The importance of merging activity for the kinetic polarization of the Sunyaev-Zel'dovich signal from galaxy clusters

Context: The polarization sensitivity of upcoming millimetric observatories will open new possibilities for studying the properties of galaxy clusters and for using them as powerful cosmological probes. For this reason it is necessary to investigate in detail the characteristics of the polarization signals produced by their highly ionized intra-cluster medium (ICM). This work is focused on the polarization effect induced by the ICM bulk motion, the so-called kpSZ signal, which has an amplitude proportional to the optical depth and to the square of the tangential velocity. <BR />Aims: We study how this polarization signal is affected by the internal dynamics of galaxy clusters and its dependence on the physical modelling adopted to describe the baryonic component. <BR />Methods: This is done by producing realistic kpSZ maps starting from the outputs of two different sets of high-resolution hydrodynamical N-body simulations. The first set (17 objects) follows only non-radiative hydrodynamics, while for each of 9 objects of the second set we implement four different kinds of physical processes. <BR />Results: Our results shows that the kpSZ signal is a very sensitive probe of the dynamical status of galaxy clusters. We find that major merger events can amplify the signal up to one order of magnitude with respect to relaxed clusters, reaching amplitudes up to about 100 nK. This result implies that the internal ICM dynamics must be taken into account when evaluating this signal because simplicistic models, based on spherical rigid bodies, may provide wrong estimates. In particular, the selection of sufficient relaxed clusters seems to be fundamental to obtain a robust measurement of the intrinsic quadrupole of the cosmic microwave background through polarization. We find that the dependence on the physical modelling of the baryonic component is relevant only in the very inner regions of clusters.