Probing Cosmology with Weak Lensing Effects Zuhui Fan Dept. of Astronomy, Peking University

Outline: Weak gravitational lensing effects Cosmological applications Systematic effects “Dark clumps” near clusters of galaxies catastrophic photo-z errors

Lensing Effects Gravitational lensing effects arise from the light deflection by the intervening structures

Weak Lensing Effects Weak distortions caused by the large-scale structures of the universe: common but weak “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

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)

Hoekstra & Jain 2008 astro-ph/08050139

Future surveys Hoekstra & Jain 2008 Sun et al. 2008 5000deg2 zm=0.9 SNAP 1000deg2 zm=1.263 zbins 3 zbins

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 * ……

“Dark clumps” around clusters (Fan, Z.H. & Liu, J.Y.)

Erben et al 2000Linden 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

-> Noise affects cluster lensing signals

cNNc

-> 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