Transcript Gravitational Lensing, SZ Effects, and Large

Probing Cosmology with
Weak Lensing Effects
Zuhui Fan
Dept. of Astronomy, Peking University
Outline:
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Weak gravitational lensing effects
Cosmological applications
Systematic effects
“Dark clumps” near clusters of galaxies
catastrophic photo-z errors
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Lensing Effects
Gravitational lensing effects arise from the light
deflection by the intervening structures
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Weak Lensing Effects
Weak distortions caused by
the large-scale structures of
the universe: common but weak
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“see” the dark matter directly
 powerful probes of the distribution of
dark matter
sensitive to the formation of large-scale structures
and the global geometry of the universe
 highly promising in dark energy studies
Observationally challenging
accurate shape measurements:
lensing induced shape
distortions are much weaker
than the intrinsic ellipticities of
galaxies
 statistical measurements of
the coherent distortions
PSF corrections
accurate calibration of the redshift distribution of
source galaxies
Observational advances
Statistical methods
theoretical studies
 Fast developing forefront of research
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Cosmological Applications
map out dark matter distribution
Bullet cluster
COSMOS Massey et al. 2007
Cosmic shear : constraining cosmological
parameters
Fu et al. 2008 A&A (CFHTLS)
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Hoekstra & Jain 2008 astro-ph/08050139
Future surveys
Hoekstra & Jain 2008
5000deg2 zm=0.9
3 zbins
Sun et al. 2008
SNAP 1000deg2 zm=1.26
3 zbins
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Systematics
Because of the weakness of the lensing signals,
systematic effects can affect their cosmological
applications considerably.
* redshift distribution of source galaxies
magnitude distribution  redshift distribution
photo-z measurement
* intrinsic alignments of source galaxies
shear-ellipticity correlation
* Nonlinear power spectrum
* observational systematics
* ……
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“Dark clumps” around clusters
(Fan, Z.H. & Liu, J.Y.)
Erben et al 2000
Linden et al. 2006
“Dark clumps” S/N ~4 M~1014Msun at z~0.2
If real, would be significant for the theory of structure formation
Galaxies are not intrinsically spherical
-> noise in the mass distribution constructed
from weak lensing effects
Real clusters vs. Noise peaks
Noise peaks have no optical counterparts
(However, Dark clumps)
On average, high S/N noise peaks are rare
Use average number density of noise peaks: P~8*10-3
Very unlikely to be a noise peak, then real “dark clumps”?
However, around real clusters,
the probability of high noise peaks can be
higher than average
Around a real cluster
Nc  N   c
-> Noise affects cluster lensing signals
-> Presence of real clusters affects the statistics of
noise peaks : analogous to the biased halo
formation (** however, mass-sheet degeneracy)
Number of high S/N noise peaks are significantly
boosted (~6 times for S/N>4.5)
Catastrophic errors
in photo-z
(Sun, L. et al.)
With SNAP standard filters, catastrophic fraction ~1.5%
3 z-bins, bias >> statistical error
with zbin~8, bias ~ 1σstatistical error
 Fine bins can help
Add in u-band filter can reduce the catastrophic fraction
efficiently (however may be difficult in space).
Weak lensing effects hold great potential
in cosmological studies
Much more investigations are needed