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Mon. Not. R. Astron. Soc. 366, 101114 (2006) doi:10.1111/j.1365-2966.2005.09782.x

Systematic errors in future weak-lensing surveys: requirementsand prospects for self-calibration

Dragan Huterer,1 Masahiro Takada,2 Gary Bernstein3 and Bhuvnesh Jain41Kavli Institute for Cosmological Physics and Astronomy and Astrophysics Department, University of Chicago, Chicago, IL 60637, USA2Astronomical Institute, Tohoku University, Sendai 980-8578, Japan3Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, USA4Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, USA

Accepted 2005 October 21. Received 2005 October 17; in original form 2005 June 20

ABSTRACTWe study the impact of systematic errors on planned weak-lensing surveys and compute therequirements on their contributions so that they are not a dominant source of the cosmologicalparameter error budget. The generic types of error we consider are multiplicative and additiveerrors in measurements of shear, as well as photometric redshift errors. In general, more power-ful surveys have stronger systematic requirements. For example, for a SuperNova/AccelerationProbe (SNAP)-type survey the multiplicative error in shear needs to be smaller than 1 per centof the mean shear in any given redshift bin, while the centroids of photometric redshift binsneed to be known to be better than 0.003. With about a factor of 2 degradation in cosmologicalparameter errors, future surveys can enter a self-calibration regime, where the mean systematicbiases are self-consistently determined from the survey and only higher order moments of thesystematics contribute. Interestingly, once the power-spectrum measurements are combinedwith the bispectrum, the self-calibration regime in the variation of the equation of state of darkenergy wa is attained with only a 2030 per cent error degradation.

Key words: cosmological parameters large-scale structure of Universe.

1 I N T RO D U C T I O N

There has been significant recent progress in the measurements ofweak gravitational lensing by large-scale structure. Only 5 yr afterthe first detections made by several groups (Bacon, Refregier &Ellis 2000; Kaiser, Wilson & Luppino 2000; van Waerbeke et al.2000; Wittman et al. 2000), weak lensing already imposes strongconstraints on the matter density relative to critical M and theamplitude of mass fluctuations 8 (Hoekstra, Yee & Gladders 2002;Jarvis et al. 2003; Heymans et al. 2004; Rhodes et al. 2004a; for areview see Refregier 2003), as well as the first interesting constraintson the equation of state of dark energy (Jarvis et al. 2005).

The main advantage of weak lensing is that it directly probesthe distribution of matter in the Universe. This makes weak lensinga powerful probe of cosmological parameters, including those de-scribing dark energy (Hu & Tegmark 1999; Huterer 2002; Heavens2003; Hu 2003a; Refregier 2003; Benabed & Van Waerbeke 2004;Ishak et al. 2004; Song & Knox 2004; Takada & Jain 2004; Takada& White 2004; Ishak 2005). The weak-lensing constraints are espe-cially effective when some redshift information is available for thesource galaxies; use of redshift tomography can improve the cos-mological constraints by factors of a few (Hu 1999). Furthermore,

E-mail: dhuterer@kicp.uchicago.edu

measurements of the weak-lensing bispectrum (BS) (Takada & Jain2004) and purely geometrical tests (Jain & Taylor 2003; Zhang, Hui& Stebbins 2003; Bernstein & Jain 2004; Hu & Jain 2004; Song& Knox 2004; Bernstein 2005) lead to significant improvements ofaccuracy in measuring the cosmological parameters. When thesemethods are combined, weak lensing by itself is expected to con-strain the equation of state of dark energy w to a few per cent, and toimpose interesting constraints on the time variation of w. Ongoingor planned surveys, such as the CanadaFranceHawaii TelescopeLegacy Survey1, the Dark Energy Survey2 (DES), PanSTARRS3 andVisible and Infrared Survey Telescope for Astronomy (VISTA)4 areexpected to significantly extend lensing measurements, while the ul-timate precision will be achieved with the SuperNova/AccelerationProbe5 (SNAP; Aldering et al. 2004) and the Large Synoptic SurveyTelescope6 (LSST).

Powerful future surveys will have very small statistical uncer-tainties due to the large sky coverage and huge number of galaxies,

1 http://www.cfht.hawaii.edu/Science/CFHLS.2 http://cosmology.astro.uiuc.edu/DES.3 http://pan-starrs.ifa.hawaii.edu.4 http://www.vista.ac.uk.5 http://snap.lbl.gov.6 http://www.lsst.org.

