Cosmic Magnification and the ISW effect

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Cosmic Magnification and the ISW effect
Columbia University, Instituto de Ciencias del
Espacio
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ISW from cross-correlation
Boughn and Crittenden 2004 Nolta et al 2004
Fosalba and Gaztanaga 2004 Fosalba, Gaztanaga
and Castander 2003 Scranton et al 2003
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ISW from cross-correlation
Boughn and Crittenden 2004 Nolta et al 2004
Fosalba and Gaztanaga 2004 Fosalba, Gaztanaga
and Castander 2003 Scranton et al 2003
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• But, at high redshifts gravitational lensing may
  become important.
• Lensing magnification
• 1. Increases the area, decreasing the galaxy
  overdensity ?n
• 2. Brightens sources promoting
  intrinsically faint objects above mlim ,
  increasing ?n

(Moessner, Jain, Villumsen 1997)
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• Together these corrections are called
• Magnification Bias

The change in ?n depends on
(Moessner, Jain, Villumsen 1997)
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Magnification bias
The measured fluctuation is a sum of two terms
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Magnification bias
The measured fluctuation is a sum of two terms
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So with magnification bias,

• tells about growth rates at lens redshifts
• ? (2.5s-1)
• s d log(N(m))/dm

• has info about structure growth at redshift
  of sample
• ? galaxy bias

Relative magnitude of the two terms is redshift,
scale and galaxy population dependent
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Questions

• How big is the effect?
• Does this alter dark energy measurements?
• Can it provide new information?

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How big is the effect?

• Magnitude and sign depend strongly on galaxy
  sample

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Consider an LSST-like survey
fsky0.5 , Ngal 108 , Mlim(R)-20.4

• Use conditional luminosity function (Cooray
  2005)
• to compute

b(z),s(z)
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Results
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Results
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Results
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Results
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Results
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Results
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Results
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Results
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• The magnification-temperature signal is large
• What are the consequences of neglecting it?

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Dark Energy parameters
(all other parameters fixed)
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Dark Energy parameters
(all other parameters fixed)
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• Magnification bias is a large systematic
• Can this systematic be turned into a signal?

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More Information

• The total signal to noise remains large at
  high redshifts

but
The high redshift signal is strongly
correlated with the low redshift signal
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More Information
Forecasted constraints on (??m0.03, h
fixed)
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More Information
Forcasted constraints on (??m0.03, h
fixed)
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Conclusions

• Magnification bias does significantly alter the
  ISW cross-correlation signal
• If not taken into account incorrect conclusions
  about cosmological parameters may be reached
• The magnification signal remains large at high
  redshifts so it provides more information than
  the galaxy-temperature correlation alone
• The magnification signal doesnt depend on galaxy
  bias so it may be a more accurate tracer of ?(z)

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Further Questions

• Is there a galaxy sample that optimizes the
  information provided by galaxy-temperature and
  magnifcation-temperature correlations?
• If we account for magnification bias, how should
  the redshift distribution be chosen to optimize
  the information learned from ISW