EXTRAGALACTIC MAGNETIC-FIELDS

Recent advances in observational techniques reveal the widespread exis tence of magnetic fields in the Universe, and produce much firmer esti mates of magnetic field strengths in interstellar and intergalactic sp ace. Ordered, microgauss-level fields are common in spiral galaxy disk s and halos, and appear to be a common property of the intra-cluster m edium of clusters of galaxies, indeed well beyond the cluster core reg ions. Strengths of ordered magnetic fields in the intracluster medium of cooling flow clusters exceed those which are typical of the interst ellar medium of the Milky Way, suggesting that galaxy formation, and e ven cluster dynamics are, at least in some circumstances influenced by magnetic forces, which also could possibly affect the global dynamics in areas of some galaxies, especially dwarf galaxies, which are rich in interstellar gas and cosmic rays. Physical processes responsible fo r the regeneration of initial seed fields in galaxies, including mecha nisms of magnetic diffusivity and dissipation which influence field am plification, are increasingly better, though far from completely, unde rstood. We review the 'conventional', slow mean field alpha-omega dyna mo theory for disk galaxies, and more recent modifications to the theo ry. Fast-acting dynamo mechanisms appear to operate in galaxies, galax y inflow and outflow, and in cooling flow clusters. Better understandi ng of the magnetic properties of extragalactic radio jets, including r ecent 3D numerical simulations, has shown how fast dynamo processes as sociated with the radio jet/lobe combination can effectivity magnetize large volumes of intergalactic space. Such processes, and starbust-dr iven outflow during the galaxy formation epoch, could have produced th e microgauss level fields now commonly seen in galaxy systems-which wo uld obviate the need for slow acting dynamos to build up field strengt h slowly over cosmic time. The observational methods for detecting and measuring extragalactic magnetic fields are discussed, along with som e new indirect methods which could be used for inferring field strengt hs at large redshifts which are otherwise beyond the reach of direct m easurement. Seed fields can be produced in battery-like processes in a variety of systems (stars, supernovae, and supernova winds), and are expelled into intergalactic space. Various cosmological seed field gen eration mechanisms are reviewed, which could generate seed fields for the subsequently formed galaxies. The question of whether the original seed fields were produced in galaxies, or the pre-recombination early Universe must await a clearer picture of how the first stars and gala xies formed, up to now, largely a 'dark' era.