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COSMOS

Weak Lensing With COSMOS: An Overview

Jason Rhodes (JPL)

May 24, 2005For the COSMOS WL team : (Justin Albert, Richard Ellis, Alexie Leauthaud, Richard Massey, Jean-Paul Kneib, Yannick Mellier, Alexandre Refregier, Ludovic van Waerbeke, Satoshi Miyazaki, James Taylor, Anton Koekemoer)

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If there is any intervening large-scale structure, light follows the distorted path (exaggerated). Background images are magnified and sheared by ~2%, mapping a circle into an ellipse. Like glass lenses, gravitational lenses are most effective when placed half way between the source and the observer.

zobserver=0

zgalaxy≈1

zlens≈0.3–0.5

Weak lensing effect cannot be measured from any individual galaxy.

Must be measured statistically over many galaxies

Weak Gravitational Lensing

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How Gravitational Lensing WorksHow Gravitational Lensing Works

Unlensed Lensed

Two component ellipticity

Positive Negative

e1

e2

•Statistical measurement on many galaxies•Lensing induced ellipticities 1-2%•Telescope Point Spread Function (PSF) is the primary systematic concern –it changes e!

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COSMOSCosmology with WL

Dark matter maps (Richard)

Galaxy properties as a function of environment (James)

Properties of galaxy DM haloes (Alexie)

Statistical measures of cosmological parameters (Ludo)

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COSMOSWL Pipeline

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COSMOSThat Pesky PSF Problem

The PSF pattern is time dependent

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COSMOSTime Dependence

Why a three week time scale

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COSMOSThe GEMS Solution

•Unique combination of high data volume is a short time•Divide data into two time bins of 5 days•Can do internal correction with stars in each time bin•Empirically derived correction using stars in the field- the typical weak lensing methodology

Why won’t it work for COSMOS??•Data too spread out in time•Not enough stars per field (or time bin) for internal correction•Usual method of using stars in the fields will not work•We need more stars!!

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COSMOSOur Solution

•Not enough stars→ Make our own!•Use TinyTim PSF modeling software•Create stars without geometric distortion (but with diffraction and other PSF effects•Required modification of TinyTim program•Distortion removal (aka multidrizzle) introduces complications that are stochastic

•Create dense stellar grids (up to 50x50 across field)•Create at range of focus positions (-10 to +5 μm)•Use stars in each COSMOS field (~10 to 20) to pick the best focus value•Use the template at that focus value to perform correction•Eliminates need for interpolation between stars

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COSMOSTT PSF Models

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COSMOSOur Solution Works… Pretty Well

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COSMOSDiagnostic Plots

From alexie

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COSMOSThe Need for Re-reduction

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“Aliasing” from pixellization

Unfortunately, a star’s sub-pixel position affects its observed shape.

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“Aliasing” from second pixellization

Then it happens again during DRIZZLE! This at least must be avoidable…

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Solution: adjust pixel scale & DRIZZLE kernel

…it is!

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COSMOS

Recommended Multidrizzle Parameters

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.01

0.011

0.015 0.025 0.035 0.045 0.055

Pixel Scale

RM

S e Square kernel

Gaussian Kernel

•Created TinyTim models with same diffraction everywhere, only distortion is different•Run through multidrizzle w/ various parameters•Large improvement going from 0.05 to 0.03 arcsecond pixels•Gaussian kernel has lower RMS(e) and is more stable

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COSMOSConclusions

•We have a workable PSF correction model•Continuing to improve it

•Better star selection•Better focus selection

•Getting science results now (see next few talks)•Will see large improvement w/re-reduction•Our pipeline is ready to incorporate all cycle 13 data•Can fold in better photo-zs easily