Accurate Cosmology from LSST Cosmic Shear

1. Accurate Cosmology from LSST Cosmic Shear

2. The great potential of cosmic shear Galaxy clustering Supernovae Cosmic shear Quality of dark energy constraint Example for optical ground-based surveys Dark Energy Task Force report astro-ph/0609591

3. The Future HSC pic KIDS pic AFTA pic?? JDEM

4. The great potential of cosmic shear Galaxy clustering Supernovae Cosmic shear Quality of dark energy constraint Example for optical ground-based surveys Dark Energy Task Force report astro-ph/0609591

5. Cosmic Shear: Potential systematics Galaxy shape measurement Intrinsic alignments Photometric redshifts Accuracy of predictions See the DESC Whitepaper for much more detail

6. Galaxy shape measurement Causes b/a ~ 0.03 Measure b/a +/- 0.3 The GREAT08 Challenge Handbook Bridle et al 2010

7. Galaxy shape measurement Causes b/a ~ 0.03 Measure this to 1% Measure b/a +/- 0.3 The GREAT08 Challenge Handbook Bridle et al 2010

8. Typical data

9. Shear TEsting Programme • Brought together international shear measurement community • Revitalised research area • Set a benchmark for shear measurement • Current methods OK for current data (then) • Very non-trivial problem for future STEP 1:

10. GREAT08: Separate the issues Bridle et al 2010

11. GREAT08 Results in Detail See also GREAT10 Kitching et al Bridle et al 2010

12. Sersic model fitting shear measurement codes • LensFit • Multifit • Im3shape See also currently popular non-Sersic methods: DEIMOS, FDNT, shapelets

13. What causes the problems?For model fitting methods • Model bias • Voigt & Bridle 2009 • e.g use wrong profile in fit • e.g. use elliptical isophote model in fit

14. Model bias due to realistic galaxy morphologies Model bias Kacprzak, SB, et al 2013

15. Model bias calibration slide from Michael

16. What causes the bias?For model fitting methods • Model bias • Voigt & Bridle 2009 • e.g use wrong profile in fit • e.g. use elliptical isophote model in fit • Noise bias • Refregier, SB et al; Kacprzak, SB et al 2012 • Maximum likelihood methods are biased • Calibration works well enough

17. GREAT3 References:Mandelbaum, Rowe et al 2013 The GREAT3 Challenge Handbook Rowe, Mandelbaum et al 2013 The GalSim Toolkit

18. GREAT3 PSFs and PSF variation Mandelbaum, Rowe et al 2013 The GREAT3 Challenge Handbook

19. Beyond GREAT3 Will be needed for LSST • Deblending • PSF reconstruction from ?? • ???? Might not be needed for LSST (tbc) • Wavelength dependent PSF • Flux dependent PSF • Star selection effects

20. Cosmic shear Face-on view Gravitationally sheared Gravitationally sheared Lensing by dark matter causes galaxies to appear aligned

21. Intrinsic alignments (II) Croft & Metzler 2000, Heavens et al 2000, Crittenden et al 2001, Catelan et al 2001, Mackey et al, Brown et al 2002, Jing 2002, Hui & Zhang 2002

22. Intrinsic alignments (II) Face-on view Intrinsically Aligned (I) Intrinsically Aligned (I) Tidal stretching causes galaxies to align Adds to cosmic shear signal

23. Intrinsic-shear correlation (GI) Hirata & Seljak 2004 See also Heymans et al 2006, Mandelbaum et al 2006, Hirata et al 2007

24. Intrinsic-shear correlation (GI) Face-on view Gravitationally sheared (G) Intrinsically aligned (I) Galaxies point in opposite directions Partially cancels cosmic shear signal

25. Effect on cosmic shear of changing w by 1% Cosmic Shear Intrinsic Alignments (IA) Normalised to Super-COSMOS Heymans et al 2004

26. Effect on cosmic shear of changing w by 1% Intrinsic Alignments (IA)

27. Ignoring Intrinsic Alignments is Bad! Badness depends on assumed model Kirk, Rassat, Host, Bridle 2012

28. Use of shear-position correlations Shear-shear correlations - Measure mostly dark matter Shear-position correlations - Measure mostly intrinsic alignments Position-position correlations - Traditional galaxy survey observable

29. Forecast Joint Constraints from Position 2-point Observables Cosmic shear Intrinsic Alignments Galaxy clustering Cosmic magnification Shear-position correlation function Angular power spectra are sourced by underlying 3D power spectra: Dark matter P(k), galaxy P(k), IA P(k), galaxy-DM cross, IA-DM cross Joachimi & Bridle 2010

30. Shear alone (ee), fixed IAs Kirk, Rassat, Host, Bridle 2012

31. Shear alone (ee), fixed IAs Kirk, Rassat, Host, Bridle 2012 Shear alone (ee), marg IAs

32. Shear alone (ee), fixed IAs ee, ne, nn, fixed IAs Kirk, Rassat, Host, Bridle 2012 Shear alone (ee), marg IAs ee, ne, nn, marg IAs

33. Intrinsic Alignment work for LSST • Observational programme of measurements • Spectra • Photoz • Theoretical developments • Analytic • Simulations • ??? • ???

34. Photometric Redshifts for LSST • Need precise redshifts to eliminate intrinsic alignments • Improved use of colour information • Use of additional e.g. morphology information?

35. Requirements on Photoz Precision More stringent requirements on photoz accuracy with IAs Kirk, Laszlo, Bridle, Bean 2011

36. Photometric Redshifts for LSST • Need precise redshifts to eliminate intrinsic alignments • Improved use of colour information • Use of additional e.g. morphology information? • Need accurate redshifts to measure cosmology • Spectra from representative training set • Development and use of cross-correlation methods

37. Requirements on Photoz Accuracy !!! Kirk, Laszlo, Bridle, Bean 2011

38. Cross-correlation of shapes and spectra Red: different sky Blue: same sky Southern Hemisphere spectra MOONS 4MOST SKA HI ????

39. Conclusions