Origin of Galactic and Extragalactic Magnetic Fields(4)

A variety of observations suggest that magnetic fields are present in all galaxies and galaxy clusters. These fields are characterized by a modest strength (10^{-7}-10^{-5} G) and huge spatial scale (~Mpc). It is generally assumed that magnetic fields in s

erved values — p = 0.1 0.2 for the typical spiral — are much smaller. There are various e ects which can lead to the depolarization of the synchrotron emission observed in spiral galaxies. These e ects include the presence of a uctuating component to the magnetic eld, inhomogeneities in the magneto-ionic medium and relativistic electron density, Faraday depolarization (see below) and beam-smearing (see, for example, Sokolo et al. 1998). Heuristic arguments by Burn (1966) suggest that for the rst of these e ects, the polarization degree is reduced by a factor equal to the ratio of the energy density of the regular eld B to the energy density of the total eld: B p = pH 2 . B2(27)(This expression is useful only in a statistical sense since one does not know a priori the direction of the regular eld.) Thus, perhaps only ～ 25% of the total magnetic eld energy in a typical spiral is associated with the large-scale component. Of course, the ratio B/B would be higher if other depolarization e ects were important.2. Faraday rotationElectromagnetic waves, propagating through a region of both magnetic eld and free electrons, experience Faraday rotation wherein left and right-circular polarization states travel with di erent phase velocities. For linearly polarized radiation, this results in a rotation with time (or equivalently path length) of the electric eld vector by an angle e3 λ2 2πm2 c4 els =ne (l)B (l)dl + 00(28)where me is the mass of the electron, λ is the wavelength of the radiation, 0 is the initial polarization angle, and B is the line-of-sight component of the magnetic eld. Here, ne (l) is the density of thermal electrons along the line of sight from the source (l = ls ) to the observer (l = 0). is usually written in terms of the rotation measure, RM:9 = (RM ) λ2 + 0 where e3 2πm2 c4 e rad m2ls(29)RM ≡ne (l)B (l)dl0 ls 0 810ne cm 3B µGdl kpc(30)In general, the polarization angle must be measured at three or more wavelengths in order to determine RM accurately and remove the ≡ ± nπ degeneracy. By convention, RM is positive (negative) for a magnetic eld directed toward (away from) the observer. The Faraday rotation angle includes contributions from all magnetized regions along the line of sight to the source. Following Kronberg & Perry (1982) we decompose RM into three basic components: RM = RMg + RMs + RMig (31)where RMg , RMs , and RMig are respectively the contributions to the rotation measure due to the Galaxy, the source itself, and the intergalactic medium. Faraday rotation from an extended source leads to a decrease in the polarization: The combined signal from waves originating in di erent regions of the source will experience di erent amounts of Faraday rotation thus leading to a spread in polarization directions. Faraday depolarization can, in fact, be a useful measure of magnetic eld in the fore

A variety of observations suggest that magnetic fields are present in all galaxies and galaxy clusters. These fields are characterized by a modest strength (10^{-7}-10^{-5} G) and huge spatial scale (~Mpc). It is generally assumed that magnetic fields in s

ground of a source of polarized synchrotron emission.3. Zeeman SplittingIn vacuum, the electronic energy levels of an atom are independent of the direction of its angular momentum vector. A magnetic eld lifts this degeneracy by picking out a particular direction in space. If the total angular momentum of an atom is J (= spin S plus orbital angular momentum L) there will be 2j + 1 levels where j is the quantum number associated with J. The splitting between neighboring levels is E = gµB where g is the Lande factor which relates the angular momentum of an atom to its magnetic moment and µ = e¯ /2me c = 9.3 × 10 21 erg G 1 is the h Bohr magneton. This e ect, known as Zeeman splitting, is of historical importance as it was used by Hale (1908) to discover magnetic elds in sunspots, providing the rst known example of extraterrestrial magnetic elds. Zeeman splitting provides the most direct method available for observing astrophysical magnetic elds. Once E is measured, B can be determined without additional assumptions. Moreover, Zeeman splitting is sensitive to the regular magnetic eld at the source. By contrast, synchrotron emission and Faraday rotation probe the line-of-sight magnetic eld. Unfortunately, the Zeeman e ect is extremely di cult to observe. The line shift associated with the energy splitting is ν = 1.4g ν B µG Hz ν.(32)For the two most common spectral lines in Zeeman-e ect observations — the 21 cm line for neutral hydrogen and the 18 cm OH line for molecular clouds — ν/ν 10 9 g (B/µG). A shift of this amplitude is to be compared with Doppler broadening, ν/ν vT /c 6 × 10 7 (T /100 K)1/2 where vT and T are the mean thermal velocity and temperature of the atoms respectively. Therefore Zeeman splitting is more aptly described as abnormal broadening, i.e., a change in shape of a thermally broadened line. Positive detections have been restricted to regions of low temperature and high magnetic eld. Within the Galaxy, Zeeman e ect measurements have provided information on the magnetic eld in star forming regions and near the Galactic center. Of particular interest are studies of Zeeman splitting in water and OH masers. Reid & Silverstein (1990), for example, used observations of 17 OH masers to map the large-scale magnetic eld of the Galaxy. Their results are consistent with those found in radio observations and, as they stress, provide in10 situ measurements of the magnetic eld as opposed to the integrated eld alo …… 此处隐藏：7459字，全部文档内容请下载后查看。喜欢就下载吧 ……