Impact of intrinsic alignments on cosmic shear Sarah Bridle, UCL (London) Concepts: II, GI Observations Theoretical models and simulations Impact on dark energy

Bluff your way in intrinsic alignments: Use these phrases with confidence! ‘GI partially cancels the cosmic shear signal’ ‘II can be removed by removing close pairs’ ‘GI comes from dark matter aligning a nearby galaxy and shearing a distant galaxy’ ‘GI is easier to measure than II’ ‘Removing elliptical galaxies will reduce GI’ ‘Removing IA requires 4 times better photozs’

Intrinsic Alignment Equations II GI SE GG Crittenden, Natarajan, Pen, Theuns 2000 but see Hirata & Seljak 2004

Gravitational Lensing Light from a distant galaxy is bent around a mass

Face-on view Galaxy image is Magnified Stretched tangentially around the mass (shear)

Cosmic lensing Lensing by large-scale structure of the Universe Cosmic shear Cosmic magnification

Lensing by dark matter causes nearby galaxies to appear aligned Cosmic shear Face-on view Gravitationally Sheared (G) Gravitationally Sheared (G) Lensing by dark matter causes nearby galaxies to appear aligned

Face-on view (zoomed out) Cosmic magnification Face-on view (zoomed out) Increased number counts behind a mass (If number counts increase as go fainter) Broadhurst, Taylor & Peacock 1995; Bartelmann 1995; Dolag & Bartelmann 1997; Sabz et al 1997; Moessner & Jain 1998; Jain 2002; Barber & Taylor 2003; Takada & Hamana 2003; Zhang & Pen 2005 Benitez & Martinez-Gonzlez 1997; Benitez et al 2001; Gaztanaga 2003; Scranton et al 2005;

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

Intrinsic alignments (II) Face-on view Intrinsically Aligned (I) Intrinsically Aligned (I) Dark matter distribution causes galaxies to align Adds to cosmic shear signal

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

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

GI from galaxies themselves Bridle & Abdalla 2007

GI from galaxies themselves Face-on view Significant on scales ~< 8 arcmin

Intrinsic Alignment Equations II GI =0? GG Crittenden, Natarajan, Pen, Theuns 2000 but see Hirata & Seljak 2004

Observing intrinsic alignments (II) Do galaxies point at each other? Need lots of physically close galaxies Don’t want contamination from cosmic shear! Use a low-z survey e.g. SuperCOSMOS, SDSS Calculate shear correlation function rp

Mandelbaum et al 2006

Observing intrinsic alignments (II) Mandelbaum et al 2006

Observing intrinsic alignments (GI) GI - do galaxies point at high mass clumps? Need mass map – or substitute galaxy map = are galaxies pointing at other galaxies? Calculate shear-galaxy correlation function Find all pairs at given separation. Find mean radial ellipticity of one galaxy. rp

Observing intrinsic alignments (GI) Low luminosity Weak signal High luminosity Significant detection Brown et al 2002; Heymans et al 2004; Trujillo, Carretero, Patiri 2006; Mandelbaum et al 2006; Hirata et al 2007

Observing intrinsic alignments (GI) Very strong signal for luminous red galaxies Brown et al 2002; Heymans et al 2004; Trujillo, Carretero, Patiri 2006; Mandelbaum et al 2006; Hirata et al 2007

Theoretical models Tidal torque theory Linear alignment model N-body simulations Fitting formulae

Tidal torque theory Quadratic alignment model Spiral galaxies Apparent ellipticity determined by angular momentum Angular momentum is from external tidal fields Contribution to GI vanishes in linear theory Crittenden, Natarajan, Pen, Theuns 2000; Catelan, Kamionkowski, Blandford 2001; Hui & Zhang 2002 Hoyle 1949; Peebles 1969; Doroshkevich 1970; White 1984; Peacock & Heavens 1985; Barnes & Efstathiou 1987; Heavens & Peacock 1988; Porciani et al 2002

Linear alignment model Elliptical galaxies Galaxy ellipticity is stretched along potential curvature Note similarity with lens equations Catelan, Kamionkowski, Blandford 2001

Intrinsic-shear (GI) Bridle & King 2007 Hirata et al 2007

N-body simulations Best model - Cosmic shear Heymans et al Heavens, Refregier & Heymans 2000; Croft & Metzler 2000; Jing 2002; Heymans et al 2006

Fitting formula HRH* (Heymans et al 2004 based on Heavens, Refregier & Heymans 2000)

Intrinsic-intrinsic (II)

Cosmic shear two point tomography

Cosmic shear tomography

Tomographic Lensing Power Spectra

Intrinsic Alignment Equations II GI =0? GG Crittenden, Natarajan, Pen, Theuns 2000 but see Hirata & Seljak 2004

Ellipticity power spectra

Ellipticity power spectra Lensing sensitivity function for z bin i Source redshift distribution in z bin j

Linear alignment model

Effect on cosmic shear of changing w by 1% Cosmic Shear Intrinsic Alignments (IA) Don’t explain details of whether GI, II Normalised to Super-COSMOS Heymans et al 2004

Effect on cosmic shear of changing w by 1% Intrinsic Alignments (IA) If consider only w then IA bias on w is ~10% If marginalise 6 cosmological parameters then IA bias on w is ~100% (+/- 1 !) Intrinsic Alignments (IA) Move along degeneracies -> 100% We have to consider intrinsic alignments, and remove them carefully See Bridle & King 2007 for details

