This document discusses gravitational lensing and some of the challenges involved in measuring it. Gravitational lensing causes the apparent deflection of light from distant background sources as it passes massive foreground objects. Precise measurements of lensing effects can provide information about dark matter distributions and the geometry and growth of the universe. However, there are three main problems: accurately measuring galaxy shapes used to detect lensing distortions, determining reliable photometric redshifts for galaxies, and accounting for intrinsic alignments of galaxy orientations unrelated to lensing.

1. Gravitational Lensing
Filipe B. Abdalla

2. Apparent deflection angle α
• = 4 G M / (c2 b)
• NB. Independent of light wavelength
b

3. Observer
Dark matter halos
Background sources
 Statistical measure of shear pattern, ~1% distortion
 Radial distances depend on geometry of Universe
 Foreground mass distribution depends on growth of structure
Dark matter halos
Background sources
Dark matter halos
Background sources
Dark matter halos
Observer
Background sources
Dark matter halos

4. Observer
Dark matter halos
Background sources
 Statistical measure of shear pattern, ~1% distortion
 Radial distances depend on geometry of Universe
 Foreground mass distribution depends on growth of structure
Dark matter halos
Background sources
Dark matter halos
Background sources
Dark matter halos
Observer
Background sources
Dark matter halos

5. Mass Mapping CFHTLS

6. Cluster Weak Lensing
• Weak Lensing
measures cluster
masses, which are
needed to use them
as Dark Energy
probes
• Preliminary cluster
mass map (contours)
from DES Weak
Lensing

7. Cosmic shear two point tomography

8. Cosmic shear two point tomography

9. Cosmic shear two point tomography

10. Data from CFHTLS

11. Three problems with lensing:
for either tomography and cluster mass
measurements:
• Measuring shapes
• Photometric redshifts
• Intrinsic alignments

12. Measuring the shear!

13. Measuring shapes
is hard

14. Galaxy spectrum at 2 different redshifts,
overlaid on griz and IR bandpasses
• Photometric redshifts
(photo-z’s) are determined
from the fluxes of galaxies
through a set of filters
• May be thought of as
low-resolution
spectroscopy
• Photo-z signal comes
primarily from strong
galaxy spectral
features, like the 4000 Å
break, as they redshift
through the filter
bandpasses
Photometric Redshifts

15. Hyper-Z: the first photo-z code
• Simple chi squared fit
of fluxes to the data
• Very simple and very
successful
• Has limitations:
- Degeneracy, flat chi
squared
- Mis-identification of
features
- contamination from
AGN
- are errors reliable?
Credit: M. Bolzonella

16. from DES Test Data in November
High Redshift Cluster Discovered by
DES

17. Cosmic shear
Additional
contributions
What we
measure
Intrinsic alignements.

18. Intrinsic-shear correlation (GI)
Hirata&Seljak
High z galaxy gravitationally
sheared tangentially
Dark matter at z1 Net anti-correlation
between galaxy
ellipticities with no
prefered scale
Galaxy at z1 is tidally sheared

19. END