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Magnetic fields are found everywhere in the cosmic Universe. They have been known to exist on the Earth for centuries. They were detected in the Sun, measured in situ in the planets, observed in stars, galaxies and clusters of galaxies. Magnetic fields are responsible for many observed effects like radio continuum emission, jets, beamed emission in pulsars, star formation, etc. Possibly the whole universe is intimately governed by the presence of magnetic fields. This review will summarize the observational results and discuss the present interpretation for the origin of the magnetic fields.

Continuum observations of the edge-on spiral galaxy NGC 4945 with the Australia Telescope National Facility's Compact Array (ATCA) and the Parkes 64-m radio telescope are presented. The ATCA maps are currently the highest resolution wide-band radio continuum maps of this galaxy. A compact, non-thermal core of less than 60 pc diameter has been detected. The luminosity of the central 90 pc is 2 × 10 erg s, equivalent to almost half the total value of the galaxy. 90 per cent of the galaxy's λ = 3 cm radio emission originates from the nuclear bulge (central 4 kpc), the radio disc measuring 23 ± 2 kpc in diameter. The nuclear bulge of the galaxy has a spectral index of a = - 0.57 ± 0.04 (S ∝ v), similar to the core value. The disc emission of the galaxy has a much steeper spectral index of α= -1.1 ± 0.1, indicative of either a high-energy cut-off to the disc population of cosmic rays or differing source energetics in the disc. The large-scale radio morphology resembles that of the majority of edge-on spirals in that no radio halo has been detected. The average total equipartition magnetic field strength of the disc is 13 μG, whilst our polarization results indicate that the regular component of the magnetic field is about 3 μG. We conclude that NGC 4945 is one of a number of edge-on spiral galaxies with a magnetic field (and hence cosmic rays) confined to the disc.

Magnetic fields are present in every corner of the Universe. The Earth, the Sun and most of the planets are known to possess dipolar magnetic fields. In the Galaxy many individual objects like stars, pulsars, bipolar nebulae and supernova remnants are found to have associated magnetic fields. It seems that the rotation plays a significant role in the ability of a cosmic object to develop a magnetic field. The magnetic field of the Galaxy is observed to be oriented along the galactic plane as evidenced by both optical and radio polarization observations. Radio maps of the Galactic centre reveal poloidal magnetic fields as ‘wisps’ or ‘strings’ around Sagittarius A. Observations of nearby galaxies give us remarkable information about the large-scale magnetic fields in these building blocks of the Universe. Magnetic fields play an important role in the formation of jets of radio galaxies. Further out, in clusters of galaxies, definitive evidence has been given for the existence of intergalactic magnetic fields.

The late-type edge-on spiral galaxy NGC 4631 is known for its high star formation rate and extended radio halo with a uniform magnetic field component ordered predominantly perpendicular to the plane of the galaxy in the inner 6 kpc (assuming a distance of 7.5 Mpc, 1′ = 2.2 kpc) (Hummel et al. 1988; Golla, Ph.D., in preparation). The strongest radio continuum source of NGC 4631 is located at the eastern edge of the central region near a giant HII region complex CM67 (Crillon and Monnet 1969). The magnetic field orientation going out from the central region and especially from the region CM67 as well as the prominent north eastern low frequency radio spur (cf. Hummel et al. 1991) indicate a close connection between the synchrotron emission of the radio halo of NGC 4631 and star forming regions/CM67 in the disk (Golla and Hummel, in preparation). Probably cosmic rays from star forming regions in the disk propagate along the magnetic field Unes into the halo.

Cosmic magnetic fields have now been studied for over 100 years. Although the initial detection was done at optical wavelengths, it was radio astronomy that gave us the bulk of information on cosmic magnetic fields in the recent years. In this review the observational results obtained by radio astronomy methods will be discussed. In particular maps of Galactic magnetic fields as well as magnetic fields in nearby galaxies will be presented. A discussion of possible developments in the near future will conclude this review.

I present the results of recent 9 mm observations of the starburst galaxy M 82 with the Effelsberg 100-m radio telescope. The magnetic fields deduced from these observations are in good agreement with previous multifrequency VLA observations. The general pattern of azimuthal magnetic fields along the disc of the galaxy and vertical fields in the central region, as seen in most nearby galaxies, is confirmed to exist also in M 82. Since the starburst galaxy M 82 is a very powerful radio source similar objects should be visible at large distances. Possibly the distant magnetic Universe could be probed by observing sources similar to M 82 with the new generation of radio telescopes. Here the ALMA telescope holds great promise for the near future. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Theoretical arguments provided the first definitive prediction for the existence of an interstellar magnetic field in our Galaxy: There was no other way to explain the observed isotropy of the cosmic radiation. Alfven, Richtmeyer and Teller (1949), and Fermi (1949) stated that the particles of the cosmic radiation have to be reflected often and this can hardly be explained other than by an interaction with a magnetic field.

Ringlike structures (in H α, CO and radio continuum) in central galactic regions have been observed in a number of objects. They seem to be a basic feature of galactic structure (for references see Dickman et al., 1988). These observations can be summarized as follows: The typical mass of these rings is 108 – 109 M ⊙. The gas distribution is smoothly decreasing from the ring into the innermost galactic regions. The rings appear always at the turnover radius where the galactic rotation turns from rigid to differential rotation.

