Studying the ICM velocity structure within galaxy clusters with simulations and X-ray observations

Galaxy clusters are optimal laboratories to test cosmology as well
as models for physical processes acting on smaller scales. X–ray
observations of the hot gas filling their dark matter potential
well, i.e. the intra–cluster medium (ICM), still provides one of
the best ways to investigate the intrinsic properties of clusters.
Methods based on X–ray observations of the ICM are commonly used to
estimate the total mass, assuming that the gas traces the
underlying potential well and satisfies spherical symmetry, and
thermal motions dominate the total pressure support. However,
non–thermal motions are likely to establish in the ICM, hence,
contribute to the total pressure and have to be taken into account
in the mass estimate. In this thesis I study the ICM
thermo–dynamical structure by combining hydrodynamical simulations
and synthetic X–ray observations of galaxy clusters. The main goal
is to study their gas velocity field and the implications due to
non–thermal motions: first, by analysing directly the velocity
patterns in simulated clusters and, secondly, by reconstructing the
internal ICM structure from mock X–ray spectra. To this aim, I
developed and applied an X–ray photon simulator to obtain synthetic
X–ray spectra from the gas component in hydrodynamical simulations
of galaxy clusters. The main findings of this work are as follows.
(i) Ordered, rotational patterns in the gas velocity field in
cluster cores can establish during the mass assembly process, but
are found to be transient phenomena, easily destroyed by passages
of gas–rich subhaloes. This suggests that in smoothly growing
haloes the phenomenon is in general of minor effect. Nonetheless,
major mergers or highly disturbed systems can indeed develop
significant ordered motions and rotation, which contribute up to
20% to the total mass. (ii) It is indeed possible to reconstruct
the thermal structure of the ICM in clusters from X–ray spectral
analysis, by recovering the emission measure (EM) distribution of
the gas as a function of temperature. This is possible with current
X–ray telescopes (e.g. Suzaku) via multi–temperature fitting of X–
ray spectra. (iii) High–precision X–ray spectrometers, such as
ATHENA, will allow us to measure velocity amplitudes of ICM
non–thermal motions, from the velocity broadening of heavy–ion
(e.g. iron) emission lines. In this work, these achievements are
obtained by applying the virtual X–ray simulator to generate ATHENA
synthetic spectra of simulated clusters. The non–thermal velocity
of the ICM in the central region is used to further characterise
the cluster and the level of deviation from the expected
self–similarity. By excluding the clusters with the highest
non–thermal velocity dispersion, the scatter of the LX −T relation
for the sample is significantly reduced, which will allow for a
more precise comparison between observations and simulations.