Simultaneous constraints on cosmology and photozs from weak lensing and large scale structure • The dream: w from cosmic shear • The problem (ii): photoz calibra on • The solu on: WL+xLSS
WL LSS WLxLSS
Sarah Bridle, University of Manchester Simultaneous constraints on cosmology and photozs from weak lensing and large scale structure • The dream: w from cosmic shear • The problem (ii): photoz calibra on • The solu on: WL+xLSS
WL LSS WLxLSS
Sarah Bridle, University of Manchester The Dream: w from Cosmic Shear
Galaxy Supernovae Cosmic clustering shear Why is the line so long?
(i) Shear calibra on
(ii) Photoz calibra on
(iii) Intrinsic alignment
(iv) Simula on predic ons
Simultaneous constraints on cosmology and photozs from weak lensing and large scale structure • The dream: w from cosmic shear • The problem (ii): photoz calibra on • The solu on: WL+xLSS
WL LSS WLxLSS
Sarah Bridle, University of Manchester The Problem (ii): Photoz Calibra on
Bonne , Troxel, Hartley, Amara, Leistedt & DES Collabora on 2016
For cosmic shear cosmology need: • n(z) of each redshi bin • Characterisa on of uncertain es in n(z)s (covariance) • Tight requirements on n(z) uncertain es
– Rule of 5: δw ~ 5δzs The Dark Energy Survey Science Verifica on photo-z distribu ons
-> Marginalise over overall offset in z per zbin with Gaussian prior of +/- 0.05
Bonne , Troxel, Hartley, Amara, Leistedt & DES Collab. 2016 The DES Collabora on 2015 The DES Collabora on 2015 KiDS-450: Hildebrandt, Viola et al 2016 KiDS-450: Hildebrandt, Viola et al 2016 The Problem (ii): Photoz Calibra on
Bonne , Troxel, Hartley, Amara, Leistedt & DES Collabora on 2016
For cosmic shear cosmology need: • n(z) of each redshi bin • Characterisa on of uncertain es in n(z)s (covariance) • Tight requirements on n(z) uncertain es
– Rule of 5: δw ~ 5δzs
à for w~1% need δzs~0.002 Simultaneous constraints on cosmology and photozs from weak lensing and large scale structure • The dream: w from cosmic shear • The problem (ii): photoz calibra on • The solu on: WL + LSS + WLxLSS
WL LSS WLxLSS
Sarah Bridle, University of Manchester Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 28 July 2016 (MN LATEX style file v2.2)
Simultaneous Constraints on Cosmology and Photometric Redshift Bias from Weak Lensing and Galaxy Clustering
S. Samuroff1?, M.A. Troxel1, S.L. Bridle1, J. Zuntz1, N. MacCrann1, E. Krause2, T. Eifler3,4, D. Kirk5 1Jodrell Bank Centre for Astrophysics, University of Manchester, Oxford Road, Manchester, M13 9PL, UK. 2Kavli Institute for Particle Cosmology and Astrophysics, Stanford University, Stanford, CA 94305, USA 3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA 4Department of Physics, California Institute of Technology, Pasadena, CA 91125, USA 5Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK.
28 July 2016
ABSTRACT We investigate the expected cosmological constraints from a combination of weak lensing and large-scale galaxy clustering using realistic redshift distributions. Introducing a systematic bias in the weak lensing redshift distributions (of 0.05 in redshift) produces a > 2 bias in the recovered matter power spectrum amplitude and dark energy equation of state, for preliminary Stage III surveys. We demonstrate that these cosmological errors can be largely removed by marginalising over unknown biases in the assumed weak lensing redshift distributions, if we assume high quality redshift information for the galaxy clustering sample. Furthermore the cosmological constraining power is mostly retained despite removing much of the information on the weak lensing redshift distribution biases. We show that this comes from complementary degeneracy directions between cosmic shear and the combination of galaxy clustering with cross-correlation between shear and galaxy number density. Finally we examine how the self- calibration performs when the assumed distributions differ from the true distributions by more than a simple uniform bias. We find that the effectiveness of this self-calibration method will depend on the details of a given experiment and the nature of the uncertainties on the estimated redshift distributions. Key words: cosmological parameters - cosmology: observations - gravitational lensing: weak - large-scale structure of Universe - dark matter - dark energy - galaxies: statistics