C 2005 The Authors. Journal compilation C 2005 RAS

102 D. Huterer et al.

and therefore an understanding of the systematic error budget willbe crucial. However, so far the rosy weak-lensing parameter accu-racy predictions that have appeared in literature have not allowedfor the presence of systematics [exceptions are Ishak et al. (2004)and Knox et al. (2005) who consider a shear calibration error, andBernstein (2005) who does the same for the cross-correlation cos-mography of weak lensing]. This is not surprising, as we are juststarting to understand and study the full budget of systematic errorspresent in weak-lensing measurements. Nevertheless, some recentwork has addressed various aspects of the systematics, both experi-mental and theoretical, and ways to correct for them. For example,Vale et al. (2004) estimated the effects of extinction on the extractedshear power spectrum (PS), while Hirata & Seljak (2003), Hoekstra(2004) and Jarvis & Jain (2004) considered the errors in measure-ments of shear. Several studies explored the effects of theoreticaluncertainties (Huterer et al. 2004; White 2004; Zhan & Knox 2005;Hagan, Ma & Kravtsov 2005) and ways to protect against their ef-fects (Huterer & White 2005; Huterer & Takada 2005). It has beenpointed out that second-order corrections in the shear predictions canbe important (Cooray & Hu 2002; Hamana et al. 2002; Schneider,Van Waerbeke & Mellier 2002; Dodelson & Zhang 2005; Dodelsonet al. 2005; White 2005).

Despite these efforts, we are at an early stage in our understand-ing of weak-lensing systematics. Realistic assessments of system-atic errors are likely to impact strategies for measuring the weak-lensing shear (Bernstein 2002; Bernstein & Jarvis 2002; Rhodeset al. 2004b; Ishak & Hirata 2005; Mandelbaum et al. 2005). Amajor effort to compare the different analysis techniques and theirassociated systematics is already underway (the Shear Testing Pro-gramme; Heymans et al. 2005). Eventually we would like to bringweak lensing to the same level as cosmic microwave background(CMB) anisotropies and type Ia supernovae, where the systematicerror budget is better understood and requirements for the control ofsystematic precisely outlined (e.g. Tegmark et al. 2000; Hu, Hedman& Zaldarriaga 2003; Kim et al. 2003; Linder & Miquel 2004).

The purpose of this paper is to introduce the framework for thediscussion of systematic errors in weak-lensing measurements andoutline requirements for several generic types of systematic error.The reason that we do not consider specific sources of error (e.g.temporal variations in the telescope optics or fluctuations in atmo-spheric seeing) is that there are many of them, they strongly dependon a particular survey considered, and they are often poorly knownbefore the survey has started collecting data. Instead we argue that,at this early stage of our understanding of weak-lensing systemat-ics, it is more practical and useful to consider three generic typesof error multiplicative and additive errors in measurements ofshear, as well as redshift error. These generic errors are useful in-termediate quantities that link actual experimental sources of errorto their impact on cosmological parameter accuracy. In fact, real-istic systematic errors for any particular experiment can in generalbe converted to these three generic systematics. Given the speci-fications of a particular survey, one can then estimate how mucha given systematic degrades cosmological parameters. This can beused to optimize the design of the experiment to minimize the ef-fects of systematic errors on the accuracy of desired parameter. Forexample, accurate photometric redshift requirements will lead to re-quirements on the number of filters and their wavelength coverage.Similarly, the requirements on multiplicative and additive errors inshear will determine how accurate the sampling of the point spreadfunction needs to be.

The plan of this paper is as follows. In Section 2, we discuss thesurvey specifications and cosmological parameters in this study. In

Section 3, we describe the parametrization of systematic errors. InSections 46, we present the requirements on the systematic errorsfor the PS, while in Section 7 we study the requirements whenboth the PS and the BS are used. We combine the redshift andmultiplicative errors and discuss trends in Section 8 and concludein Section 9.

2 M E T H O D O L O G Y, C O S M O L O G I C A LPA R A M E T E R S A N D F I D U C I A L S U RV E Y S

We express the measured convergence PS as

Ci j () = Pi j () + i j 2

ni, (1)

where Pi j () is the measured PS with systematics [see the nextsection and equation (20) on how it is related to the no-systematicPS Pi j ()],

2 is the variance of each component of the galaxy shear

and ni is the average number of resolved galaxies in the ith redshiftbin per steradian. The convergence PS at a fixed multipole and forthe ith and jth redshift bins is given by

Pi j () =

0

dzWi (z) W j (z)

r (z)2 H (z)P

(

r (z), z

), (2)

where r(z) is the comoving angular diameter distance and H(z) isthe Hubble parameter. The weights Wi are given by

Wi ( ) = 32

M H20 gi ( ) (1 + z), (3)

where gi ( ) = r ( )

dsni (s)r (s )/r (s), is the comov-

ing radial distance and ni is the fraction of galaxies assigned to theith redshift bin. We employ the redshift distribution of galaxies ofthe form

n(z) z2 exp(z/z0), (4)where z0 is survey-dependent and specified below. The cosmologicalconstraints can then be computed from the Fisher matrix

Fi j =

(C

pi

)TCov1

C

p j, (5)

where C is the column vector of the observed power spectra andCov1 is the inverse of the covariance matrix between the powerspectra whose elements are given by

Cov[Ci j (

), Ckl ()]

= (2 + 1) fsky

[Cik()C

jl () + Cil ()Cjk()

]. (6)

Here is the bandwidth in multipole we use, and f sky is the frac-tional sky coverage of the survey.

