1. Glenn van de Ven Institute for Advanced Study glenn@ias.edu
Probing dark matter in galaxies with gravitational lensing and kinematics
Glenn van de Ven
Institute for Advanced Study

2. Cold dark matter simulations
…galaxies are embedded in extended dark matter distributions with a close to universal profile and triaxial shape
Almost all observations based on collecting light…
Need conversion from dark matter to (luminous) baryons:
‘adding’ baryons using empirical prescriptions
‘subtracting’ baryons from total mass distribution inferred through luminous tracers of the gravitational potential a b c
(b/a)dm ~ 0.7
(c/a)dm ~ 0.5
(Jing & Suto 2002)
The now-standard cold DM cosmological model predicts that …
Since almost all observations are based on collecting light, we can test these predictions only by converting between dark and luminous baryonic matter.
This conversion can be done in two ways…
The main goal of my proposal is to use this second approach to constrain both the profile and shape of the DM distribution.
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

3. Photometric observations
M51 = NGC 5194
NGC 4365
If we then look for these luminous tracers, its is obvious that the spiral galaxies have a very complex appearance with lost of obscuring dust.
On the other hand, elliptical galaxies appear very simple with mainly stars as clean luminous tracers.
Late-type galaxies: complex with lots of obscuring dust
Early-type galaxies: simple with (mainly) stars as clean tracers
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

4. Integral-field spectroscopy
To measure the kinematics of the stars we can use integral-field spectroscopy.
An array of lenslets provides at every position on the plane of the sky a spectrum, from which we can extract, for example, the kinematics of the stars.
… a spectrum at every position on the plane of the sky
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

5. Stellar kinematic observations
NGC 4660 [-150/+150 km/s]
NGC 4365 [-58/+58 km/s] z x z
triaxial
oblate axisymmetric
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

6. Stellar dynamical models
Most early-type galaxies consistent with oblate axisymmetry
SAURON paper IX: Emsellem et al. (2007,MNRAS,379,401)
SAURON paper X: Cappellari et al. (2007,MNRAS,379,418)
SAURON paper XII: Krajnovič et al. (2008,MNRAS,390,93)
Triaxial dynamical models of giant ellipticals…
van de Ven, de Zeeuw & van den Bosch (2008,MNRAS,385,614)
van den Bosch, van de Ven, et al. (2008,MNRAS,385,647)
van den Bosch & van de Ven (2008,MNRAS,arXiv: )
…still dominated by short-axis tube orbits
“Simple” axisymmetric models at high(er) redshift?
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

7. Stellar kinematics at higher-z
In a very recent project, I used another integral-field spectrograph, GMOS at the GEMINI south telescope, to obtain kinematics of the inner parts of a galaxy at a distance about 10 times as far as our typical kinematic observations so far.
Although quite challenging we were able to extract a very nice velocity and dispersion field.
From fitting a dynamical model we find an M/L…
Applying stellar population models to the spectra and images at different wavelengths we estimate a stellar M/L…
This provides with at most 20% DM in the inner parts.
Perhaps even more important is that this proofs the feasibility of detailed stellar dynamical modeling at increasing redshift.
GMOS spectrograph ~4 hours on-source z=0.04 (D=155 Mpc)
van de Ven et al. (2008,ApJ,arXiv: )
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

8. Axisymmeric dynamical model
Mass-anisotropy degeneracy: radial variation in l.o.s. velocity dispersion due to change in mass or velocity anisotropy
Total mass-to-light ratio: M/L=3.7±0.5 M/L
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

9. Strong gravitational lensing
zl= (D=155 Mpc)
zs=1.695
However, as you have noticed, this is a rather special galaxy, since it also acts as a lens galaxy that deflects the light of a distant quasar into four images in a cross-like configuration: well-known as the Einstein Cross
Since GL is sensitive to the total mass, the images provide alternative luminous tracers.
Below, on the left, is again the observed LIGHT distribution (with HST), and on the right is the TOTAL MASS distribution as obtained by fitting a lens model to the positions and fluxes of the four quasar images.
This yields also a M/L that is consistent with the dynamical one, and again, at most a 20% DM fraction.
This is a crucial test of both kinematics and GL as luminous tracers.
The strong advantage of GL over kinematics is that it “only” requires (good) imaging.
In particular, the upcoming deep and wide imaging surveys like … are expected to reveal a vast number of these lensing events, allowing constraints on the DM distribution at high redshift.
The Einstein Cross
QSO
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

10. Strong gravitational lens model
source position
flux ratio
Mass-sheet degeneracy: additional mass along line-of-sight
Scale-free projected potential
(Evans & Witt, 2003,MNRAS,345,1351)
Predict image positions and flux ratios: 2x4+3 = 11 constraints
Slope , quasar position (ξ,η) + 7 Fourier coeff. = 10 parameters
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

