Filtering techniques for the detection of Sunyaev-Zel'dovich clusters in multifrequency maps

Abstract

The problem of detecting Sunyaev-Zel'dovich (SZ) clusters in multifrequency CMB observations is investigated using a number of filtering techniques. A multifilter approach is introduced that optimizes the detection of SZ clusters on microwave maps. An alternative method is also investigated in which maps at different frequencies are combined in an optimal manner so that existing filtering techniques can be applied to the single combined map. The SZ profiles are approximated by the circularly-symmetric template τ(x) = [1 + (x/rc)2]-λ, with λ ≃ 1/2 and x ≡ |x|, where the core radius rc and the overall amplitude of the effect are not fixed a priori, but are determined from the data. The background emission is modelled by a homogeneous and isotropic random field, characterized by a cross-power spectrum Pν1ν2(q) with q ≡ |q|. The filtering methods are illustrated by application to simulated Planck observations of a 12.8° × 12.8° patch of sky in 10 frequency channels. Our simulations suggest that the Planck instrument should detect ≈ 10 000 SZ clusters in 2/3 of the sky. Moreover, we find the catalogue to be complete for fluxes S > 170 mJy at 300 GHz.