Removal of intrinsic alignments Intrinsic – intrinsic (II) Weight down close pairs (King & Schneider 2002, Heymans & Heavens 2003, Takada & White 2004) Fit parameterized models (King & Schneider 2003, Bridle & King 2007) Shear – intrinsic (GI) Fit parameterized models (King 2005, Bernstein DETF. Bridle & King 2007) Redshift weighting (Schneider) The effect I just described can be classified as a type of shear-intrinsic (GI) intrinsic alignment. Various methods have been suggested for removing the intrinsic alignments. The intrinsic-intrinsic alignment is the alignment of close pairs of galaxies. So we can remove it by simply removing all close pairs! Or simultaneously fit a model for the intrinsic alignments at the same time as fitting the cosmological model. Its much harder to remove the shear-intrinsic correlation. For the intrinsic-intrinsic correlation we need to remove all close pairs of galaxies. Whereas, to first order, for the shear-intrinsic term we need to remove all the pairs that aren’t close, which would leave us with no galaxies! Lindsay King and Gary Bernstein have suggested fitting parameterised models, and on Monday Peter briefly described a new redshift weighting method. Whichever method you use, the quality of the redshift information is absolutely crucial. So Lindsay King and I recently investigated the effect of photometric redshift errors on the model fitting techniques. Redshift quality is crucial!

Dark energy Figure of Merit Perfect redshifts Least flexible model considered FoM is improved! Redshift dependence of IA (# bins) 2 3 5 No Intrinsic Alignments Dark energy Figure of Merit Reasonable model? (14 IA pars) Similar FoM to no IA case Very flexible (100 IA pars) FoM is roughly halved Scale dependence of IA (# bins)

Dark energy Figure of Merit Perfect redshifts Redshift dependence of IA (# bins) 2 3 5 Dark energy Figure of Merit Scale dependence of IA (# bins)

Realistic photozs σz=0.05(1+z) Redshift dependence of IA (# bins) 2 3 5 Dark energy Figure of Merit Scale dependence of IA (# bins)

No Intrinsic Alignments FoM / FoM(specz) Relatively flat Bridle & King arXiv:0705.0166 (e.g. Hu 1999, Ma, Hu, Huterer 2006, Jain et al 2007, Amara & Refregier 2007 ....) Photoz error σz / (1+z)

FoM / FoM(specz) Photoz error σz / (1+z) Reasonable model? (14 IA pars) Very flexible (100 IA pars) FoM / FoM(specz) Bridle & King arXiv:0705.0166 Photoz error σz / (1+z)

Redshift accuracy σz / (1+z) With intrinsic alignments, need 4x better redshifts to get 80% of the DE information 0.8 FoM / FoM(specz) Bridle & King arXiv:0705.0166 0.02 0.08 Redshift accuracy σz / (1+z)

Bluff your way in intrinsic alignments: Use these phrases with confidence! ‘GI partially cancels the cosmic shear signal’ ‘II can be removed by removing close pairs’ ‘GI comes from dark matter aligning a nearby galaxy and shearing a distant galaxy’ ‘GI is easier to measure than II’ ‘Removing elliptical galaxies will reduce GI’ ‘Removing IA requires 4 times better photozs’

More research needed! Theoretical Can IA be predicted accurately enough to help? How complicated are IA as fn (scale, z)? Necessary to model IA as fn(magnitude, type)? Can intrinsic cosmic magnification be removed? Observational How do intrinsic alignments (IA) evolve with z? How big are IA for blue galaxies? Correlated with other properties e.g. photozs? Can cosmic magnification overcome e.g. extinction?

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Shearing by elliptical galaxy halos Plan: Calculate shear from elliptical halo Calculate contribution to shear correlation fn Average over a population of lenses Compare with cosmic shear signal Consider effect of halo profile Investigate redshift dependence Bridle & Abdalla 2007

Shear correlation function z1=0.3 z2=0.8 Cosmic shear signal NFW Shear correlation function Average over population visible to R=24 ^

Shear correlation function z1=0.3 z2=0.8 Cosmic shear signal Singular isothermal ellipsoid NFW Shear correlation function Average over population visible to R=24 ^

Shear correlation function zlens=0.3 zsource=0.8 Bridle & Abdalla M200=1x1012 h-1 Mo Shear correlation function

How good to photozs need to be to remove intrinsic alignments? Plan: Remove GI, II by marginalising over some flexible model Look at the effect of GI, II on dark energy errors Dependence on flexibility of model? Dependence on photoz errors? Bridle & King 2007

σz / (1+z)

Contribution to ellipticity correlation function: Bridle & Abdalla Contribution to ellipticity correlation function: Average shear around circular annulus Does not average to zero →net contamination

Shear correlation function z1=0.3 z2=0.8 Cosmic shear signal Bridle & Abdalla Shear correlation function Average over population visible to R=24

Shear correlation function z1=0.3 z2=0.8 Cosmic shear signal Bridle & Abdalla Shear correlation function Average over population visible to R=24 Change in cosmic shear signal for  w = 0.05 We do need to consider this effect at scales smaller than about 10 arcmins

Intrinsic-shear (GI) Hirata et al 2007 Bridle & King 2007