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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.

We have observed the continuum emission of the interacting galaxy NGC4631 at 0.87 and 1.23mm using the Heinrich-Hertz-Telescope on Mt. Graham and the IRAM 30-m telescope on Pico Veleta. We have obtained fully sampled maps which cover the optical emission out to a radius of about 7' at both wavelengths. For a detailed analysis, we carefully subtracted the line contributions and synchrotron and free-free emission from the data, which added up to 6% at 1.23mm and 10% at 0.87mm. We combined the flux densities with FIR data to obtain dust spectra and calculate dust temperatures, absorption cross sections, and masses. Assuming a ``standard'' dust model, which consists of two populations of big grains at moderate and warm temperatures, we obtained temperatures of 18K and 50K for the both components. However, such a model suffers from an excess of the radiation at 1.23mm, and the dust absorption cross section seems to be enhanced by a factor 3 compared to previous results and theoretical expectations. At large galactocentric radii, where the galaxy shows disturbances as a result of gravitational interaction, this effect seems to be even stronger. Some possibilities to resolve these problems are discussed. The data could be explained by a very cold dust component at a temperature of 4-6K, an increased abundance of very small grains, or a component of grains with unusual optical properties. We favour the latter possibility, since the first two lead to inconsistencies., 12 pages, 6 figures, accepted for publication in Astronomy & Astrophysics. Updated version with minor errors corrected (typos, LaTeX formatting, missing citation)

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)

The Revised Shapley-Ames Catalog (Sandage & Tamman, 1981) contains 335 spiral galaxies brighter than BT = 12. A subsample of these galaxies with δ ≥ -25° and total flux densities at 1.49 GHz (Condon, 1987) above 10 mJy were observed at λ2.8 cm using the 100-m radio telescope of the MPIfR Bonn. Depending on the expected flux density and extent of the source two observational methods were used: cross-scanning and mapping. In total, radio flux densities of 192 galaxies have been derived (Niklas et al., 1995). Additionally, a literature search was made in order to get flux densities at other frequencies.

This southern galaxy with its fairly regular pattern is well suited to study the influence of a bar onto the magnetic field in a galaxy. With its low inclination of 24° it provides a geometry similar to M 51. Also the magnetic field strength of 11 ± 2 μG is in the same range.

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.

Continuum observations of the southern extension of the radio Arc located near 1~0.2° have been carried out at λ20 and 6cm using the VLA in its hybrid B/C and C/D array configurations. A number of long and narrow filaments have been identified on the negative latitude side of the plane. Some of the filaments appear to extend continuously into the radio continuum Arc and suggesting strongly that they are associated physically with the Arc. Other filaments appear isolated and thus have characteristics similar to those of the radio “threads” which have been seen near the Galactic center. These new threads and filaments are highly polarized at λ6cm and show rotation measures which vary between 300 and 3000 rad m−2. The details present in the high-resolution images of this region strengthen the hypotheses that the large field strength is dynamically important and that the large-scale geometry of the magnetic field is poloidal near the Galactic center.

We have begun a long-range project to study southern galaxies using the radio telescopes at Parkes and Molonglo, the Siding Spring optical facilities and soon, the Australia Telescope. Here we present the results of polarization mapping at two wavelengths of the galaxies NGC 55, 253,4945, M 83 and the Circinus Galaxy.

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

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.

Global configuration of magnetic field in several spiral galaxies were determined by analysing characteristic variation of Faraday rotation within the galaxy disks. The majority has an open spiral, bisymmetric field configuration, while some (10–20%) have a ring field.

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.

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.