In addition to any nuisance parameters describing the systemat-ics, we consider six or seven cosmological parameters and assumea flat universe throughout. The six standard parameters are energydensity and equation of state of dark energy DE and w, spectralindex n, matter and baryon physical densities Mh2 and Bh2, andthe amplitude of mass fluctuations 8. Note that w = constant pro-vides useful information about the sensitivity of an arbitrary w(z),since the best-measured mode of any w(z) is about as well measuredas w = constant and therefore is subject to similar degradations inthe presence of the systematics. It is this particular mode, being themost sensitive to generic systematics, that will drive the accuracy re-quirements we explicitly illustrate this in Fig. 2. In addition to theconstant w case, we also consider a commonly used two-parameter

C 2005 The Authors. Journal compilation C 2005 RAS, MNRAS 366, 101114

Systematic errors in future weak-lensing surveys 103

Table 1. Fiducial sky coverage, density of source galaxies, varianceof (each component of) shear of one galaxy, and peak of the sourcegalaxy redshift distribution for the three surveys considered.

DES SNAP LSST

Area (deg2) 5000 1000 15 000n (gal arcmin2) 10 100 30

0.16 0.22 0.22zpeak 0.5 1.0 0.7

description of dark energy w(z) = w0 + wa z/(1 + z) (Chevallier &Polarski 2001; Linder 2003) where wa becomes the seventh cosmo-logical parameter in the analysis. Throughout we consider lensingtomography with 710 equally spaced redshift bins (see below), andwe use the lensing power spectra on scales 50 3000. We holdthe total neutrino mass fixed at 0.1 eV; the results are somewhatdependent on the fiducial mass. We compute the linear PS usingthe fitting formulae of Eisenstein & Hu (1999). We generalize theformulae to w = 1 by replacing the Lambda cold dark matter(CDM) growth function of density perturbations with that for ageneral w(z); the latter is obtained by integrating the growth equa-tion directly (e.g. equation 1 in Cooray, Huterer & Baumann 2004).To complete the calculation of the full non-linear PS we use thefitting formulae of Smith et al. (2003).

The fiducial surveys, with parameters listed in Table 1, are: theDark Energy Survey; SNAP and LSST. Note that there is someambiguity in the definition of the number density of galaxies ng;it is the quantity 2 /ng that determines the shear measurementnoise level, where is the intrinsic shape noise of each galaxy.Galaxies implicitly assumed for the DES shear measurements arethose with the largest angular sizes, and therefore they have corre-spondingly smaller intrinsic shape noise than the SNAP and LSSTgalaxies. The surveys are assumed to have the source galaxy dis-tribution of the form in equation (4) which peaks at zpeak = 2z0.For the fiducial SNAP and LSST surveys, we assume tomographywith 10 redshift bins equally spaced out to z = 3, as future pho-tometric redshift accuracy will enable relatively fine slicing in red-shift. For the DES, we assume a more modest seven redshift binsout to z = 2.1, reflecting the shallower reach of the DES whilekeeping the redshift bins equally wide (z = 0.3) as in the othertwo surveys. Finally, we do not use weak-lensing information be-yond = 3000 in order to avoid the effects of baryonic cooling(Huterer et al. 2004; White 2004; Zhan & Knox 2004) and non-Gaussianity (White & Hu 2000; Cooray & Hu 2001), both of whichcontribute more significantly at smaller scales. While there may beways to extend the useful -range to smaller scales without riskingbias in cosmological constraints (Huterer & White 2005), extendingthe measurements to max = 10 000 would improve the marginal-ized errors on cosmological parameters by only about 30 per cent.7

The parameter fiducial values and accuracies are summarized inTable 2. The fiducial values for the parameters not listed in Table 2are Mh2 = 0.147, Bh2 = 0.021, n = 1.0, and m = 0.1 eV.

It is well known that measurements of the angular PS of the CMB,such as those expected by the Planck experiment, can help weaklensing to constrain the cosmological parameters. In particular, themorphology of the peaks in the CMB angular PS contains useful

7 On the other hand, especially for the DES, the Gaussian covariance as-sumption may be somewhat optimistic for the range 1000 < < 3000 (e.g.White & Hu 2000).

information on the physical matter and baryon densities, while thelocations of the peaks help to constrain the dark energy parameters.However, we checked that, when the Planck prior added, all sys-temat...