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Weak Lensing with LSST

Weak Lensing with LSST. Weak lensing intro Predicted performance Challenges for LSST Should the UK join LSST Corp for the sake of WL? Questions International LSST Weak Lensing Working Group led by Wittman (UCDavis) and Jain (UPenn)

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Weak Lensing with LSST

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  1. Weak Lensing with LSST • Weak lensing intro • Predicted performance • Challenges for LSST • Should the UK join LSST Corp for the sake of WL? • Questions International LSST Weak Lensing Working Group led by Wittman (UCDavis) and Jain (UPenn) Based on slides from http://www.lsst.org/guest/LSST-General.ppt

  2. Cosmic shear

  3. 3d map of the dark matter • Dark energy • Nature of dark matter • Galaxy formation

  4. LSST and Cosmic Shear Ten redshift bins yield 55 auto and cross spectra useful range baryons + higher order

  5. LSST Precision on Dark Energy [in DETF language] Zhan 2006 Combining techniques breaks degeneracies. Requires wide sky area deep survey.

  6. LSST Survey 6-band Survey: ugrizy 320–1100 nm Frequent revisits: grizy Sky area covered: >20,000 deg2 0.2 arcsec / pixel Each 10 sq.deg field revisited ~1000 times Limiting magnitude: 27.6 AB magnitude @5s 25 AB mag /visit = 2x15 seconds Photometry precision: 0.005 mag requirement, 0.003 mag goal

  7. Dark Energy Task Force report astro-ph/0609591

  8. Cosmic Shear: Potential systematics Shear measurement Measurement Photometric redshifts Astrophysical Intrinsic alignments Accuracy of predictions Theoretical

  9. Challenges for LSST Weak Lensing • Accurate galaxy shape measurement • Small PSF • Stable PSF • High precision PSF deconvolution techniques • High quality photometric redshifts • Well-calibrated photometry • Many filters • Spectroscopic follow-up of representative sample

  10. Critical Issues • WL shear reconstruction errors • Show control to better than required precision using existing new facilities • Photometric redshift errors • Develop robust photo-z calibration plan • Undertake world campaign for spectroscopy () • Photometry errors • Develop and test precision flux calibration technique

  11. Challenges for LSST Weak Lensing • Accurate galaxy shape measurement • Small PSF • Stable PSF • High precision PSF deconvolution techniques • High quality photometric redshifts • Well-calibrated photometry • Many filters • Spectroscopic follow-up of representative sample

  12. HST ACS data LSST: ~50 galaxies per sq.arcmin

  13. LSST image quality

  14. Comparing HST with Subaru

  15. Comparing HST with Subaru

  16. space weak lensing shear ground Shear Data: Ground vs Space Typical cosmic shear is ~ 1%, and must be measured with high accuracy Space: small and stable PSF: larger number of resolved galaxies  reduced systematics

  17. Residual shear correlation Test of shear systematics: Use faint stars as proxies for galaxies, and calculate the shear-shear correlation after correcting for PSF ellipticity via a different set of stars. Compare with expected cosmic shear signal. Conclusion: 200 exposures per sky patch will yield negligible PSF induced shear systematics. Wittman (2005) Cosmic shear signal Stars

  18. ellipticity correlation of the residual PSF: single exposure Averages down like 1 / number of exposures Simulation of 0.6” seeing. J. Jee 2007

  19. Challenges for LSST Weak Lensing • Accurate galaxy shape measurement • Small PSF • Stable PSF • High precision PSF deconvolution techniques • High quality photometric redshifts • Well-calibrated photometry • Many filters • Spectroscopic follow-up of representative sample

  20. LSST six color system Includes sensor QE, atmospheric attenuation, optical transmission functions Relative system throughput (%) Wavelength (nm)

  21. Calibrating photometric redshifts Cross-correlation LSS-based techniques can reconstruct the true z distribution of a photo-z bin, even with spectroscopy of only the brightest galaxies at each z. These techniques meet LSST requirements with easily attainable spectroscopic samples, ~104 galaxies per unit z. Newman 2008

  22. Should the UK join LSST Corp for the sake of Weak Lensing? • Will good shears and photozs be in public catalogue? Yes -> 6 No -> 2 • Is PS4 going to do it all? Yes -> 6 (but how do we get into PS4?) No -> 3 • Is Euclid going to happen? Yes: still want photozs from LSST (+LSST deeper?) -> 5 No -> 4 • Can shear be done well enough from the ground? Yes -> 5 No -> 3 • Yes • No

  23. Questions about Weak Lensing with LSST for the UK • Would the shears be in the public catalogue? • SDSS shears are not public. Ditto CFHTLS. • Would we trust a public shear catalogue? • Would we want to influence the survey strategy? • Probably we have similar interests to other weak lensers in LSST • Possibly the multiple short exposures are not optimal but no chance of changing this surely? • Would the UK have intellectual leadership? • Could be • Synergy / competition with other intstruments • Especially PS4 and Euclid • ?

  24. Potential UK Contributions for Weak Lensing • Algorithms and simulations • Shear measurement algorithms • Taking into account many short exposures • Photometric redshift measurement • Intrinsic alignment simulations • Simulations on the effect of baryons • ? • Data • Infrared imaging from Euclid • Spectra from Euclid and/or WFMOS • ? • ?

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