Searching for massive clusters in weak lensing surveys

We explore the ability of weak lensing surveys to locate massive clusters. We use both analytic models of dark matter haloes and mock weak lensing surveys generated from a large cosmological N-body simulation. The analytic models describe the average properties of weak lensing haloes and predict the number counts, enabling us to compute an effective survey selection function. We argue that the detectability of massive haloes depends not only on the halo mass but also strongly on the redshift where the halo is located. We test the model prediction for the peak number counts in weak lensing mass maps against mock numerical data, and find that the noise resulting from intrinsic galaxy ellipticities causes a systematic effect which increases the peak counts. We develop a correction scheme for the systematic effect in an empirical manner, and show that, after correction, the model prediction agrees well with the mock data. The mock data is also used to examine the completeness and efficiency of the weak lensing halo search by fully taking into account the noise and the projection effect by large-scale structures. We show that the detection threshold of S/N = 4 ∼ 5 gives an optimal balance between completeness and efficiency. Our results suggest that, for a weak lensing survey with a galaxy number density of n g = 30 arcmin -2 with a mean redshift of z = 1, the mean number of haloes which are expected to cause lensing signals above S/N = 4 is N balo (S/N > 4) = 37 per 10 deg 2 , whereas 23 of the haloes are actually detected with S/N > 4, giving the effective completeness as good as 63 per cent. Alternatively, the mean number of peaks in the same area is N peak = 62 for a detection threshold of S/N = 4. Among the 62 peaks, 23 are caused by haloes with the expected peak height S/N >4, 13 result from haloes with 3 < S/N < 4 and the remaining 26 peaks are either the false peaks caused by the noise or haloes with a lower expected peak height. Therefore the contamination rate is 44 per cent (this could be an overestimation). Weak lensing surveys thus provide a reasonably efficient way to search for massive clusters.