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Systematic Errors in Weak Lensing Measurements

Systematic Errors in Weak Lensing Measurements. Jun Zhang, Eiichiro Komatsu (The University of Texas at Austin). arXiv : 1002.3614; 1002.3615; 0901.4781, Astro-ph/0612146. IHEP, Nov 8, 2011. Outline:. Introduction of Weak Lensing Problems in Weak Lensing Measurement

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Systematic Errors in Weak Lensing Measurements

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  1. Systematic Errors in Weak Lensing Measurements Jun Zhang, Eiichiro Komatsu (The University of Texas at Austin) arXiv: 1002.3614; 1002.3615; 0901.4781, Astro-ph/0612146. IHEP, Nov 8, 2011

  2. Outline: • Introduction of Weak Lensing • Problems in Weak Lensing Measurement • The Form of Cosmic Shear Estimator (general) • The Reduced Shear (general) • The Pixelation Effect (specific) • The Photon Noise (specific) • Summary

  3. Introduction Credit: NASA / WMAP Science Team

  4. Introduction Credit: NASA/WMAP

  5. Introduction Credit: Wittman et al. , (LSST team)

  6. Introduction Credit: Clowe et al. (2006) Image Credit: Hoekstra & Jain (2008)

  7. Introduction

  8. Introduction y y’ Lensing x’ x

  9. Introduction y y’ Lensing x x’ y y’ Lensing x x’ y y’ Lensing x x’

  10. Introduction

  11. Introduction DES EUCLID HSC KDUST WFIRST LSST

  12. Problems in Weak Lensing Measurements How to accurately measure the cosmic shear? The shear signal: a few percent V.S. Intrinsic variance of the galaxy ellipticity (shape noise): of order one Systematic errors are not tolerable!!!

  13. Lensing PSF 引力透镜所致形变 观测所致图像模糊 Pixelation 有限像素 光子噪音 Noise Image Credit: GREAT08 team

  14. Problems in Weak Lensing Measurements

  15. Problems in Weak Lensing Measurements Credit: Huterer et al. 2006, MNRAS, 366, 101

  16. Problems in Weak Lensing Measurements Credit: Huterer et al. 2006, MNRAS, 366, 101

  17. Problems in Weak Lensing Measurements • Sources of Systematic Errors: • Corrections due to the Point Spread Function (PSF); • Photon Noise; • Pixelation Effect; • High Order Terms in Shear/Convergence; • Charge Transfer Inefficiency; • PSF determination from Star Shapes; • Source Selection Bias; • Photometric Redshift Errors; • Galaxy Intrinsic Alignment.

  18. Problems in Weak Lensing Measurements • Sources of Systematic Errors: • Corrections due to the Point Spread Function (PSF); • Photon Noise; • Pixelation Effect; • High Order Terms in Shear/Convergence; • Charge Transfer Inefficiency; • PSF determination from Star Shapes; • Source Selection Bias; • Photometric Redshift Errors; • Galaxy Intrinsic Alignment.

  19. Problems in Weak Lensing Measurements • The Form of Cosmic Shear Estimator (general) • The Reduced Shear (general) • The Pixelation Effect (specific) • The Photon Noise (specific)

  20. The Form of Shear Estimator ? STEP I, STEP II, GREAT 08, GREAT 10

  21. The Form of Shear Estimator

  22. The Form of Shear Estimator ✗

  23. The Form of Shear Estimator ✗ ✔

  24. The Form of Shear Estimator Warming Up Exercise

  25. The Form of Shear Estimator Warming Up Exercise

  26. The Form of Shear Estimator Are there ideal/conventional shear estimators existing in convenient forms? ?

  27. The Form of Shear Estimator Coordinate Rotation:

  28. The Form of Shear Estimator

  29. The Form of Shear Estimator

  30. The Form of Shear Estimator

  31. The Form of Shear Estimator Indeed, we only need to consider spin-2 shear estimators because of the following:

  32. The Form of Shear Estimator On spin-2 shear estimators:

  33. The Form of Shear Estimator On spin-2 shear estimators: Scalar Pseudo-Scalar Spin-4 Spin-4

  34. The Form of Shear Estimator On spin-2 shear estimators:

  35. The Form of Shear Estimator Consider a special type of galaxies: ) , (

  36. The Form of Shear Estimator Consider a special type of galaxies:

  37. The Form of Shear Estimator Consider a special type of galaxies:

  38. The Form of Shear Estimator Consider a special type of galaxies:

  39. The Form of Shear Estimator Consider a special type of galaxies:

  40. The Form of Shear Estimator Consider a special type of galaxies: Bartelmann & Schneider 2001, Phys. Rept., 340, 291

  41. The Form of Shear Estimator In the Presence of PSF: ? ✗

  42. The Form of Shear Estimator ✗

  43. The Form of Shear Estimator ✗ ✔

  44. The Form of Shear Estimator ✗ ✔

  45. The Form of Shear Estimator ✗ ✔

  46. The Form of Shear Estimator Ref: Astro-ph/0612146, arXiv: 0901.4781, arXiv: 1002.3614

  47. The Form of Shear Estimator Ref: Astro-ph/0612146, arXiv: 0901.4781, arXiv: 1002.3614

  48. The Form of Shear Estimator • Correct for the Point Spread Function; • No Assumptions on the Morphologies of the Galaxies or the PSF; • Exact & Simple Math; • Well Defined Treatment of Background Noise; • Accurate to the 2nd Order in Shear/Convergence; • Immune to Misidentification of Sources (faint/small galaxies, stars) • < 10-2 seconds/Galaxy • S / N per galaxy Increased.

  49. The Reduced Shear

  50. The Reduced Shear If one plans to calibrate the multiplicative factor “a+bκ”, one should pay attention to its dependence on “κ”, which, though, is not known a priori in observations.

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