Weak Gravitational Flexion from HST GEMS and STAGES

1
Weak Gravitational Flexionfrom HST GEMS and
STAGES
Barnaby Rowe with David Bacon (Portsmouth), Andy
Taylor (Edinburgh), Catherine Heymans (U.B.C.),
Richard Massey (Caltech), Dave Goldberg (Drexel)
2
STAGES TEAM
Meghan Gray (PI) (Nottingham) David Bacon
(Portsmouth) Michael Balogh (Waterloo) Marco
Barden (MPIA) Fabio Barazza (UTexas) Eric Bell
(MPIA) Asmus Boehm (AIP) John Caldwell
(UTexas) Boris Häußler (MPIA) Catherine Heymans
(UBC) Knud Jahnke (MPIA) Shardha Jogee
(UTexas) Eelco van Kampen (Innsbruck) Sergey
Koposov (MPIA) Kyle Lane (Nottingham) Daniel
McIntosh (UMass) Klaus Meisenheimer (MPIA) Chien
Peng (STScI) Hans Walter Rix (MPIA) Sebastian
Sanchez (CAHA) Rachel Somerville (MPIA) Andy
Taylor (Edinburgh) Lutz Wisotzki (AIP) Christian
Wolf (Oxford) Xianzhong Zheng (PMO)
GEMS TEAM
Hans Walter Rix (PI) (MPIA) Marco Barden
(MPIA) Steven Beckwith (STScI) Eric Bell
(MPIA) Andrea Borch (MPIA) John Caldwell
(UTexas) Boris Häußler (MPIA) Catherine Heymans
(UBC) Knud Jahnke (MPIA) Shardha Jogee
(UTexas) Sergey Koposov (MPIA) Daniel McIntosh
(UMass) Klaus Meisenheimer (MPIA) Chien Peng
(STScI) Sebastian Sanchez (CAHA) Rachel
Somerville (MPIA) Lutz Wisotzki (AIP) Christian
Wolf (Oxford) Xianzhong Zheng (MPIA)
3
Simulations exhibit an abundance of dark matter
substructure at a wide range of scales
Cluster halo
they also suggest that halos follow a certain
profile (e.g. the NFW density profile see
Navarro, Frenk White 1997)
Galaxy halo
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
4
Traditional weak lensing
source
Image transformations can often be described by a
simple, locally linearized mapping
lens
observer
g g1 ig2
5
Weak lensing to higher order flexion
6
Flexion is sensitive to matter variations
at smaller scales than shear its like a high
pass filter for mass structure
k
x
Cosmological predictions (see Bacon et al. 2006)
show that flexion is particularly sensitive to
dark matter structure at small scales
7
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).

8
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
9
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
10
Shear and flexion measurements
F
11
Quick and dirty STEP
12
Galaxy-galaxy lensing is a useful tool for
studying galaxy halo mass distributions
source galaxy
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
13
Galaxy-galaxy shear
14
Galaxy-galaxy F
15
Galaxy-galaxy G
16
Flexion correlation statistics
Massive foreground halo
In a Universe in which matter is correlated
(clumpy) we also expect correlations between the
shears and flexions of pairs of galaxies, varying
as a function of angular separation.
Background galaxy
Predictions for lensing correlation functions
C(q) (for gg, FF, GG etc.) exist, and can be used
to constrain cosmological parameters.
17
Cosmic flexion
F
G
18
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

19
(No Transcript)
20
(No Transcript)
21
(No Transcript)