Impact of supermassive black holes on galaxy clusters

In the semi-analytical work presented here the feedback from
supermassive black holes on galaxy clusters is investigated. In
particular we aim at providing simple diagnostics tools to
constrain the characteristic velocities and spatial scales of the
hot Intra Cluster Medium (ICM) motions. In the so-called "cold
core'' clusters these motions are believed to be driven by the
activity of a central black hole. The methods developed here,
together with present-day and future observations, are designed to
help to solve the puzzle of cooling flow clusters (see section
$1.3$) and understand better the AGN/gas interaction in smaller
systems (down to individual galaxies).\\ Clusters of galaxies are
the largest gravitationally bound systems in the Universe: they are
composed of hundreds to thousands of galaxies, moving in a deep
potential well set by the dominating dark matter. The whole volume
of clusters is filled with hot (temperature $\sim 10^7-10^8$~K) and
rarefied (electron density $10^{-4}-10^{-1} {\rm cm^{-3}}$) gas. In
such a high-temperature regime even heavy elements (e.g. silicon,
sulfur, iron etc.) are highly ionized up to [H]- or [He]-like ions
and they emit in bright lines with energies from $\sim 0.7$ to
$\sim 8$ keV. Using X-ray observations one can reliable measure all
the major gas properties: the temperature, density and abundance of
heavy elements.\\ A significant fraction of clusters (called "cool
core'' clusters) show distinct signatures in the central region:
the gas temperature drops inward, while the gas density increases.
The central gas radiative cooling time in such clusters is much
shorter than the age of the cluster and without any external source
of energy the gas would cool well below X-ray temperatures. However
observations suggest that the gas temperature drops only to 1-2
keV. One plausible explanation of this problem is that the activity
of a central supermassive black hole deposits large amounts of
mechanical energy into the cluster gas and that this balances the
gas radiative losses. A direct implication of this hypothesis is
that the hot gas is not at rest, but it is continuously stirred by
the AGN activity.\\ The same class of cool core clusters is
characterized by a centrally peaked distribution of the heavy
elements abundance (usually measured using the He-like iron 6.7 keV
line). The peaked abundance profiles are likely associated with the
metals ejection by the stars of very massive elliptical galaxies,
that are always present at the centers of these clusters. However
the observed abundance distributions are significantly broader than
the central galaxy light profiles, suggesting that some gas motions
are spreading the metals ejected from the galaxy. We treat this
process in a diffusion approximation to derive, from the X-ray
observations, constraints on the characteristic velocities and
spatial scales of the gas motions for a sample of cool core
clusters and groups (Chapters $2$ and $3$). The parameters derived
from a simple semi-analytic model are then compared with the
results of numerical simulations of the AGN/gas interaction in the
cluster core (Chapter $4$).\\ In Chapter $5$ we discuss the impact
of the gas motions on the width of the strongest X-ray emission
lines. Since the characteristic thermal velocities of heavy ions
(e.g. iron) are much smaller than the sound speed of the gas, the
width of the lines sensitively depends on the presence of gas
motions. We show that both the absolute value of the linewidth and
its dependence on the projected distance from the cluster center
provide valuable diagnostics of the gas motions. Such measurements
will soon become possible with the launch of X-ray
micro-calorimeters in space.\\ This work has been done in
collaboration with E.Churazov, R.Sunyaev, H.B\"ohringer,
M.Br\"uggen, W.Forman and E.Roediger.