Scientific background

Scientific background

Magnetic fields in galaxies

Magnetic fields are a major agent in the interstellar medium (ISM) of spiral, barred, irregular and dwarf galaxies. They contribute significantly to the total pressure, which balances the ISM against gravity. They may affect the gas flows in spiral arms, around bars and in galaxy halos. Magnetic fields are essential for star formation as they enable the removal of angular momentum from a protostellar cloud during its collapse. Turbulence in the magnetised interstellar gas distributes energy from supernova explosions within the ISM. "Reconnection" of magnetic lines with opposite polarity is a possible heating source for the ISM and the halo gas. Furthermore, magnetic fields control the density and distribution of cosmic rays in the ISM.

The strongest radio emission is found in "radio galaxies", powered by violent processes around massive black holes in their centres. They show "jets" expanding from their centres far into intergalactic space and "lobes" at the end of the jets. Strong magnetic fields are crucial to form the jets in radio galaxies. The optical counterparts of radio galaxies are elliptical galaxies. Jets can also emerge from the active nuclei of spiral galaxies.

Observations of magnetic fields in galaxies

Galactic magnetic fields can be observed in the optical range via starlight that is polarized by interstellar dust grains in the foreground. These grains are elongated and can be aligned by magnetic fields, where the major axis of the grains becomes perpendicular to the field lines. Measurements of many stars reveal an overall picture of the magnetic field in the Milky Way near the Sun. Aligned dust grains give also rise to polarized infrared emission, which is useful to map magnetic fields in dust clouds of the Milky Way. Zeeman splitting of radio spectral lines allows measurements of relatively strong fields in nearby, dense gas clouds in the Milky Way. For those three techniques, observations in external galaxies are still difficult to obtain. The fourth technique, measuring synchrotron emission, is the most powerful one to observe magnetic fields and can be applied to the Milky Way, nearby galaxies and also to distant galaxies, as presented in this Atlas.

Synchrotron emission

Cosmic-ray electrons in galaxies, accelerated in the shock fronts of supernova explosion or in jets, are spiraling around interstellar magnetic field lines with almost the speed of light. They emit electromagnetic waves, called "synchrotron emission", over a large range of radio wavelengths. The intensity of synchrotron emission increases with the observation wavelength to a power of about 0.8. The most energetic electrons can emit synchrotron waves in the infrared, optical or even in the X-ray ranges.

The intensity of synchrotron emission is a measure of the density of cosmic-ray electrons and of the strength of the total magnetic field component in the sky plane. The degree of linear polarization of synchrotron emission can be as high as 75% in a completely ordered field, which is a field with a constant orientation within the volume traced by the telescope's beam. Any variation of the field orientation within the beam reduces the degree of polarization. Regular fields are believed to be generated by a dynamo mechanism. Polarized emission can also emerge from "anisotropic turbulent" fields (with random orientations in two dimensions), which are generated from isotropic turbulent fields (with random orientations in three dimensions) by compressing or shearing gas flows. Anisotropic turbulent fields frequently reverse their field direction by 180 degrees on scales smaller than the telescope beam. Unpolarized synchrotron emission indicates isotropic turbulent fields that can be generated by turbulent gas flows. Hence, three components of the total field are distinguished by observations: regular, anisotropic turbulent and isotropic turbulent fields.

Typical degrees of polarization are 10-20% on average, indicating that isotropic turbulent fields dominate in galaxies. Locally, 50% is observed (e.g. in the interarm regions of NGC 6946); the regular and/or anisotropic turbulent field dominates in such regions.

Thermal radio emission

Radio continuum emission of galaxies is not purely of synchrotron origin, but also contains thermal radiation. Hot gas in galaxies emits unpolarized radio emission due to random motions of free electrons. The intensity is almost independent of wavelength. The average fraction of thermal radio emission in spiral galaxies is 30% at 3cm, 20% at 6cm and 10% at 20cm. Locally, the thermal fraction can be up to three times larger or smaller. Irregular and dwarf galaxies tend to have larger thermal fractions than spiral galaxies.

Faraday rotation

The intrinsic orientation of the observed polarization plane of an electromagnetic wave is perpendicular to the field orientation. When the wave travels through a magnetised plasma, the orientation of the polarization plane is changed by "Faraday rotation". The rotation angle increases with the plasma density, the strength of the component of the regular field along the line of sight, and the square of the observation wavelength. Fields directed towards the observer cause an anticlockwise sense of rotation, fields directed away from the observer a clockwise rotation. Anisotropic and isotropic turbulent fields do not cause a net Faraday rotation when the emission is integrated along the line of sight. For typical plasma densities and regular field strengths in the interstellar medium of galaxies, Faraday rotation becomes significant at wavelengths larger than a few centimetres (frequencies below a few GHz). At decimetre wavelengths (frequencies below about 1 GHz), Faraday rotation is generally strong and leads to "Faraday depolarization". In the metrewave range (below frequencies of about 300 MHz), polarized emission from galaxies is generally too weak to be detected.