11. surface brightness = light
Surface density
surface brightness = light
… “mass follows light” in projected shape
lens model = mass
RE~1”
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

12. Mass distribution projected mass Projected flat ME/LE=3.4 M/L round
lens models
ME/LE=3.4 M/L
luminous density
round
density
ellipticity
flat
Rein Re
round
Intrinsic
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"
circular velocity

13. Dark matter fraction Dynamical mass-to-light ratio: 3.7 ± 0.5 M/L
Mass and light within Einstein radius: 3.4 ± 0.1 M/L
Stellar mass-to-light ratio:
HST central V, I, R, H and K colors
SSP models with Kroupa IMF, 0.01–120 M (Vazdekis et al. 2006)
…best-fit t=8 Gyr and [Fe/H]=0.2: M*/L = 3.3 M/L
…range t=7–14 Gyr and [Fe/H]=0.0–0.3: M*/L=2.8 – 4.1 M/L
Inner R<4” ~ 2/3 Re bulge-dominated region of ~L* lens galaxy: mass follows light, at most ~20% constant dark matter fraction
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

14. Isothermal conspiracy?
Total mass density isothermal? (e.g., Koopmans et al. 2006)
Strong lensing provides constraints only around Einstein radius!
Einstein Cross: projected
Galaxy models: projected slope
lens models
luminous density
lens models
luminous density
dark matter
isothermal
isothermal
stars
van de Ven, Mandelbaum & Keeton (2008,MNRAS,arXiv: )
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

15. Isothermal conspiracy?
Total mass density isothermal? (e.g., Koopmans et al. 2006)
Strong lensing provides constraints only around Einstein radius!
Einstein Cross: projected
Galaxy models: deflection curve
lens models
luminous density
isothermal
isothermal
dark matter
stars
van de Ven, Mandelbaum & Keeton (2008,MNRAS,arXiv: )
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

16. Lensing cross section σ(x)
Selection biases
strong lens galaxies
all galaxies
+ magnification bias
Lensing cross section σ(x)
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

17. Orientation and shape
Total cross section: higher long-axis along l.o.s.
Four-image: higher intermediate axis along l.o.s.
Three-image: only for very flat projections, but possibly misclassified quads
Total: no shape bias if averaged over viewing angles
4/2-ratio: probe of shape, but dependence on e.g. source luminosity function
total
4/2-ratio
projected flattening
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

18. Dark matter inner slope
total
4/2-ratio
Strong selection bias with dark matter inner slope (> adiabatic contraction)
Total: independent of shape and magnification bias
4/2-ratio: weak dependence shape + magnification bias
Degenerate with concentration…
DM inner slope
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

19. Dark matter concentration
Total cross section
total
4/2-ratio
DM concentration
DM inner slope
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

20. Lensing and kinematics
Lensing in photometric surveys, and kinematics at increasing redshift: constraints on dark matter distribution
…but first understand selection and modeling biases
Simulation pipeline for lensing and kinematics by galaxy models (van de Ven, Mandelbaum & Keeton, 2008,MNRAS,arXiv: )
Selection biases in strong lensing surveys (Mandelbaum, van de Ven & Keeton, 2008,MNRAS,arXiv: )
Strong lensing modeling biases (Keeton, Mandelbaum & van de Ven, in prep.)
Stellar kinematics modeling biases (cf. van de Ven, de Zeeuw & van den Bosch 2008,MNRAS,385,614)
Strong lensing and stellar kinematics biases
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

21. Summary in figures HFS, March 10, 2009
Einstein Cross
NGC 4365
NGC 3379
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

22. END
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

23. Isothermal conspiracy?
Total mass density isothermal? (e.g., Koopmans et al. 2006)
Strong lensing provides constraints only around Einstein radius!
Galaxy models (stars + DM)
van de Ven, Mandelbaum & Keeton (2008,MNRAS,arXiv: )
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

24. Image separation: spherical
Singular isothermal sphere (SIS): Δθ=2Rein
Distributions highly skewed towards SIS
Consistent with “isothermal conspiracy”
Nearly independent of DM inner slope or magnification bias
DM inner slope
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

25. Image separation: non-spherical
Long-axis in plane highest mean image separation
Intermediate axis along l.o.s. highest width image sep. distribution
edge-on
face-on
side-on
end-on
l.o.s.=long-axis
l.o.s.=intermediate-axis
l.o.s.=short-axis
DM inner slope
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

26. Sersic: cross section vs. index
Lower mass
Higher mass dm dm
stars
stars
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"

27. Sersic: Einstein radius
total
stars
dark matter
Einstein radius
HFS, March 10, 2009
Glenn van de Ven, "Probing dark matter in galaxies"