On the role of cosmic rays in clusters of galaxies

We take a multi-faceted approach to study the relativistic cosmic
ray (CR) proton population in galaxy clusters. CR protons may be
accelerated by structure formation shock waves, injected from radio
galaxies into the intra-cluster medium, or result from supernova
driven galactic winds. This thesis addresses the following
questions: do CR protons exist in galaxy clusters? What is the
dynamic and cosmological impact of CRs? How can we observe them?
How can we describe CRs and their interactions? The first major
part of this thesis investigates the question of the dynamic
influence of CRs on the intra-cluster medium and searches for
unbiased tracers of their existence using multi-frequency
observational results. To this end, I develop an analytical
framework to describe the hadronic interactions of CR protons with
the ambient thermal plasma. In the second part, a description of CR
gas for cosmological applications is presented that is especially
suited for hydrodynamical simulations. During the course of this
work, I focus on developing a formalism for instantaneously
identifying and estimating the strength of structure formation
shocks during cosmological simulations to accelerate CRs through
diffusive shock acceleration. Since the energetically dominant CR
population is trapped by cluster magnetic fields, it can only be
observed indirectly through non-thermal radiative processes. CR
protons interact hadronically with the ambient plasma and produce
mainly neutral and charged pions that successively decay into
gamma-rays, secondary electrons, and neutrinos. I develop an
analytic formalism which describes the induced radio synchrotron,
inverse Compton, and gamma-ray emission. Comparing the expected
gamma-ray flux to the upper limits obtained by the gamma-ray
observatory EGRET, I am able to constrain the CR proton energy
density in nearby cooling core clusters to < 20% relative to the
thermal energy density. In this context, I study the hypothesis
that the diffuse radio synchrotron emission of galaxy clusters is
produced by hadronically originating relativistic electrons and I
develop a non-parametric criterion to obtain the minimum energy
state for an observed radio synchrotron emission: the excellent
agreement between the observed and theoretically expected radio
surface brightness profile of the Perseus mini-halo and the small
amount of energy density in CR protons needed to account for the
observed radio emission makes this hadronic model an attractive
explanation of radio mini-halos found in cooling core clusters. To
explain the giant radio halo of Coma within the hadronic model of
secondary electrons, the CR proton-to-thermal energy density
profile has to increase radially up to moderate CR energy
densities. Cosmological simulations that self-consistently follow
CR acceleration at shock waves predict such an energy density
profile: strong shock waves, that occur predominantly in low
density regions, are able to efficiently accelerate high-energetic
CRs, whereas weak central flow shocks inject only a low-energetic
CR population which is strongly diminished by Coulomb interactions.
This implies that the dynamic importance of the shock-injected CR
energy density is largest in the low-density halo infall regions,
but is less important for the weaker shocks occurring in central
high-density cluster regions. As an extension of this work, I
propose a new method in order to elucidate the content of the radio
plasma bubbles located at cool cores of galaxy clusters. Using the
Sunyaev-Zel'dovich (SZ) effect, the Atacama Large Millimeter Array
and the Green Bank Telescope should be able to infer the
dynamically dominant CR component of the plasma bubbles in suitable
galaxy clusters within short observation times. Future
high-sensitivity multi-frequency SZ observations will be able to
infer the energy spectrum of the dynamically dominant electron
population. This knowledge can yield indirect indications for an
underlying composition of relativistic outflows of radio galaxies
because plasma bubbles represent the relic fluid of jets. In the
second major part of my thesis, I address the problem of
constructing an accurate and self-consistent model for the
description of CRs that aims at studying the dynamic influence of
CRs on structure formation and galaxy evolution. This will not only
allow the production of realistic non-thermal emission signatures
of galaxies and clusters of galaxies, but also allow in-vivo
studies of dynamic effects driven by relativistic particles and the
star formation history. The developed model self-consistently
traces relativistic protons originating from various kinds of
sources, such as structure formation shock waves and supernovae
driven galactic winds, and also accounts for dissipative processes
in the relativistic gas component. To this end, I develop a
formalism for the identification and accurate estimation of the
strength of structure formation shocks during cosmological smoothed
particle hydrodynamics simulations. Shocks not only play a decisive
role for the thermalization of gas in virializing structures but
also for the acceleration of CRs through diffusive shock
acceleration. The formalism is applicable both to ordinary
non-relativistic thermal gas and to plasmas composed of CRs and
thermal gas. I apply these methods to studying the properties of
structure formation shocks in high-resolution hydrodynamic
simulations of the LambdaCDM model and find that most of the energy
is dissipated in weak internal shocks which are predominantly
central flow shocks or merger shock waves traversing halo centers.
Collapsed cosmological structures are surrounded by external shocks
with a much higher Mach number, but they play only a minor role in
the energy balance of thermalization. I show that after the epoch
of cosmic reionization, the Mach number distribution is
significantly modified by an efficient suppression of strong
external shock waves due to the associated increase of the sound
speed of the diffuse gas.