Milestones in the Observations of Cosmic Magnetic Fields

Abstract

Magnetic fields are observed everywhere in the universe. In this review, we concentrate on the observational aspects of the magnetic fields of Galactic and extragalactic objects. Readers can follow the milestones in the observations of cosmic magnetic fields obtained from the most important tracers of magnetic fields, namely, the star-light polarization, the Zeeman effect, the rotation measures (RMs, hereafter) of extragalactic radio sources, the pulsar RMs, radio polarization observations, as well as the newly implemented sub-mm and mm polarization capabilities. The magnetic field of the Galaxy was first discovered in 1949 by optical polarization observations. The local magnetic fields within one or two kpc have been well delineated by starlight polarization data. The polarization observations of diffuse Galactic radio background emission in 1962 confirmed unequivocally the existence of a Galactic magnetic field. The bulk of the present information about the magnetic fields in the Galaxy comes from analysis of rotation measures of extragalactic radio sources and pulsars, which can be used to construct the 3-D magnetic field structure in the Galactic halo and Galactic disk. Radio synchrotron spurs in the Galactic center show a poloidal field, and the polarization mapping of dust emission and Zeeman observation in the central molecular zone reveal a toroidal magnetic field parallel to the Galactic plane. For nearby galaxies, both optical polarization and multifrequency radio polarization data clearly show the large-scale magnetic field following the spiral arms or dust lanes. For more distant objects, radio polarization is the only approach available to show the magnetic fields in the jets or lobes of radio galaxies or quasars. Clusters of galaxies also contain widely distributed magnetic fields, which are reflected by radio halos or the RM distribution of background objects. The intergalactic space could have been magnetized by outflows or galactic superwinds even in the early universe. The Zeeman effect and polarization of sub-mm and mm emission can be used for the study of magnetic fields in some Galactic molecular clouds but it is observed only at high intensity. Both approaches together can clearly show the role that magnetic fields play in star formation and cloud structure, which in principle would be analogous to galaxy formation from protogalactic clouds. The origin of the cosmic magnetic fields is an active field of research. A primordial magnetic field has not been as yet directly detected, but its ⋆

Magnetohydrodynamic flows as Newtonian-type gravitational motions

“…3 In the next section we will expressρ in terms of the quantities of the original magnetised fluid. 3 It should be noted that the virtual fluid does not necessarily satisfy the continuity equation. In particular, ∂ ρ + ∂ α (ρ α ) = 0, unless further constraints are imposed on the magnetised medium.…”

“…Whether this happens or not depends on the specifics of the situation and on the strength of the B-field in particular. For example, the mean density within an area of 1 pc around the galactic centre is approximately 3×10 5 M /pc 3 . In that case the α-parameter exceeds unity provided the magnetic field is of the order of 1 G or stronger.…”

“…The widespread presence of magnetic fields in the universe has been repeatedly verified [1][2][3]. Despite this, however, the role and the implications of magnetism are very often bypassed.…”

Properties of the warm magnetized ISM, as inferred from WSRT polarimetric imaging

“…However, the magnetic field, and therefore the synchrotron emissivity of the thin disk, is far from uniform (e.g. Beck et al 1996;Han & Wielebinski 2002). Therefore, the smoothness of the synchrotron total intensity cannot be due to homogeneous synchrotron emission.…”

The mystery of cosmic magnetogenesis

“…The Milky Way and other spiral galaxies have coherent magnetic fields of μG-order magnitude, while galaxy clusters and remote protogalactic clouds also carry fields of similar strength (Han & Wielebinski 2002, Vallée 2011. Moreover, recently, there have been independent reports of relatively strong magnetic fields (around 10 −15 G) in intergalactic voids (Neronov 2010, Tavecchio et al 2010, Ando & Kusenko 2010.…”