibrium assumptions• Direct measurement of projected mass distribution
– Cluster masses (X-ray, dynamics, lensing) agree
• Weak shear: measure shapes of halos as well as overall power spectrum of dm (not average density though)
`Lens pushes sources away’
`Radial squeezing’
Gdansk Jul 02 2005
Gdansk Jul 02 2005
Alternative• MOND (Milgrom 1984)
– Below accelerations of ca 10-10 gravity gets stronger: ((|g/a0|)g)=4G where g= and 1 for large g
– (x) x for small x gives for weak accelerations g(GMa0/r2)1/2 1/r
– Relativistic version ‘TeVeS’ (Bekenstein 2004)
TAK!
• Rotation curve shapes and amplitudes well-explained
• Pioneer effect?• Naturally explains Tully-Fisher
NIE!
• Cosmic expansion as if there is no dark matter
• Unclear how well it does on clusters• Halo shapes?• Galaxy stability?
Excuse me?Przepraszam?
Gdansk Jul 02 2005
Co to jest?
• Baryons?– Nucleosynthesis and CMB bounds– Brown dwarf, cool white dwarf counts
• Compact objects (MACHO’s)?– Microlensing experiments– LMC results (MACHO, EROS): 0-20% of dark halo can
be made up of objects with masses of planets-stars– Detailed interpretation complex because of unknown 3-
D structure of LMC.
Nie!!
Gdansk Jul 02 2005
M31 microlensing
• Pixel lensing• Higher optical depth than to LMC• Compare near & far side of disk
– Very different M31 halo path lengths– Discriminate MW vs M31 halo vs M31 disk– Constrain M31 halo flattening
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Gdansk Jul 02 2005
MEGA project (Crotts, P.I.; de Jong, PhD thesis)
• INT monitoring, 1999-2004• Find variables in PSF-matched difference
images• 14 events• Consistent with lensing by bulge and disk
only• AGAPE team used same data,
claim ~ 20% halo fraction
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Gdansk Jul 02 2005
Doubts
• Let’s detect the particle!
• Has dynamical friction against a dark halo ever been seen?– Satellites (clouds), bars,
warps, polar ring formation
• Do all galaxies have dark halos?– NGC 3379
• What are the shapes of dark halos?
Prospects
• Direct detection experiments continue
• Improved constraints from CMB
• PNe as tracers of outer dynamics probe galaxy halos
• Weak lensing measurements for projected shapes and radial profiles
ZŁY DOBRY
Gdansk Jul 02 2005
The KIDS survey and dark matter• VST/OmegaCAM survey
• 1700 sq deg. ugriz + YJHK• Median z ~ 0.8• Weak lensing
– Galaxy halo masses, radii, shapes– Power spectrum of large-scale mass
distribution– Evolution of angular diameter distance
• Halo objects– Faint high proper-motion stars (white,
brown dwarfs)
Gdansk Jul 02 2005
• Overlaps:– UKIDSS– SDSS– 2dFGRS– CFHLS– COSMOS
• 960 sq deg.
2dFGRSSDSS DR2
CFHLS
KIDS(Leiden, Groningen, Munchen, Bonn, Paris, Naples, Imperial, Edinburgh, Cambridge)
Gdansk Jul 02 2005
• Overlaps:– 2dFGRS– VISTA!
• 720 sq deg.
• Perfect for VLT and AAT, APEX, ALMA
2dFGRS
KIDS(Leiden, Groningen, Munchen, Bonn, Paris, Naples, Imperial, Edinburgh, Cambridge)
Gdansk Jul 02 2005
KIDS vs. SDSS
Gdansk Jul 02 2005
Weak gravitational lensing
`Lens pushes sources away’
`Radial squeezing’
80,000,000 background galaxies
200,000 foreground galaxies (z<0.2)
Gdansk Jul 02 2005
Galaxy-galaxy lensing45 sq. deg from RCS survey (Hoekstra, Yee, Gladders 2004)
Galaxy-mass correlation
Halo radii
Halo shapes
KIDS:
7x smaller errors (#pairs)
Good photo-z’s (b/g), spectroscopic z’s (lenses)
Study effect by galaxy type
Gdansk Jul 02 2005
‘w’ (weak lensing)
• Weak lensing constraints– Lensing effect depends on relative distances
of source and lens
– Measure lensing strength as function of redshift
– Deduce distance as function of redshift– Geometrical test of expansion history: w (5%)– Needs well-controlled photo-z’s!
Gdansk Jul 02 2005
Summary • Dark matter is with us
– CMB, large-scale structure formation– Galaxy dynamics– Gravitational lensing
• It is mostly non-baryonic– CMB, nucleosynthesis arguments
• Halos do not consist of MACHO’S– Microlensing experiments to LMC and M31
• Evidence for ‘live’ dark halos would be nice– Shapes– Dynamical friction
• Laboratory detection of a DM particle would be nice!
PNe as astrophysical tool!
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