Weak Lensing from GEMS

Berkeley, 16th – 18th May 2004 Wide-Field Imaging from Space

Weak lensing with GEMS

Galaxy Evolution from Morphologies and SEDS

Catherine Heymans Max-Planck-Institute for Astronomy, Heidelberg

& the GEMS collaboration: Michael Brown, Marco Barden, John Caldwell, Boris Haussler, Knud Jahnke, Hans-Walter Rix (PI), Steve Beckwith, Eric Bell,

Andrea Borch, Sharda Jogee, Dan McIntosh, Klaus Meisenheimer, Chien Peng, Sebastian Sanchez, Rachel Somerville, Andy Taylor,

Lutz Wisotski, Chris Wolf.

Wide-Field Imaging from SpaceBerkeley, 16th – 18th May 2004

GEMS: HST’s largest colour mosaic

GEMS two-colour imaging.

mAB(F606W) = 28.3

mAB(F850LP) = 27.1

(5σ depths for compact sources).(Rix et al. 2004)

z = F850LPV = F606W

GOODS imaging.

Area ~ 28 x 28 arcmins centred on the Chandra Deep Field South ~ 150 HDF

HST imaging: resolve morphology, mergers and structural properties.

A large sample ~10,000 galaxies (Rvega< 24) have redshifts accurate to 2%, and rest frame luminosities from COMBO-17 (Wolf et al. 2004)

Wide-Field Imaging from SpaceBerkeley, 16th – 18th May 2004GEMS: www.mpia.de/GEMS/gems.htm SkyWalker: www.aip.de/~ssa/gems/sw


GEMS science goals : galaxy evolution since z~1Analyse the frequency of galaxies as a function of multi-parameter space

(z,L,r,morphology,M,SED….)

• Bulges and spheroids • Disk galaxies• Interactions and mergers• Bell et al. 2004, McIntosh et al. 2004, Barden et al. 2004, Wolf et al.

2004, Jogee et al. 2004, Jahnke et al. 2004; Sánchez et al. 2004

• GEMS is perfect test data for future space-based weak lensing surveys

• Weak gravitational lensing studies:

– Cosmic shear (Heymans et al. 2004)– Galaxy-galaxy lensing– 3D lensing incorporating COMBO-17 redshifts


The ACS camera imaging properties

1. Geometric distortions

• Tilted focal surface of ACS wrt optical axis yields strong geometric distortions

• Additional small corrections required for the effect of differential velocity aberration.

• During frame combination with multidrizzle, images are reinterpolated onto a regular grid

• Residual distortions < 0.01%

Meurer et al. 2002


• The PSF is strongly non-Gaussian and varies for different filters

• Kaiser Squires & Broadhurst 1995 (KSB) PSF correction is applied as a function of galaxy size

V band

z band

Jahnke et al. 2004

The ACS point spread function distortion


PSF anisotropy

• 95% of GEMS imaging taken within the space of 20 days → stable PSF

• Wide-field ~1000 stars map anisotropy of PSF consistenty within data set without relying on models (Tiny-Tim) or archived stellar cluster imaging

• Distortions ~ 5%

• After correction < 0.1%

Before PSF correction

After PSF correction

PSF for GOODS & GEMS differ in magnitude and direction ~2% due to different dithering patterns and time variation in the PSF. CTE signature not seen.


E/B mode decomposition

E/B correlators

Crittenden et al. 2001 Schneider et al. 2002

B-mode = Noise + systematics

E B

E-mode = Lensing signal + noise + systematics


Cosmic Shear analysis of GEMSWeak lensing by large scale structure distorts background images, inducing correlations in the observed ellipticities of galaxies.


Cosmological parameter estimation

σ8(Ωm/0.27)0.67 = 0.74 ± 0.17

Note this does not include cosmic variance! CDFS is a factor of 2 underdense in massive galaxies.

Marginalised over zm=0.95 ± 0.1. We assume WMAP priors on H0, Ωm + ΩΛ

= 1, and a Smith et al. 03 non-linearity correction


Space-based imaging vs ground

• Space-based imaging has a significantly higher surface density of resolved sources, which can probe the matter density power spectrum at higher redshifts than will ever be feasible from the ground.

GEMS COMBO-17 (Brown et al. 2003)

~ 100 galaxies per sq arcmin ~ 35 galaxies per sq arcmin


Galaxy ellipticity measured in COMBO-17 and GEMS

e1, e2 COMBO-17 & GEMS

KSB shape determination from space-based data is less noisy

Lower redshift sub-sample of galaxies imaged by GEMS and resolved in COMBO-17


Conclusions• HST ACS is an excellent instrument for weak lensing studies• But the ACS PSF varies with time – GOODS and GEMS have

different PSF patterns – important for COSMOS to model time variation

• This anaylsis has used KSB. A wealth of shape information is avaliable with space based data – future analysis will take full advantage of our high resolution imaging using shapelets (Refregier & Bacon 2003) and maximum likelihood model fitting ( Miller, Heymans & Heavens 2004).

• GEMS combined with COMBO-17 is a great test case to compare ground and space-based imaging for weak lensing studies

• We find a higher S/N estimates of the shear correlation function from a subsample of COMBO-17 resolved galaxies when the galaxy shapes are determined from space based imaging.

• Cosmic shear has been detected from ¼ sq degree HST mosaic and used place joint constraints on σ8 and Ωm.


E/B mode decomposition

Crittenden et al. 2001 Schneider et al. 2002

E-mode = Lensing signal + noise + systematics

B-mode = Noise + systematics

E B


Ωm = 0.3


Mimicking weak lensing1. PSF distortions

2. Geometric distortions

3. CTE degradation

4. Object selection bias – tested with simulations

5. Intrinsic galaxy alignments <2% (Heymans et al. 2004)

Good agreement between galaxy ellipticity parameters measured in the z and V band images


Space-based imaging vs groundCOMBO-17GEMS


E/S0Sa-SmPeculiar/InteractingWeak Interg/Compact

Barden et al. 2004

Surface brightness dimming of disk galaxies with cosmic time

Bell et al. 2004

Galaxies have a bi-modal colour distribution to z~1, and are also roughly bi-modal by morphology

Weak Lensing from GEMS

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