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 A&A (CFHTLS)

Hoekstra & Jain 2008 astro-ph/

Future surveys Hoekstra & Jain 2008 Sun et al deg 2 zm=0.9 SNAP 1000deg 2 zm= 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 2000 Linden et al “ Dark clumps ” S/N ~4 M~10 14 M sun 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

-> 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