1. Galaxy-Galaxy Lensing: Observational Constraints on Dark Matter Halos
Tereasa Brainerd
Boston University, Institute for Astrophysical Research

2. Outline
Highlights of recent gg lensing studies (not a comprehensive review!!)
Results from RCS, SDSS, COMBO-17, GEMS
Improvements over early efforts
GG lensing as function of Hubble type and luminosity of the lens
Halo density profiles (SIS vs. NFW)
Mass-to-light ratios and virial vs. stellar mass fraction in lenses
Field galaxy halos vs. cluster galaxy halos
Halo shapes (spherical or flattened?)

3. How do you improve detection of the signal?
Bigger data sets (generally = wider)
Smarter shape measurements (see Henk’s talk)
Smarter lens-source separation (spectroscopic z when possible, photometric z if nothing else)
Combinations of the above

4. Red Cluster Sequence (RCS) Hoekstra, Yee & Gladders 2004, ApJ, 606, 67
RCS was wide field imaging survey designed primarily to detect clusters to z ~ 1.4
22 widely-separated patches of ~2.1o X 2.3o, obtained with CFHT and CTIO 4-m telescopes
GG lensing result comes from ~42 sq. deg. of northern RCS data
Lens-source separation based on apparent magnitude (bright=close, faint=far)
Strong detection of circularly averaged shear, first constraint on virial radius and first to show consistency with NFW halos
First constraint on halo flattening (will come back to this…)

5. Hoekstra et al. (2004) ~1.2x105 lenses with 19 < RC < 21
~1.5x106 sources with
21.5 < RC< 24
Signal on very large scales is a reflection of the clustering of the lenses, not their masses (i.e., galaxy-mass cross-correlation)

6. Hoekstra et al. (2004)
BBS halo model:
Best fit halo:

7. Slice from one of Ben Moore’s LCDM simulations (~0.3 Gpc long)

8. “Navarro, Frenk & White” (NFW) halos; not quite isothermal
CDM halos are triaxial and self-similar (i.e., cluster halos look like galaxy halos but with different scale sizes)
“Navarro, Frenk & White” (NFW) halos; not quite isothermal
Can we actually discriminate between NFW and isothermal halos by gg-lensing?
“Milky Way” halo from one of Ben Moore’s simulations

9. Navarro, Frenk & White (NFW) Halos
3-d mass density:
where
Concentration parameter:
Virial mass:
Surface mass density:
where x = R/rs
(Bartelmann 1996, A&A, 313, 697; Wright & Brainerd 2000, ApJ, 534, 34)

10. Wright & Brainerd 2000, ApJ, 534, 34 x = R/rs SIS = dashed line
NFW = solid line
x = R/rs

11. Hoekstra et al. (2004)
Best-fit NFW halo model:

12. COMBO-17 Kleinheinrich et al. 2006, A&A, 455, 441
COMBO-17 = “Classifying Objects by Medium-Band Observations in 17 Filters”
Five 34’x33’ fields using Wide Field Imager on the 2.2-m MPG/ESO telescope; wavelength coverage of filters is ~350nm to ~930nm
Accurate photo-z’s used for lens-source separation
GG lensing results from 3 of the 5 fields
At fixed luminosity, the halos of red galaxies are more massive than those of blue galaxies by a factor of ~2 (but with large error bars)

13. Kleinheinrich et al. (2006); BBS SIS model with variable Tully-Fisher index
all galaxies
Galaxies with <U-V> <= z -0.08(MV - 5log h + 20) are “blue”, all others are “red”

14. Kleinheinrich et al. (2006); NFW halo model
In analogy with Tully-Fisher:
Warning! Different groups define the virial radius in different ways (can make the mass constraints seem to be inconsistent when they aren’t).

15. Kleinheinrich et al. (2006); NFW halo models
all galaxies

16. Sheldon et al. 2004, AJ, 127, 2544
SDSS galaxies:
~1.2x105 lenses with spectroscopic z
~9.0x106 sources with zphot
Spectroscopic z for lenses allows measurement of signal as a function of physical radius on the sky

17. Sheldon et al. 2004, AJ, 127, 2544
Galaxy-mass cross correlation function from gg lensing in SDSS, corrected for halo clustering on large scales

18. Sheldon et al. 2004, AJ, 127, 2544
GG lensing as a function of lens luminosity

19. Sheldon et al. 2004, AJ, 127, 2544
Galaxy-galaxy lensing as a function of lens color and morphology

20. RCS + Hoekstra, Hseih, Yee, Linn & Gladders 2005, ApJ, 635, 73
~33.6 sq. deg. of RCS RC, z’ + B, V for photo-z
94,509 “isolated” lens galaxies with 0.2 < zphot < 0.4, 18 < RC < 24, <zphot> ~ 0.32
lenses have 109 < LB < 5x109 h-2 LB,sun and no galaxy with LB > 5x109 h-2 LB,sun within 30 arcsec
sources have zphot < 1
NFW halo model
Constrain M/L and Mvir/Mstar

21. Hoekstra et al. 2005, ApJ, 635, 73
Dotted line is model from van den Bosch et al. (2003)

22. Rest-frame M/L for blue galaxies is ~30 to ~35
Hoekstra et al. 2005, ApJ, 635, 73
Rest-frame M/L for blue galaxies is ~30 to ~35

23. Hoekstra et al. 2005, ApJ, 635, 73
Stellar mass fraction is a factor of ~2 lower in early-type galaxies than in late-type galaxies
Conversion of gas into stars was rather ineffiecient (~33% in late types, ~14% in early types)