1 INTRODUCTION nary Stage III datasets, containing 10 million galaxies (DES Col- ⇠ lab. 2015; Hildebrandt et al. 2016, see also Heymans et al. 2013; Cosmic shear is potentially the most powerful tool available to cos- Jee et al. 2016). The increase in the number of galaxies with reliable mologists today. As an unbiased probe of the mass distribution, it shape measurements has allowed tighter cosmology constraints, but offers powerful constraints on the mean density of the Universe and also requires better control of systematic biases. In this Letter we the clustering of dark matter. It is also expected to shed new light on arXiv:1607.07910v1 [astro-ph.CO] 26 Jul 2016 focus on a potential Achilles’ heel of galaxy imaging surveys for the late-time accelerated expansion of the Universe and thus mea- cosmology: the use of photometric redshifts to estimate distances sure the dark energy equation of state and test General Relativity to galaxies. on the largest scales. A three decade programme aiming to extract unprecedented Tomographic cosmic shear analyses bring a number of ben- constraints on our cosmological model from cosmic shear is now efits (Hu 1999), but place stringent requirements on our knowl- midway to completion. It began soon after the first detection in edge of galaxy redshift distributions. Amara and Refr´ egier´ (2007); 2000 (Bacon et al. 2000; Van Waerbeke et al. 2000; Wittman et al. Abdalla et al. (2008); Jouvel et al. (2009) and Ishak et al. (2006) 2000; Kaiser et al. 2000) using 10000 galaxies and is expected to present detailed studies of the requirements for spectroscopic fol- ⇠ culminate in catalogues of more than a billion galaxies by the end low up of Stage IV cosmology surveys, while Ma et al. (2006); of the coming decade (Stage IV, Albrecht et al. (2006)). Logarith- Huterer et al. (2006); Bernstein (2009) offer numerical forecasts of mically, we are halfway there, with ongoing analyses of the prelimi- cosmological impact from photometric redshift (photo-z) biases. Many others (e.g. Bordoloi et al. 2012; Cunha et al. 2014) present detailed studies of specific photo-z systematics, albeit with less fo- ? [email protected] cus on the ultimate cosmological impact. Tightening systematics
c 0000 RAS The Original Plan
• Forecast requirements on δzs for DES Year 1 WL+xLSS by:
– Plot δσ8 versus δzs
– Overplot δσ8 DES Year 1 WL+xLSS forecast
– Read off requirement on δzs DES Y1 Forecast Machinery
Fiducial photozs: DES SV fiducial (SkyNet)
Galaxy clustering sample from luminous red galaxies (DES SV redMaGiC) Cosmology from WL+xLSS
Cosmic shear Intrinsic Alignments
Galaxy clustering Cosmic magnifica on Shear-posi on correla on func on
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
Bernstein 2009, Joachimi & Bridle 2010 Cosmology from WL+xLSS
WL LSS WLxLSS
Shear-shear Galaxy clustering Shear-position correlations correlations
All from a single imaging survey The Original Plan
• Forecast requirements on δzs for DES Year 1 WL+xLSS by:
– Plot δσ8 versus δzs
– Overplot δσ8 DES Year 1 WL+xLSS forecast
– Read off requirement on δzs WL
WL+xLSS
Samuroff, Troxel, Bridle et al 2016 DES Y1 Forecast Machinery
Fiducial photozs: DES SV fiducial (SkyNet)
Simply perturbed photozs: Fiducual – 0.05
Galaxy clustering sample from luminous red galaxies (DES SV redMaGiC) Samuroff, Troxel, Bridle et al 2016 Samuroff, Troxel, Bridle et al 2016 Samuroff, Troxel, Bridle et al 2016 Robustness to Assump ons
• Stochas c bias le free – yes • Addi onal n(z) stretch parameter – yes • Include smaller scales in LSS analysis – yes • Wider prior on WL shears (0.05 cf 0.02) – yes • Wider prior on LSS n(z) shi s (0.05 cf 0.01) – no • Realis c redshi distribu on? – Differs by much more than just a shi in n(z)s DES Y1 Forecast Machinery
Fiducial photozs: DES SV fiducial (SkyNet)
Simply perturbed photozs: Fiducual – 0.05
Realis cally perturbed BPZ uncalibrated (~0.05 below SkyNet)
Galaxy clustering sample from luminous red galaxies (DES SV redMaGiC) Samuroff, Troxel, Bridle et al 2016 Samuroff, Troxel, Bridle et al 2016 Conclusions • To realise the great poten al of w from cosmic shear need to understand photoz uncertain es to extremely high accuracy
– Rule of 5: δw ~ 5δzs
• Combined WL+xLSS is much less sensi ve to δz than cosmic shear (WL) alone • At the level of DES Year 1 cosmology WL+xLSS can self-correct a simple bias of δz=0.05 • WL+xLSS may be able to correct for more realis c redshi distribu ons too