We use new multiwavelength radio observations, made with the VLA and Effelsberg telescopes, to study the magnetic field of the nearby galaxy M51 on s

Context.The origin of the spiral pattern of magnetic fields in disc galaxies is an open question.Aims.Comparison of the regular magnetic field orientation with the gaseous spiral arm pitch angles can tell us whether spiral shock compression is responsible for the magnetic spirals. We also wish to see whether the ridges of different components of the ISM show the large-scale, systematic shifts expected from density wave theory.Methods.We have developed a technique of isolating elongated structures in galactic images, such as spiral arms, using anisotropic wavelets and apply this to maps of the CO, infrared and radio continuum emission of the grand-design spiral galaxy M 51. Results.Systematic shifts between the ridges of CO, infrared and radio continuum emission that are several long are identified, as well as large variations in pitch angle along spiral arms, of a few tens of degrees. We find two types of arms of polarized radio emission: one has a ridge close to the ridge of CO, with similar pitch angles for the CO and polarization spirals and the regular magnetic field; the other does not always coincide with the CO arm and its pitch angle differs from the orientation of its regular magnetic field.Conclusions.The offsets between ridges of regular magnetic field, dense gas and warm dust are compatible with the sequence expected from spiral density wave triggered star formation, with a delay of a few tens of millions of years between gas entering the shock and the formation of giant molecular clouds and a similar interval between the formation of the clouds and the emergence of young star clusters. At the position of the CO arms the orientation of the regular magnetic field is the same as the pitch angle of the spiral arm, but away from the gaseous arms the orientation of the regular field varies significantly. Spiral shock compression can explain the generation of one type of arm of strong polarized radio emission but a different mechanism is probably responsible for a second type of polarization arm.

In a search for the signature of turbulence in the diffuse interstellar medium (ISM) in gas density distributions, we determined the probability distribution functions (PDFs) of the average volume densities of the diffuse gas. The densities were derived from dispersion measures and H i column densities towards pulsars and stars at known distances. The PDFs of the average densities of the diffuse ionized gas (DIG) and the diffuse atomic gas are close to lognormal, especially when lines of sight at |b| < 5° and |b| ≥ 5° are considered separately. The PDF of 〈nH i〉 at high |b| is twice as wide as that at low |b|. The width of the PDF of the DIG is about 30 per cent smaller than that of the warm H i at the same latitudes. The results reported here provide strong support for the existence of a lognormal density PDF in the diffuse ISM, consistent with a turbulent origin of density structure in the diffuse gas.

Hydrostatic equilibrium of the multiphase interstellar medium in the solar vicinity is reconsidered, with the regular and turbulent magnetic fields t

Narrow depolarized canals are common in maps of the polarized synchrotron emission of the Milky Way. Two physical effects that can produce these canals have been identified: the presence of Faraday rotation measure (RM) gradients in a foreground screen, and the cumulative cancellation of polarization known as differential Faraday rotation. We show that the behaviour of the Stokes parameters Q and U in the vicinity of a canal can be used to identify its origin. In the case of canals produced by a Faraday screen we demonstrate that, if the polarization angle changes by 90° across the canal, as is observed in all fields to-date, the gradients in RM must be discontinuous. Shocks are an obvious source of such discontinuities and we derive a relation of the expected mean separation of canals to the abundance and Mach number of supernova-driven shocks, and compare this with recent observations by Haverkorn, Katgert & de Bruyn. We also predict the existence of less common canals with polarization angle changes other than 90°. Differential Faraday rotation can produce canals in a uniform magneto-ionic medium, but as the emitting layer becomes less uniform the canals will disappear. We show that for moderate differences in emissivity in a two-layer medium, of up to 1/2, and for Faraday depth fluctuations of standard deviation ≲1 rad, canals produced by differential rotation will still be visible.