Galaxy clusters are the largest gravitationally bound structures in the Universe. About 80 per cent of their mass is due to dark matter, 15 per cent due to hot gas and only a few per cent of the mass corresponds to stars. Hot gas is therefore the dominant baryonic component of clusters and the largest mass constituent which can be observed directly. Along the line of sight, the motion of the gas inside the cluster can in principle be measured by analysing the Doppler shift of X-ray spectral lines. However, given that the expected shift is very small, X-ray spectrometers with high spectral resolution are needed. Measuring the gas motion perpendicular to the line of sight through the usual spectroscopic techniques is even more difficult, as the shift is about 100 times smaller. We therefore investigated the effects of gas motions on the polarization in the emitted X-ray light.

The hot gas (10⁷ - 10⁸ K) in galaxy clusters emits X-ray radiation in a continuum and in emission lines of ionized heavy elements. As iron is the most abundant element, the resonant lines of ionized iron are especially strong and bright. In these lines the scattering cross section is much larger than in the continuum and the optical depth can be of the order of one or even larger, which means that on average each photon emitted suffers at least one scattering. For instance, the optical depth of the iron line at an energy of 6.7 keV in the brightest clusters in the sky, the Perseus and Virgo clusters, is approximately 3 and 1.4, respectively.

The atomic structure of the ions defines the process of line scattering, i.e. the absorption and re-emission of a photon, as a combination of isotropic and Rayleigh scattering. It is well known that the Rayleigh component leads to a polarization in the emitted radiation (just like in the case of Thomson scattering) if there is a quadrupole moment in initial radiation field. In galaxy clusters such a quadrupole moment arises naturally if either the scattering takes place far from the bright central core of the cluster and/or if there are gas motions. The expected degree of polarization is high: in the Perseus cluster it reaches about 7 per cent in the iron line at 6.7 keV; in the Virgo cluster it is several per cent in the most promising lines (see Fig. 1).

Transverse gas motions in the cluster can change the expected degree and direction of the polarization, since the cross section of scattering along the direction of the motion is decreasing. Using modern, fully three- dimensional simulations of galaxy clusters we calculated the polarization degree of the X-ray lines expected when taking gas motions into account. In Fig. 2 one can see that the polarization degree in the iron line reaches about 25 per cent within a distance of 500 kpc from the core if the cluster gas is at rest. Including gas motions decreases the polarization down to about 10 per cent and causes a rotation of the polarization plane.

A new era of high resolution X-ray spectroscopy and polarimetric studies is coming, driven by progress in the development of a new generation of X-ray detectors. The first polarimetric mission GEMS , based on the photo-effect principle, is already approved and funded and will be launched in the near future. Several more missions are under discussion. Measuring the polarization degree of the bright X-ray emission lines and comparing these with simulations will provide us with new information on the bulk and turbulent gas motions in clusters of galaxies.

Irina Zhuravleva, Eugene Churazov, Sergey Sazonov, Rashid Sunyaev, Klaus Dolag

Further Reading:

Zhuravleva I.V., Churazov E.M., Sazonov S.Y., Sunyaev R.A., Forman W., Dolag K., 'Polarization of X-ray lines from galaxy clusters and elliptical galaxies - a way to measure the tangential component of gas velocity', MNRAS, 403, 129—150 (2010)