Impact of intrinsic alignments on cosmic shear

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Impact of intrinsic alignments on cosmic shear
Sarah Bridle, UCL (London)

• Concepts II, GI
• Observations
• Theoretical models and simulations
• Impact on dark energy

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Bluff your way in intrinsic alignmentsUse 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

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Intrinsic Alignment Equations
GI SE
II
GG
Crittenden, Natarajan, Pen, Theuns 2000 but see
Hirata Seljak 2004
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Gravitational Lensing
Light from a distant galaxy is bent around a mass
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Face-on view

• Galaxy image is
• Magnified
• Stretched tangentially around the mass (shear)

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Cosmic lensing

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

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Cosmic shear Face-on view
Gravitationally Sheared (G)
Gravitationally Sheared (G)
Lensing by dark matter causes nearby galaxies to
appear aligned
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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
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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
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Intrinsic alignments (II) Face-on view
Intrinsically Aligned (I)
Intrinsically Aligned (I)
Dark matter distribution causes galaxies to
align Adds to cosmic shear signal
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Intrinsic-shear correlation (GI)
Hirata Seljak 2004 See also Heymans et al
2006, Mandelbaum et al 2006, Hirata et al 2007
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Intrinsic-shear correlation (GI) Face-on view
Gravitationally sheared (G)
Intrinsically aligned (I)
Galaxies point in opposite directions Partially
cancels cosmic shear signal
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GI from galaxies themselves
Bridle Abdalla 2007
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GI from galaxies themselves Face-on view
Significant on scales lt 8 arcmin
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Intrinsic Alignment Equations
GI
II
0?
GG
Crittenden, Natarajan, Pen, Theuns 2000 but see
Hirata Seljak 2004
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Observing intrinsic alignments (II)

• Do galaxies point at each other?
• Need lots of physically close galaxies
• Dont want contamination from cosmic shear!
• Use a low-z survey
• e.g. SuperCOSMOS, SDSS
• Calculate shear correlation function

rp
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Mandelbaum et al 2006
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Observing intrinsic alignments (II)
Mandelbaum et al 2006
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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
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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
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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
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Theoretical models

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

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Tidal torque theoryQuadratic 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
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Linear alignment model

• Elliptical galaxies
• Galaxy ellipticity is stretched along potential
  curvature
• Note similarity with lens equations

Catelan, Kamionkowski, Blandford 2001
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Intrinsic-shear (GI)
Bridle King 2007
Hirata et al 2007
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N-body simulations
Heymans et al
Best model
- Cosmic shear
Heavens, Refregier Heymans 2000 Croft
Metzler 2000 Jing 2002 Heymans et al 2006
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Fitting formula

• HRH (Heymans et al 2004 based on Heavens,
  Refregier Heymans 2000)

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Intrinsic-intrinsic (II)
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Cosmic shear two point tomography
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Cosmic shear tomography
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Tomographic Lensing Power Spectra
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Intrinsic Alignment Equations
GI
II
0?
GG
Crittenden, Natarajan, Pen, Theuns 2000 but see
Hirata Seljak 2004
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Ellipticity power spectra
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Ellipticity power spectra
Lensing sensitivity function for z bin i
Source redshift distribution in z bin j
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Linear alignment model
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Effect on cosmic shear of changing w by 1
Cosmic Shear
Intrinsic Alignments (IA)
Normalised to Super-COSMOS Heymans et al 2004
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Effect on cosmic shear of changing w by 1
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)
See Bridle King 2007 for details
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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)

Redshift quality is crucial!
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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)
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Perfect redshifts
Redshift dependence of IA ( bins) 2 3 5
Dark energy Figure of Merit
Scale dependence of IA ( bins)
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Realistic photozs sz0.05(1z)
Redshift dependence of IA ( bins) 2 3 5
Dark energy Figure of Merit
Scale dependence of IA ( bins)
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No Intrinsic Alignments
FoM / FoM(specz)
Relatively flat
Bridle King arXiv0705.0166
(e.g. Hu 1999, Ma, Hu, Huterer 2006, Jain et al
2007, Amara Refregier 2007 ....)
Photoz error sz / (1z)
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Reasonable model? (14 IA pars)
Very flexible (100 IA pars)
FoM / FoM(specz)
Bridle King arXiv0705.0166
Photoz error sz / (1z)
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With intrinsic alignments, need 4x better
redshifts to get 80 of the DE information
0.8
FoM / FoM(specz)
Bridle King arXiv0705.0166
0.02
0.08
Redshift accuracy sz / (1z)
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Bluff your way in intrinsic alignmentsUse 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

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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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END
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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
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z10.3 z20.8
Cosmic shear signal
NFW
Shear correlation function
Average over population visible to R24

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z10.3 z20.8
Cosmic shear signal
Singular isothermal ellipsoid
NFW
Shear correlation function
Average over population visible to R24

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zlens0.3 zsource0.8
Bridle Abdalla
M2001x1012 h-1 Mo
Shear correlation function
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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
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sz / (1z)
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Bridle Abdalla
Contribution to ellipticity correlation
function Average shear around circular annulus
Does not average to zero ?net contamination
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z10.3 z20.8
Cosmic shear signal
Bridle Abdalla
Shear correlation function
Average over population visible to R24
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z10.3 z20.8
Cosmic shear signal
Bridle Abdalla
Shear correlation function
Average over population visible to R24
Change in cosmic shear signal for ? w 0.05
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Intrinsic-shear (GI)
Hirata et al 2007
Bridle King 2007