THEORETICAL RESEARCH

Involved IRA Scientists and Collaborators: G. Brunetti, R. Cassano, G. Setti,  F. Vazza.

The theoretical group at IRA mainly focuses on models that try to understand the origin and the evolution of extragalactic radio sources at different scales, from radio galaxies and quasars to radio halos and relics in clusters of galaxies. Major research topics are:

- Emission Proccesses in Astrophysics
- Cosmic Ray Physics
- Theory of Particle Acceleration in Astrophysics
- Extragalactic Radio Sources
- Plasma Physics
- Non-Thermal Emissions from Galaxy Clusters
- Cosmological Numerical Simulations

X-RAY EMISSION AND ENERGETICS OF RADIOGALAXIES

Radio galaxies and quasars are extraordinary laboratories to study the acceleration of relativistic particles and the physics of the plasma. Synchrotron emission in the radio band gives indirect information on the mixing of relativistic particles and magnetic fields, while Inverse Compton (IC) measurements in the X-ray band can provide direct information on the energy and spectrum of the relativistic particles. X-ray emission comes from two main mechanisms: the electron scattering with the CMB photons (e.g. Harris et al. 1979) and the  scattering between nuclear electrons and photons (Brunetti et al. 1997; Brunetti 2000).

The latter mechanism is interesting because the X-ray emission is due to electrons with a Lorentz factor of 100-300, which contain the bulk of the leptonic energies from the radio lobes. The study of IC emission is particularly useful to measure the energetics of the lobes and magnetic field because the IC properties depend only on the number density of the emitting electrons and thus allow us to break the so called ``synchrotron degeneracy''. In recent years several studies have been performed using combined radio and X-ray data. The group is also involved in the modelling of the broad band emission from compact features in radio galaxies (e.g., hot spots) which are the sites of particle acceleration in these sources (see Hot Spots in Radio Galaxies in the research section of Extragalactic Radiosources). To find out more see:  Comastri et al. 2003; Migliori et al. 2007.

ORIGIN OF RADIO HALOS IN GALAXY CLUSTERS

Radio halos originate from synchrotron emission of relativistic electrons and positrons (with energy E > a few GeV) which interact with the cluster magnetic field (see observational results in Observational Studies of the Properties of Radio Halos in the research Section - Clusters of Galaxies. The origin of these particles is rather complex and still far from being understood. A promising model today is the “re-acceleration'' model, originally proposed by the IRA group (Brunetti et al. 2001) and later developed in many other papers. The model predicts that the turbulence, originated and developed in the Intra-Cluster-Medium during cluster mergers, accelerates relativistic particles via second order Fermi processes. However, since these processes are not very efficient in accelerating thermal particles, one hypothesis requires the presence of a reservoir of super-thermal or relativistic particles in the ICM (e.g., from cosmic rays sources in galaxy clusters, such as AGN, galaxies or proton-proton interactions).

The interaction between turbulence and particles, which ultimately causes the final acceleration of the particles themselves, is a very complex process. The understanding of the physics of such interactions is important because they are relevant not only to galaxy clusters, but to many other astrophysical environments as well (e.g. the Sun, ISM).   The IRA theoretical group has developed a formalism capable of modeling in a self-consistent way the non-linear interaction between Alfven modes, relativistic protons, primary and secondary electrons and positrons (Brunetti et al. 2004,  Brunetti et al. 2005).
The formalism, applied to galaxy clusters, allowed us to understand for   the first time the radio halo formation when appropriate physical conditions exist. A similar study has recently been carried out even in the case of magnetosonic waves, considering not only the non-linear interactions between the waves and particles, but also the physics of such waves in the typical conditions of the ICM (Brunetti et al. 2007).

STATISTICAL PROPERTIES AND COSMOLOGICAL EVOLUTION OF NON-THERMAL PROCESSES IN GALAXY CLUSTERS

Observational data suggest a connection between non-thermal phenomena in galaxy clusters (such as radio halos, relics, hard X-ray tails) and the assembly of the clusters themselves (see cluster observational studies in the research Section - Clusters of Galaxies). Indeed, there is evidence that a fraction of the energy dissipated during cluster formation can be conveyed and used into amplification of magnetic fields and acceleration of non-thermal particles. Models which follow both the formation of clusters and the acceleration of particles have allowed us to better understand most of the statistical properties of  radio halos (Cassano et al. 2005; Cassano et al. 2006) and to predict and test new scaling relations present in these sources (see Figure 20a and Figure 20b from Cassano et  al. 2007).

Fig 20a - Expectation for the synchrotron and Inverse Compton emissions from a Coma-like galaxy cluster, according to the magneto-sonic reacceleration mechanism and for various epochs (from Cassano et al. 2005).

Fig 20b - Expectation for the radio-halo luminosity function for various assumed configurations of the magnetic fields within a cluster, compared with an estimate from Ensslin et al. (2002)  (from Cassano et al. 2006).

The relevance of these models rests on the fact that it will be possible to predict in some details the properties of synchrotron and IC emission from galaxy clusters at different cosmological epochs. Such properties will be tested by future new instruments in the radio band such as LOFAR, LWA and SKA, or Simbol-X and GLAST in the X-ray and gamma band, respectively.

COSMOLOGICAL NUMERICAL SIMULATIONS

Simulations are a unique tool to follow the formation and evolution of cosmic structure in the non-linear regime and to study in detail the heating and mixing processes in the interiors of galaxy clusters. During a merger event a fraction of the accreted energy can be dissipated driving large-scale turbulent motions. We made use of Lagrangian cosmological simulations of the SPH (Smoothed Particle Hydrodynamics) fashion (Gadget-2) with a reduced viscosity to study this complex process (Dolag et al. 2005; Vazza et al. 2006).
The most interesting research topic for the group is the study of cosmological shocks associated with the formation of large-scale structures. Shocks are very important since they are responsible for the thermalization of baryonic matter within clusters and they also are thought to be the main source of cosmic rays particles within virialized structures.  Making use of Eulerian cosmological simulations with the ENZO code (developed for cosmological simulations of the early Universe; see http://lca.ucsd.edu/portal/software/enzo) we are now studying the statistical properties of cosmological shocks, and their dynamics during clusters assembly; about 15% of the volume of the Universe is filled by these shocks which are particularly prominent in the external regions of the galaxy clusters and in the filaments where most of the cosmic rays acceleration takes place (see Figure 21) while only a small number of shocks (1/1000) takes place within the virial radius of clusters (see Figure 22).

Fig 21 - A simulated datacube of 80 Mpc on a side, showing: projected X-ray luminosity (left), thermalized kinetic energy through shocks (center) and shock strength (right) - Courtesy of F. Vazza.

Fig 22 - Evolution of accretion shocks around a massive galaxy cluster, from z=1.0 (upper left corner) to z=0.0 (bottom center) -  Courtesy of F. Vazza.