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.
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.
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.
/>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.