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3D Weak Lensing with COSMOS

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					L&P United: The Leiden Lensing (& Photo-z) Group




Tim Schrabback: 3D Weak Lensing with COSMOS
                       3D Weak Lensing with COSMOS

                                          Tim Schrabback


                 Leiden Observatory Science Day, September 3 2009




Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS   Results   Conclusions




The Universe is full of dark stuff... How can we study it?




                                                        WMAP
Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS           Results         Conclusions




Look at the bright side to trace dark matter?




SDSS galaxy distribution                       But is light an unbiased tracer of mass?


Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS            Results        Conclusions




The direct path to the dark side: Use the force!
                     For my ally is the force, and a powerful ally it is (Yoda)


                                                Gravitational lensing
                                                        Physics well understood:
                                                        General Relativity
                                                        Light rays are deflected in
                                                        the gravitational potential of
                                                        matter concentration
                                                        Light bundles are magnified
                                                        and anisotropically sheared
Strong lensing by a galaxy cluster; credit:             by the tidal gravitational field
NASA


Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS             Results           Conclusions




                 Strong lensing by galaxy cluster RXJ1347–1145
             (HST/ACS image; credit: NASA/ESA/AIfA; PI: Erben, Image: Schrabback)

Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS   Results   Conclusions




Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS       Results        Conclusions




                                                        Weak lensing distor-
                                                        tions can be mea-
                                                        sured     statistically
                                                        from an ensemble
                                                        of coherently lensed
                                                        galaxies.



Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS      Results        Conclusions




Assumption: Galaxies randomly oriented without lensing




At some level intrinsic alignments will become important, but they can probably be
                 dealt with using photometric redshift information.



Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS              Results            Conclusions




Cosmic shear: Weak lensing by large-scale structure
                                                        Ωm = 1
                                                        ΩΛ = 0
                                                        Cosmological weak lensing
                                                        probes the density field free
                                                        of assumptions on the
                                                        relative distribution of
                                                        luminous and dark matter.
                                                        Ωm = 0.3
                                                        ΩΛ = 0
                                                        Ray-tracing through N-body simula-
                                                        tions by Jain, Seljak, & White (2000).
                                                        Left: magnification (map similar to pro-
                                                        jected mass), Right: projected shear
                                                        field.


Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS   Results   Conclusions




How about Dark Energy?




      DE increases distances (z) and
      dampens growth of structure.
      WL sensitive to both ⇒
      constrain w and test for
      modified gravity
      Requires 3D lensing ⇒ Split
      according to photo-z

Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS   Results   Conclusions




What do we need to measure?

 1. Shears (very weak: ∼ 1%)




 2. Photo-zs ⇒ only available for most recent surveys


Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS             Results          Conclusions




Why COSMOS?

      Largest area ever imaged with HST:
      1.64 deg2 with ACS in i814
      ⇒ sharp PSF (FWHM∼0. 11)
                                                        HST during SM 3B (NASA/ESA).
      ⇒ ∼ 80 resolved galaxies/arcmin2
      Excellent photo-zs from ground:
      COSMOS-30, CFHTLS-D2
      Area too small for competitive DE
      constraints. But:
      Crucial proof of concept!
      No other survey has yielded
      comparable results so far!

                                                        Tiling (Koekemoer et al. 2007).
Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS            Results   Conclusions




Why a new analysis?
     Earlier analysis: Massey et al. (2007)
             3 redshift bins; No Dark Energy constraints
             Limited treatment for some systematic effects

     New analysis: Schrabback et al. (in prep)
             New photo-z catalogs
             6 redshift bins
             Independent data reduction
             New treatment of systematic effects:
                   PSF correction with PCA interpolation
                   CTI correction for stars and galaxies
                   Proper covariance and error marginalization

Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS   Results   Conclusions




Systematics I: PSF anisotropy
        Introduces spurious signal if uncorrected
        HST/ACS PSF varies between exposures
        COSMOS fields contain too few (∼ 20)
        stars for direct interpolation
        New correction scheme with Principal
        Component Analysis (PCA)
        97% of variation 1-parametric (focus)
        Additional residual correction



Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS   Results   Conclusions




Systematics II: Charge transfer
inefficiency
        Cosmic ray bombardment creates
        charge traps
        Introduces spurious ellipticity
        New parametric correction for
        galaxies and stars




Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS          Results   Conclusions




Test: E/B-mode decomposition




                               Lensing creates only E-modes!


Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS         Results         Conclusions




Test: E/B-mode decomposition




Massey et al. (2007)                           Schrabback et al. (in prep.)


Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS          Results        Conclusions




3D Weak lensing: The principle




                                                    Cross-correlate galaxy elliptici-
Redshift bins                                       ties between redshift bins.

Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS         Results       Conclusions




3D Weak lensing: Shear-shear cross-correlation




                                               Shear-shear cross-correlations with
Redshift bins
                                               bin 6. Model: ΛCDM, σ8 = 0.8

Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS       Results     Conclusions




3D Weak lensing: Combined redshift-scaling




                                               Combined redshift-scaling.
Redshift bins
                                               3D Lensing works as expected!

Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS          Results       Conclusions




Cosmological constraints
                                                        Ωm : Matter density
                                                        σ8 : Power spectrum normal-
                                                        ization (“clumpiness”)


                                                            CMB and WL
                                                            constraints nearly
                                                            orthogonal!
                                                            DE constraints in
                                                            progress (non-linear
                                                            corrections...)


                                                        NL-power spectrum: Smith et
                                                        al. 2003
Tim Schrabback: 3D Weak Lensing with COSMOS
Motivation             Gravitational lensing   COSMOS      Results       Conclusions




Conclusions
             Weak Lensing allows us to study the DM distribution directly and
             will potentially be the most powerful probe of DE.
             COSMOS provides a crucial proof of concept for future DE
             missions like CFHTLS, KIDS, DES, JDEM, LSST.
             Weak Lensing redshift scaling works!


                           Thank you for your kind attention!

   I would like to thank my collaborators on this project:
   Patrick Simon, Jan Hartlap, Benjamin Joachimi, Martin Kilbinger, Maruˇa s
         c
   Bradaˇ, Tim Eifler, Thomas Erben, Chris Fassnacht, William High, Hendrik
   Hildebrandt, Henk Hoekstra, Konrad Kuijken, Phil Marshall, Eric Morganson,
   Peter Schneider, Elisabetta Semboloni & Ludovic Van Waerbeke.

Tim Schrabback: 3D Weak Lensing with COSMOS

				
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