24. COMBO-17 + GEMS Heymans et al. 2006, MNRAS, 371, L60
Combine COMBO-17 (accurate photo-z) with HST GEMS survey (accurate images)
696 high-mass, high luminosity lenses, 0.2 < zlens < 0.8
For lenses, <Mstar> = 7.2x1010 Msun, <Lr> = 2.4L*, L* = 1010 h-2 Lsun
75% of lenses are early type
NFW halo model
Mvir/L = 123 +/- 36 h Msun/Lsun
Investigate evolution of Mvir/Mstar

25. Heymans et al. 2006, MNRAS, 371, L60
Solid curve is best-fit NFW model with zlens = 0.65, rvir = 204 h-1 kpc

26. Triangles = Mandelbaum et al. 2006 (SDSS)
Heymans et al. 2006, MNRAS, 371, L60
Star = Hoekstra et al. 2005
Triangles = Mandelbaum et al (SDSS)
Hashed region = formal 1-sigma error from Heymans et al. 2006

27. Something a little different - gg lensing through clusters…

28. Limousin et al. 2006, astro-ph/0609782
Lenses are truncated isothermal distributions
5 clusters: A1763, A1835, A2218, A383, A2390
Halos of L* cluster galaxies do appear to have smaller radial extents than those of L* field galaxies

29. Halo Shapes from GG Lensing
CDM predicts that halos should be mildly-flattened, <e> ~ 0.3
in principle, this should be detectable from galaxy-galaxy lensing
big question is whether or not mass and light are aligned sufficiently well in the lens galaxies
to date, only 2 groups brave enough to publish an observational result!

30. Hoekstra et al. (2004); RCS data
Simple model of halo flattening + assume mass and light are aligned within the lenses:
Round lenses (f=0) are ruled out at the 99.5% confidence level!
Best fit model:

31. This is a hard experiment to do…
Mandelbaum et al. 2006, MNRAS, 370, 1008
SDSS data
~2x106 lenses with r < 19 and photo-z
~32x106 sources
Using non-Gaussian error distributions and NFW halos compute f = ehalo/elight separately for red and blue lenses
fred = 0.60+/-0.38
fblue =
This is a hard experiment to do…

32. Anisotropic Galaxy-Galaxy Lensing
At fixed angular separation, sources closer to the major axis of the mass will experience greater shear than sources close to the minor axis of the mass.
At large impact parameters (compared to virial radius), the anisotropy drops sharply
Are mass and light truly aligned in the lenses?
Can we assume ehalo = f elight for all lenses??

33. Agustsson & Brainerd 2006, ApJ, 650, 550
ehalo = f elight is a poor assumption for DISK lenses
Projected ellipticity of lens halo is nearly independent of projected ellipticity of lens disk
Most galaxy lenses are spirals, BUT ellipticals are responsible for most of the galaxy-galaxy lensing signal…

34. Agustsson & Brainerd 2006, ApJ, 650, 550
1st panel, disk J aligned with halo major axis
2nd panel, disk J aligned with halo minor axis
3rd panel, disk J aligned with halo J
4th panel, disk J aligned with major axis of local LSS
Luminous disks will have their major axes offset from that of the halo mass to at least some degree
Most galaxy lenses are spirals, BUT ellipticals account for most of the galaxy-galaxy lensing signal…
Might expect to see less anisotropic lensing around spirals than around ellipticals

35. Satellite galaxies ought to tell us something about halo shapes…
In the “Holmberg Effect”, satellite galaxies are found preferentially close to the minor axes of their host galaxies.

36. Agustsson & Brainerd 2007, in prep.
Satellites of isolated host galaxies in the SDSS are anisotropically distributed about their hosts, with a preference for being close to the host’s MAJOR axis
Satellite locations trace the projected mass (i.e., the halo) of the host galaxy, not the light

37. Satellites of blue host galaxies are isotropically distributed!
Red SDSS Hosts
Blue SDSS Hosts
Satellites of blue host galaxies are isotropically distributed!
Could be misalignment of mass and light, intrinsically rounder halos, or lower mass halos for blue hosts
Should expect to see less “flattening” in gg lensing around blue lenses

38. Another thing you ought to see in gg lensing by flattened halos…
Wright 2002 (PhD Thesis)
Wright 2002 (PhD Thesis)
Pairs of lenses and sources can (and will) be lensed by more nearby galaxies, inducing correlated images
Mass and light in the lenses become misaligned! (cosmic shear from large k part of power spectrum)
Ray tracing simulations with singular isothermal ellipsoid halos and Ilim= 25
Major axis of LIGHT used here
Major axis of MASS used here

39. Howell & Brainerd 2007, in prep.
13.5 square degrees of I-band imaging data
BTC on the CTIO 4-m, median seeing ~ 1 arcsec (FWHM)
“Lenses” have 18 < I < 20, zmed ~ 0.35
“Sources” have 20 < I < 22.5, zmed ~ 0.68
~ 15,400 “lenses”
~ 180,000 “sources”
~ 535,000 lens-source pairs

40. Howell & Brainerd 2007, in prep.
Sources within 45o of the major axis of the light have shear consistent with zero on scales greater than ~10 arcsec
Sources within 45o of the minor axis of the light have shear consistent with zero on scales greater than ~60 arcsec
This is the differential shear! Data points and error bars are independent.
We think it’s right…