Introductory Remarks

Our aim in the talks given at the XXV Canary Islands Winter School of Astrophysics in 2013 was to present models for the generation of primordial magnetic fields in the early Universe and the imprints that these leave in cosmological observables, with an emphasis on Cosmic Microwave Background (CMB) anisotropies.

Introduction

The origin of the large-scale magnetic fields (LSMF) observed in galaxies, clusters of galaxies, with also hints of their presence in cosmic voids and on even larger scales in filaments, is an open issue of great importance in modern astrophysics (seeWidrow, 2002, for reviews). Both large-scale and stochastic components are present in magnetic fields observed in galaxies (usually dependent on the morphology of the host) with magnitudes from a few to several microGauss (μG). In clusters of galaxies, stochastic magnetic fields from a few to several μG strength are present with a correlation scale of the order of magnitude of ten kiloparsec (Clarke et al., 2001; Enslin & Vogt, 2006). More recently, the presence of intergalactic magnetic fields even in cosmic voids of the large-scale structure was proposed as a possible explanation for the gamma-ray observations of a couple of blazars (Neronov & Vovk, 2010; Taylor et al., 2011; Vovk et al., 2012; Tavecchio et al., 2010, 2011; Dolag et al., 2011). As a possible explanation for the lack of TeV photons observed in the high-energy spectrum from these blazars a lower bound of 10−18−10−15 G was derived for such pervasive intergalactic magnetic fields.

In light of this multitude of observations of LSMF of different magnitudes and different coherence lengths, a primordial hypothesis for generating the seed magnetic fields, which are amplified afterwards by adiabatic compression and dynamo during structure formation, is a viable possibility (Widrow, 2002), also taking in consideration the recent observations of strong magnetic fields in galaxies at high redshift (Bernet et al., 2008; Wolfe et al., 2008). Recently, initial seeds motivated as primordial magnetic fields (PMF) have been used in N-body simulations to reproduce magnetic fields in clusters of galaxies (Govoni et al., 2013; Xu et al., 2012).

Abstract

Active galactic nuclei (AGNs), the luminous, compact core regions of galaxies where accretion occurs onto supermassive black holes, can dramatically influence their entire host galaxy evolution by a process referred to as AGN feedback. Energy feedback to the galaxy is the result of combined radiation fields and directed outflows, and especially radio-loud active galaxies show pronounced jets and lobes. Their synchrotron radio emission indicates that dynamically important magnetic fields are at play in AGN jet collimation, stability, energy transfer to the intergalactic medium and their overall morphological appearance. Current knowledge on the launching mechanisms for such highly energetic relativistic jets, as well as the near black-hole accretion processes themselves, all invoke magnetic fields as active agents in angular momentum, mass and energy redistributions. In this review, we cover aspects of AGN feedback and the role played by magnetic fields, almost necessarily studied at vastly different length and timescales. We emphasize how typical large-scale galaxy interaction studies rely on parametric prescriptions for feedback, while detailed dedicated studies for near black-hole dynamics and relativistic jet propagation exist which take full account of magnetic field influences. We discuss representative hydro to magnetohydrodynamic (MHD) numerical simulations that exploit analogies with less energetic X-ray binary sources or even protostellar accretion-ejection systems, emphasize relativistic MHD descriptions, and point out that magnetic fields in accretion disks yield many linear instability routes to turbulence that have scarcely been recognized in the astrophysical community. In combination, they serve to show that magnetic field influences in AGN accretion, jet launch, energy feedback, and overall evolution are still far from completely understood, although many aspects have been disclosed by advanced analytical and numerical relativistic MHD studies.

Motivation: Astrophysical Jets

Radio galaxies confront us with dramatic views on energy redistributions at all scales, as mediated by central massive black holes lurking in their nucleus. A clear example is provided by the elliptical galaxy NGC5532, a nearby (red shift z = 0.0237, type S0) galaxy where the stellar distribution is in sharp contrast with its double-jetted appearance in radio images.

To calculate quantum state densities when a plasma is not in thermodynamic equilibrium, it is necessary to examine the individual processes populating and de-populating the quantum states. We have already considered the radiative processes (spontaneous radiative decay, photo-excitation and stimulated emission in Section 4.2 and free-bound radiative recombination in Section 5.4). Typical radiative reactions involved are listed in Table 11.1.

Other processes which need to be taken into account to evaluate quantum state population densities are collision induced. In colliding with an ion, electrons (and to a lesser extent other ions) can cause transitions between quantum states and between discrete quantum states and free electrons. A list of collisional reactions affecting quantum state populations is given in Table 11.2. Models calculating plasma quantum state densities and consequent radiation emission and absorption properties using rates of radiative and collisional processes are known as collisional radiative models [89].

Collisions in Plasmas

When the electrons, ions and atoms in a plasma undergo collisions they share energy and momentum so that a thermal distribution (Equation 1.22) characterised by a temperature and number density is produced. We considered the idea of a collision of an electron with an ion in Section 5.2 where the acceleration of the electron produces bremsstrahlung emission. Only a small loss of energy to radiation occurs with each collision.

In collisions between similar mass particles such as electron–electron and ion– ion collisions, the charged particle momentum and energy can be readily exchanged between the colliding particles. Collision between electrons and ions also transfer energy, but at a rate which is slower by the relative mass of the ions and electrons. Where the kinetic energy of colliding particles is conserved (apart from the small bremsstrahlung emission), the collision is referred to as being elastic.

Inelastic collisions involve changes in the bound quantum state of an ion or atom as a result of the collision. Energy is transferred from an electron kinetic energy to the potential energy of a bound electron (or vice versa).

An important measure with all collisions is the value of the cross-section for the collision.

The opacity of a medium is its impenetrability to radiation. For electromagnetic radiation in plasmas, the opacity arises due to absorption by free and bound electrons or due to scattering (where some radiation is re-emitted). We discussed scattering in Chapter 3, absorption of radiation in Chapter 4 and the particular physics of absorption by free electrons in Chapter 5.

Radiation opacity in plasma is important at higher densities or in larger plasmas. However, even low-density plasma may have some frequencies of radiation, say in the microwave or radiofrequency spectral range where radiation absorption is significant. Plasma opacity can limit the ability to diagnose conditions within the interior of a plasma and plasma opacity can slow the outflow of energy within a plasma and lead to non-local radiation heating.

Radiation transport calculations use a measure of dimensionless distance known as the optical depth. The optical depth of a thickness of material is the natural logarithm of the ratio of the incident to transmitted radiation power through the material. In the absence of any source of radiation, the radiation power falls exponentially with optical depth. When the optical depth approaches zero, a medium is said to be optically thin for the specified radiation and the opacity can be neglected. Large values of optical depth #x226B; 1 for a plasma or other medium suggest that opacity and radiation transport is significant: a condition known as ‘optically thick’.

We explore in this chapter the issue of opacity and the movement of radiation energy in plasmas. After some general treatment of opacity, we consider plasmas which are in, or close to, thermal equilibrium, so that the radiation field can be described by the Planck black-body distribution. Opacity modeling and radiation transport in plasmas close to thermal equilibrium is important in solar and stellar radiation modelling and in laser-produced plasmas.

The Equation of Radiative Transfer

The radiation intensity within a plasma exchanges energy with the bound and free electrons by emission and absorption processes. Electron transitions between bound discrete quantum states associated with the energy states of an ion or atom produce spectral-line emission, while transitions from free electrons to other free quantum states or to bound quantum states produce continuum emission.