1304.2873 (Xiangkun Liu et al.)

Mask effects on cosmological studies with weak lensing peak statistics    [PDF]

Xiangkun Liu, Zuhui Fan

In this paper, we analyze in detail with numerical simulations how the mask effect can influence the weak lensing peak statistics reconstructed from the shear measurement of background galaxies. It is found that high peak fractions are systematically enhanced due to masks, the larger the masked area, the higher the enhancement. In the case with about 13% of the total masked area, the fraction of peaks with SNR $\nu\ge 3$ is $\sim 13%$ in comparison with $\sim 9%$ of the mask-free case in our considered cosmological model. This can induce a large bias on cosmological studies with weak lensing peak statistics. Even for a survey area of $9\hbox{deg}^2$, the bias in $(\Omega_m, \sigma_8)$ is already close to $3\sigma$. It is noted that most of the affected peaks are close to the masked regions. Therefore excluding peaks in those regions can reduce the bias but at the expense of loosing usable survey areas. Further investigations find that the enhancement of high peaks number can be largely attributed to higher noise led by the fewer number of galaxies usable in the reconstruction. Based on Fan et al. (2010), we thus develop a two-noise-level model that treats the areas close to and away from the masked regions separately. It is shown that the model can account for the mask effect very well, and the parameter bias is significantly reduced. We also analyze the systematic effects on peak statistics resulting from the nonlinear convergence reconstruction, including the smoothing order problem and the mass-sheet degeneracy.

View original: http://arxiv.org/abs/1304.2873