High-resolution, high-sensitivity observations of the CO(J=2-1) lines in the galaxy M82 have revealed that the rotation of the gas disk is Keplerian, which suggests that the galaxy has no massive halo. The extended molecular gas halo, extending up to ±2 kpc above the galactic plane, shows very slow rotation and indicates that the halo gas has been ejected from the central region in angular momentum conservation

Radio continuum surveys give us the fundamental information about the distribution of the radio intensity across the sky. The radio waves originate in three fundamental emitting processes and are measured superposed in the antenna beam. At the lowest radio frequencies (below 10 GHz) the nonthermal emission process (synchrotron radiation) predominates. This radio emission comes from supernova remnants, from the “galactic background” (relativistic electrons diffusing in galactic magnetic fields), but also from extragalactic objects (galaxies, quasars, radio galaxies, etc.). In the frequency range 10 GHz < f < 300 GHz the thermal (free-free) process becomes dominant. In the Galaxy we see the HII regions along the galactic plane. In nearby galaxies we can identify the thermal emission in the disks. Above 300 GHz the dust is seen. This is the boundary region between radio and far-infrared radiation.

Seven new pulsars are reported from the Molonglo radio observatory. Some conclusions can be drawn about the distribution of pulsars in the galaxy.

The existence of ‘haloes’ in clusters of galaxies was deduced by Ryle and Windram (1968) for the Perseus cluster and by Willson (1970) for the Coma cluster at 408 MHz by comparing total flux measured by a single dish with the sum of fluxes of radio sources found in the field. A direct measurement of the extended source Coma C was made by Jaffe et al. (1976) at 610 MHz. The failure to detect the halo of Coma at higher frequencies is attributed by all authors to a steep spectrum of this extended component.

Publisher Summary This chapter discusses the basic concepts related to antenna calibration. Direct antenna calibrations of radio telescopes—such as the measurement of the polar pattern on a turntable—are usually not possible. Present-day radio telescopes are too large for these methods, which are generally employed in the testing of antennas for radio communication purposes. To achieve meaningful calibration by scaling, both a far-field measurement of the polar pattern of the model and near-field probing of the aperture field in the model and of the full antenna need to be combined. Consequently, very specialized and intricate measurements must be made before a successful antenna calibration is available. The criteria for the selection of calibration sources are described in the chapter, and lists of calibrators, both for positional and for flux scale calibration, are given. With a view of the discrepancies between observatories at frequencies of 408 MHz and below, discussion of present-day accuracy of the flux scale is undertaken. The techniques of calibration are described, as used in the tests of the 100-m fully steerable, paraboloidal radio telescope at Effelsberg. A description of the aims of a complete pointing theory is also presented.

Total-power and polarization observations of NGC 253 at 10.7 GHz have been performed with the 100-m MPIfR radio telescope. The observed arm/interarm polarization contrasts are discussed in the context of possible field configurations in spiral arms.

A NEW series of observations of the galactic radio emission has been conducted at the frequencies of 20, 30 and 85 Mc/s. These low-resolution observations made with scaled aerials were intended to investigate the spectral index of the galactic radio emission in the frequency region 10–100 Mc/s.

High-resolution surveys of the galactic radio emission show detailed structure both near the galactic plane and at high galactic latitudes. Some of this structure is describable as ‘spurs’ while the rest comprises isolated features. We describe here measurements of the spectral index of the background radiation with a resolution of 3°.5. These measurements are aimed at a detailed understanding of the high-latitude synchrotron emission.

FOUR further pulsars have been discovered at the Molonglo radio observatory during a pulsar search at 408 MHz. One of the four pulsars, MP 0254, a high galactic latitude pulsar, was discovered in November 1968, but failed to appear on any of some twenty subsequent transits. In a recent observing session this pulsar was observed twice, allowing measurement of the dispersion and pulse width. Two of the pulsars (MP 1706 and MP 1911) were among many suspected pulsars noted during a close re-examination of the search records made since October 1968 using the two beam technique which led to the discovery of eighteen pulsars1–4. Recent observations have confirmed the existence of MP 1706 and MP 1911, and have enabled their properties to be measured. The fourth pulsar, MP 1154, was discovered during a search for high dispersion pulsars along the galactic plane using a two channel dispersion remover which was described in connexion with the discovery of MP 1240 (ref. 3).

WE recently described1 the discovery of seven pulsars found in a search at the Molonglo Radio Observatory using the 408 MHz radio telescope. A conclusion reached on the basis of this search was that pulsars are galactic objects associated with the disk or the spiral arms, but with a somewhat anomalous distribution. A zone of avoidance along the inner regions of the galactic plane was suggested, based on the argument that dispersion in the thin layer of an ionized medium closely confined to the galactic plane results in the loss of the distant pulsars. This communication describes the discovery of further pulsars which follow the same general pattern of galactic distribution.

PULSAR surveys carried out at the Molonglo Radio Observatory during February and March, 1969, have resulted in the discovery of three new pulsars. They have been designated MP 0450, MP 1642 and MP 1818. The last two were found in areas of the sky which had not previously been searched for pulsars using the Molonglo radio telescope. The position of MP 0450, however, had been searched twice before during the pulsar survey programme, but the pulsar was not detected presumably because of long term variations of its pulse amplitude.

In the framework of the diffusive shock acceleration of relativistic electrons in extragalactic jets, we show that it is possible to derive the speed of the jet. For this purpose, we transform continuity relations through an oblique shock front into the reference frame of the observer. We apply these calculations to knot A of the M87 jet. Measuring the deviation of the fluid through the shock front from high-resolution radio maps and the deviation of the magnetic field from optical polarization maps (Fraix-Burnet et al., 1989), we derive speeds of about 0.01 c (Fraix-Burnet and Biermann, in preparation). The compression ratio is most probably 4 and the magnetic field is nearly parallel to the shock front both upstream and downstream.