1. Galaxy-Galaxy lensing
Alexie Leauthaud
Jean-Paul Kneib, Olivier Le Fevre, Lidia Tasca
LAM (Laboratoire d’Astrophysique de Marseille)
Jason Rhodes (JPL), Richard Massey (Caltech)

2. Galaxy-Galaxy Lensing
200 kpc
Tangential ellipticity is proportional to tangential shear, γt
γt = εt
Shear is a measure of the total mass (dark and luminous)
γt (r) ~ Σ(<r) - Σ(r)

3. …is a weak lensing technique
Weak shear cannot be detected from a single background source due to shape noise Galaxies are intrinsically elliptical with
< e > ~ 0.2
< e2 > ~ 0.32
Lensing induces shape correlations that can be measured by averaging over many lenses (~ 1000 )
< γt > = < εt >
γ ~ 0.007
Sensitivity: 0.3 / (N)1/2
Nice!…but rare

4. Probing Dark Matter Halos
Dynamical methods work well within optical radius
r < 20 kpc
Satellite dynamics works at larger scales
- System may not be in equilibrium
- Interlopers make interpretation difficult
Galaxy-Galaxy lensing is a unique method to probe galaxy Dark Matter Haloes ( DMH ) out to large radii
100h-1 kpc ~ 200 h-1 kpc

5. Aims : Galaxy mass function
Measure Galaxy Bias: how does total mass relate to luminous mass?
Understand how DMH vary with - Morphological type - Environment - Redshift
Constrain low mass end of Mass Function - critical ingredient for galaxy formation - predictions by N-body simulations - analytical predictions Few observational constraints.
Jenkins et al. 2000

6. Key to Galaxy Mass function
Galaxy-galaxy lensing requires that Lenses must be stacked.
Therefore, we probe DMH’s of a certain population.
Previous results limited by area and depth of surveys. Cosmos will allow fine binning so that specific populations can be studied.
If we assume stellar mass traces dark matter, then we can bin galaxies according to their stellar mass.
In Progress: Galaxy-Galaxy lensing analysis of H-band selected Nicmos catalog prepared by Lidia Tasca ~ 6000 galaxies

7. Current state of work
Compiled ACS cycle 12/13 catalog Suggestions on this catalog are welcome!
Area : 1.2 sqr degre Number of objects : Source density : ~ 60 sqr armin Lens density : ~ 10 sqr arcmin
Correlated with PhotoZ catalog: 68% correlation
Shapes of galaxies are measured with RRG method (Rhodes, Refregier and Groth, 2000)
PSF correction is performed using Tiny Tim models

8. Preliminary results
Sheldon et al. astro-ph/

9. Single lens approximation
Direct method
Can we assume a Clean LOS ?
Probability that a source will undergo multiple weak deflections must increase with the depth of the data set.
Multiple weak deflections lead to:
- Larger net and tangential shear
Overestimation of halo mass
Use Mockcatalogs with randomized ellipticities and simulate lensing (lenstool) to quatify error on mass.
NEXT STEP: Inverse methods
200 kpc
Teresa Brainerd et al. astro-ph/

10. Galaxy-galaxy flexion
Flexion is a third order lensing term
Combining shear + flexion leads to improved measures of galaxy masses
Bacon et al. astro-ph/
Flexion can be measured using the Shapelets formalism

11. Anisotropic lensing or Evidence for non sphericql halos
(X-ray isophotes, strong lensing)
Stack lenses according to optical axis
()
- Dissipationless CDM models predict flattened halos
 ~ 0.3
- Alternative models of gravity predict isotropic signal or