1.2. The extent and implications of extragalactic magnetic fields
Observations over the last ten years have produced magnetic field detections not only in galaxy disks, but also in galaxy halos, clusters of galaxies, and in some very distant galaxy systems which produce both absorption lines and Faraday rotation of the radiation from background quasars. Generally, the more we look for extragalactic magnetic fields, the more ubiquitous we find them to be.
It is important to understand how galaxy formation and evolution is influenced by the existence of magnetic fields, whose presence has been so far largely ignored in the modeling of these processes. In particular, we would like to know how magnetic fields have been amplified over cosmic time to the µG levels observed in present-epoch galaxy disks. Was the original weak seed field a product of the Big Bang, or were the first magnetic fields created in stars by Biermann's `battery' process (Biermann 1950), and subsequently ejected into the early intergalactic medium? To provide some answer to this question, we need to estimate intergalactic field strengths both at the present, and earlier cosmological epochs.
Studies of the Sun's magnetic field have led to a good deal of understanding of the amplification, or regeneration, of magnetic fields, in particular the dynamo mechanism (Parker 1955, Steenbeck and Krause 1969). The MHD dynamo proposed for galaxy disks is driven by two fundamental components, one due to non-uniform rotation, called the -effect, and the second due to cyclonic motions, called the -effect. Other important phenomena are diffusivity, caused by resistive dissipation, and/or magnetic reconnection. For the Sun, and for Sun-Earth interactions both of these latter are important. Certainly dynamo processes, and possibly reconnection appear to operate on galactic and intergalactic scales, and it is currently a major challenge to both observation and theory to elaborate and better understand how they function.
We would like to know if the intergalactic voids are permeated by a widespread magnetic field, and if there is any evidence for or against field strength evolution in galaxies since the time of galaxy formation. If there is a widespread intergalactic field, was it frozen into the baryonic IG gas? If so, the evolution of its strength over cosmic time should be derivable from the co-moving, uncondensed matter density, (z) over the observable redshift range. If magnetic fields were widespread since the early universe were they dynamically significant, and if so at what crucial periods of galaxy evolution? Or, restating the question: how have widespread magnetic fields affected the evolution of stars and galaxies since the early universe?
Some a priori statements of expectation can be made. First, intergalactic magnetic fields will constrain the heat conductivity of the gas. They also provide an additional component of pressure to the interstellar and intergalactic gas. They couple cosmic ray particles to the non-relativistic gas, and depending on density of magnetic energy relative to the bulk dynamical and thermal energy densities of the gas, magnetic fields could strongly constrain the dynamics of galaxy formation. Purely the effect of an (even weak) magnetic field on the conductivity of the interstellar gas will have important consequences for galactic evolution. From an observational standpoint, the magnetic field structure in disks and halos can serve as a tracer of the disk dynamics and, though less directly, of the past dynamics thanks to the high conductivity of interstellar gas and consequent long lifetime of the interstellar fields. The following sections review the state of our knowledge of these fields and some of their implications.