Docstoc

Rowe

Document Sample
Rowe Powered By Docstoc
					 Weak Gravitational Flexion
from HST GEMS and STAGES
                     Barnaby Rowe
                          with
   David Bacon (Portsmouth), Andy Taylor (Edinburgh),
  Catherine Heymans (U.B.C.), Richard Massey (Caltech),
                Dave Goldberg (Drexel)
       STAGES TEAM            Meghan Gray (PI) (Nottingham)
                              David Bacon (Portsmouth)
                              Michael Balogh (Waterloo)
                              Marco Barden (MPIA)
   GEMS TEAM                  Fabio Barazza (UTexas)
                              Eric Bell (MPIA)
Hans Walter Rix (PI) (MPIA)   Asmus Boehm (AIP)
Marco Barden (MPIA)           John Caldwell (UTexas)
Steven Beckwith (STScI)       Boris Häußler (MPIA)
Eric Bell (MPIA)              Catherine Heymans (UBC)
Andrea Borch (MPIA)           Knud Jahnke (MPIA)
John Caldwell (UTexas)        Shardha Jogee (UTexas)
Boris Häußler (MPIA)          Eelco van Kampen (Innsbruck)
Catherine Heymans (UBC)       Sergey Koposov (MPIA)
Knud Jahnke (MPIA)            Kyle Lane (Nottingham)
Shardha Jogee (UTexas)        Daniel McIntosh (UMass)
Sergey Koposov (MPIA)         Klaus Meisenheimer (MPIA)
Daniel McIntosh (UMass)       Chien Peng (STScI)
Klaus Meisenheimer (MPIA)     Hans Walter Rix (MPIA)
Chien Peng (STScI)            Sebastian Sanchez (CAHA)
Sebastian Sanchez (CAHA)      Rachel Somerville (MPIA)
Rachel Somerville (MPIA)      Andy Taylor (Edinburgh)
Lutz Wisotzki (AIP)           Lutz Wisotzki (AIP)
Christian Wolf (Oxford)       Christian Wolf (Oxford)
Xianzhong Zheng (MPIA)        Xianzhong Zheng (PMO)
Simulations exhibit an abundance
of dark matter substructure at a
wide range of scales…

                …they also suggest
                that halos follow a
                certain profile
                (e.g. the NFW density
                profile – see Navarro,
                Frenk & White 1997)

Detailed predictions exist for halo
properties and substructure as a function
of mass, formation time and
environment…                                (Moore et al. 1999)



 …but these predictions remain untested
“Traditional” weak lensing
          source
                        Image transformations can
                        often be described by a
                        simple, locally linearized
   lens
                        mapping:
             observer




i  Aij        '
                   j
                          g = g1 + ig2
Weak
lensing
to higher
order:
flexion

 =  1 + i 2 '
i 1 Aij  j
    *             + Dijk  
                       1             '       '
 = 2               2             j       k

g = 2 
    1

F = 
G = g
                   Spin 1   Spin 2       Spin 3
        Flexion is sensitive to matter variations at
                 smaller scales than shear
     – it’s like a high pass filter for mass structure







                               x
Cosmological predictions (see Bacon et al. 2006) show
that flexion is particularly sensitive to dark matter
structure at small scales
Flexion from space
We are using the GEMS and
STAGES surveys for a
combined shear-flexion weak
lensing analysis

The fields each offer:
• >800 arcmin2 of deep (~60
  galaxies per arcmin2) space
  imaging from the HST-ACS.
• >8 000 high-quality
  photometric redshifts from the
  COMBO-17 survey (see Wolf
  et al. 2004).
    Measuring galaxy shapes
We can make accurate
measurements of galaxy
shapes using the Shapelet
formalism (see e.g. Refregier
2003, Massey & Refregier 2005)

• Using this method we can
  decompose each image
  into a sum of orthogonal
  2D basis functions
• All shape information can
  then be easily quantified
                                 Massey et al. 2006
PSF correction




                                We built a detailed shapelet
                                model of each star

Using these models we can estimate the PSF across the survey
images– then deconvolve our galaxies in shapelet space
Shear and flexion measurements
Quick and dirty “STEP”
Galaxy-galaxy lensing is a useful tool for
studying galaxy halo mass distributions


For shear, we may first
look at the mean
tangential shear within
angular bins around
foreground lenses…
                                                 

               For flexion the median provides a
               better statistic, being less sensitive to
               the broad wings in F and G
Galaxy-galaxy shear
Galaxy-galaxy F
Galaxy-galaxy G
  Flexion correlation statistics
In a Universe in which matter is                    Massive
correlated (clumpy) we also                     foreground halo
expect correlations between
the shears and flexions of                         Background
pairs of galaxies, varying as a                      galaxy
function of angular separation.




                                   Predictions for lensing
                                   correlation functions C()
                                   (for gg, FF, GG etc.) exist,
                                   and can be used to constrain
                                   cosmological parameters.
Cosmic flexion




F                G
               Summary
• Flexion is a promising tool for studying dark
  matter structure on small scales
• Measurements of flexion from HST GEMS
  and STAGES demonstrate that the signal
  can be accurately recovered
• Maximum-likelihood analysis of galaxy-
  galaxy and cosmic flexion signals is
  underway; these will place new constraints
  upon small-scale dark matter structure

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:1
posted:2/22/2012
language:
pages:21