Measurements of the Faraday rotation from multi-wavelength observations allow us to determine the strength and direction of the regular field component along the line of sight; combined with the orientations of the polarization planes this yields a fully three-dimensional picture of the magnetic field.

Magnetic field strengths

Total magnetic field strengths can be determined from the intensity of total synchrotron emission, assuming energy balance (equipartition) between magnetic fields and cosmic rays. This assumption seems valid on large spatial and time scales, but deviations occur on local scales in galaxies. The typical average equipartition strength for spiral galaxies is about 10 μG (1 nT). For comparison, the Earth's magnetic field has an average strength of about 0.3 G (30 μT). Radio-faint galaxies like M 31 and M 33, our Milky Way's neighbors, have weaker fields (about 5 μG), while gas-rich galaxies with high star-formation rates, for example M 51, M 83 and NGC 6946, have 15 μG on average. In prominent spiral arms, in regions where cold gas and dust are concentrated, the field strengths can be up to 25 μG.

Even stronger fields (50-100 μG) were found in starburst galaxies, for example in M 82 and the Antennae galaxies NGC 4038/9, and in nuclear starburst regions, for example in the centres of NGC 1097 and other barred galaxies. The magnetic field strengths in the jets of radio galaxies can be several mG.

The ATLAS images

The ATLAS OF GALAXIES presents images of the total and linearly polarized radio emission of many nearby galaxies. Total intensity is a signature of total magnetic fields (consisting of regular, anisotropic turbulent and isotropic turbulent fields), while linearly polarized intensity is a signature of ordered magnetic fields (consisting of regular and anisotropic turbulent fields). Total intensity is presented in rainbow colors (from blue=low intensity to red=high intensity), polarized emission in "heat" colors (from brown to white). The range of intensity values has been chosen for best visibility of details and is different for each galaxy image. The orientations of the polarization are not plotted.

The galaxy properties

The galaxies are separated into four classes: spiral galaxies with low or moderate inclination, spiral galaxies with high inclination (see almost "edge-on"), irregular galaxies and galaxies of the Virgo Galaxy Cluster. In each class, the galaxies are ordered according to their catalogue numbers in the Messier or NGC catalogues.

The classifications of the galaxies are taken from the NASA Extragalactic Database (NED). The distance estimates vary over a large range; the median values from the NED are given. The inclinations of the galaxy disks are taken from the cited publications; they are based on measurements of rotation velocities and may suffer from uncertainties.

Radio galaxies form a separate class. This Atlas is restricted to "normal" galaxies, where radio emission and magnetic fields are related to star-formation processes in the interstellar medium.

Elliptical galaxies without an active nucleus cannot form jets or radio lobes. As they lack cold interstellar matter, they cannot trigger star formation and generate interstellar magnetic fields. As a consequence, quiet elliptical galaxies are invisible in the radio range.

The wavelengths

The images are based on observations obtained at various radio wavelengths, most commonly at 3.6 cm (8.6 GHz), 6.2 cm (4.8 GHz), 11 cm (2.7 GHz) and around 20 cm (1.4-1.5 GHz), which are radio bands that are mostly free of terrestrial interference. The images at 3.6 cm and 6.2 cm hardly suffer from Faraday depolarization, so that the polarized intensity is an excellent measure of the strength of the ordered field. At around 20 cm, Faraday depolarization strongly reduces the polarized intensity, especially in the inner parts of a galaxy's disk, and the remaining polarization emerges from the upper disk, the outer disk and/or the halo of a galaxy.

The radio telescopes

The most sensitive instruments for radio polarization measurements are the 100-m single-dish telescope in Effelsberg (Germany), the 64-m dish in Parkes (Australia), the synthesis (interferometer) telescopes in Westerbork (WSRT, Netherlands), the Jansky Very Large Array (JVLA, USA) and the Australia Telescope Compact Array (ATCA). Low-frequency instruments like the Low Frequency Array (LOFAR, Europe) has not yet detected polarization from galaxies because of strong Faraday depolarization at long wavelengths. A major increase in sensitivity and angular resolution is expected from the Square Kilometre Array (SKA).

Synthesis telescopes like the JVLA cannot detect large-scale radio emission, especially at shorter wavelengths, so that several of the images in this Atlas are based on combined data from the JVLA and the Effelsberg single-dish telescope.

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