Extragalactic magnetic fields

Abstract

Recent advances in observational techniques reveal the widespread existence of magnetic fields in the Universe, and produce much firmer estimates of magnetic field strengths in interstellar and intergalactic space. Ordered, microgauss-level fields are common in spiral galaxy disks and halos, and appear to be a common property of the intra-cluster medium of clusters of galaces, indeed well beyond the cluster core regions. Strengths of ordered magnetic fields in the intracluster medium of cooling flow clusters exceed those which are typical of the interstellar medium of the Milky Way, suggesting that galaxy formation, and even 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 for the regeneration of initial seed fields in galaxies, including mechanisms of magnetic diffusivity and dissipation which influence field amplification, are increasingly better, though far from completely, understood. We review the 'conventional', slow mean field a-w dynamo theory for disk galaxies, and more recent modifications to the theory. Fast-acting dynamo mechanisms appear to operate in galaxies, galaxy inflow and outflow, and in cooling flow clusters. Better understanding of the magnetic properties of extragalactic radio jets, including recent 3~ numerical simulations, has shown how fast dynamo processes associated with the radio jetilobe combination can effec!ivity magnetize large volumes of inlergalactic space. Such processes, and starbust-driven outflow during the galaxy formation epoch, could have produced the microgauss level fields now commonly seen in galaxy systemswhich would obviate the need for slow acting dynamos to build up field strength slowly over cosmic time.The observational methods for detecting and measuring extragalactic magnetic fields are discussed, along with some new indirect methods which could be used for inferring field strengths at large redshifts which are otherwise beyond the reach of direct measurement.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 generation 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 galaxies formed, up to now, largely a 'dark' era.

Primordial large-scale electromagnetic fields from gravitoelectromagnetic inflation

“…The existence, strength and structure of these fields in the intergalactic plane, within the Local Superclusted, has been scrutinized recently [2]. Many spiral galaxies are endowed with coherent magnetic fields of µG (micro Gauss) strength [3,4,5,6,7,8], having approximately the same energy density as the Cosmic Microwave Background Radiation (CMBR). In particular, the field strength of our galaxy is B ≃ 3 × 10 −6 G, similar to that detected in high redshift galaxies [9] and damped Lyman alpha clouds [10].…”

On the effect of cyclotron emission on the spectral distortions of the cosmic microwave background

“…The average field strength is found to be ∼ 3−4 µG. Such a strength corresponds to an approximate energy equipartition between the magnetic field, the cosmic rays confined in the Galaxy, and the small-scale turbulent motion (Kronberg, 1994) …”

Prospect on intergalactic magnetic field measurements with gamma-ray instruments

“…For variations of the IGMF and of the density on scales much smaller than c/H 0 , the variance of the rotation measures should increase with the redshift and be detectable for B I G ∼ 6 × 10 −9 G (Kronberg & Perry (1982), Blasi et al (1999)), which is not observed. A strict upper limit of B I G < 10 −9 G can be deduced in such a case (Kronberg (1994)). However such limits need to be revisited with a more accurate description of the large scale structure and taking into account evolutionary effects in intrinsic rotation measures (Kronberg et al (2008)).…”

“…In this regard, an astrophysical origin of the IGMF appears as an interesting alternative. A substantial IGMF could be formed later on by ejection of magnetized plasmas into the intergalactic space, from galaxies, AGN, starbursts, Pop III stars and large scale shocks, as discussed for instance by Kronberg (1994), Widrow et al (2012), Ryu et al (2012) and Lilly (2012). For recent overviews on the primordial magnetic field and the IGMF, see for instance reviews by Widrow (2002), Kulsrud & Zweibel (2008), Kandus et al (2011 and Ryu et al (2012).…”

“…Magnetic fields up to a few tens of μG have been detected in cooling flows like in the Hydra cluster (Kronberg (1994)). Recent promising scenarios for the generation of such 10 μG-field consider galactic mergers driving cluster-scale dynamos.…”