Observations of magnetic fields in the Milky Way and in nearby galaxies with a Square Kilometre Array
Introduction
A full understanding of galactic structure and evolution is impossible without understanding magnetic fields. Magnetic fields fill interstellar space, contribute significantly to the total pressure of interstellar gas, are essential for the onset of star formation, and control the density and distribution of cosmic rays in the interstellar medium (ISM). However, because magnetic fields cannot be directly observed, our understanding of their structure and origin lags significantly behind that of the other components of the ISM.
Radio astronomy has long led the way in studying astrophysical magnetic fields. Synchrotron emission measures the field strength; its polarization yields the field’s orientation in the sky plane and also gives the field’s degree of ordering; Faraday rotation provides a measurement of the mean direction and strength of the field along the line of sight; the Zeeman effect provides an independent measure of field strength in cold gas clouds. All these effects have been effectively exploited. However, the study of magnetism in the Milky Way and in galaxies is a field still largely limited to examination of specific interesting regions, bright and nearby individual sources, and gross overall structure. Here, we describe how exciting new insights into magnetic fields can be provided by the unique sensitivity, resolution and polarimetric capabilities of the Square Kilometre Array (SKA).
Section snippets
Background
While synchrotron emission and its polarization are useful tracers of magnetic fields, they are only easily detected in regions where the density of cosmic rays (i.e., relativistic gas) is relatively high, or where the magnetic field is strong. Many regions of interest for magnetic field studies are far from sites of active star formation and supernova activity, and thus cannot be studied through these techniques.
A much more pervasive probe of interstellar magnetic fields is Faraday rotation,
Scientific applications of an RM grid
The densely-spaced RM grid which would result from the experiment proposed above will have numerous applications: the magnetic properties of any extended foreground object will be able to be mapped in detail. Before discussing specific projects, we make a few general comments about such analyses:
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The RM signature of an extended foreground object can only ever be identified provided that its contribution to the total RM dominates the average intrinsic RM of the background sources. Since the
Faraday tomography
Major progress in detecting small structures has recently been achieved with decimetre-wave polarization observations in the Milky Way (Duncan et al., 1997, Duncan et al., 1999, Gaensler et al., 2001, Haverkorn et al., 2003a, Haverkorn et al., 2003b, Reich et al., 2004, Uyanıker et al., 1998, Uyanıker et al., 1999, Uyanıker et al., 2003). A wealth of structures on parsec scales has been discovered: filaments, canals, lenses, and rings (e.g., Fig. 4). Their common property is that they appear
Dynamo versus primordial field origin
The observation of large-scale patterns in RM in many galaxies (Beck, 2000) proves that the regular field in galaxies has a coherent direction and hence is not generated by compression or stretching of irregular fields in gas flows. In principle, the dynamo mechanism is able to generate and preserve coherent magnetic fields, and they are of appropriate spiral shape (Beck et al., 1996) with radially decreasing pitch angles (Beck, 1993). However, the physics of dynamo action is far from being
Summary
A 1.4-GHz all-sky survey of Faraday rotation will accumulate tens of millions of rotation measure measurements toward background radio sources. This will allow us to characterize the overall magnetic geometry and turbulent properties of the disk and halo of the Milky Way, and of embedded individual objects such as Hii regions and SNRs. In a highly complementary fashion, mapping of diffuse polarized emission from the Milky Way in many narrow bands over a wide frequency range will allow us to
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Cited by (89)
Radio galaxies and feedback from AGN jets
2020, New Astronomy ReviewsCitation Excerpt :Detailed studies with current and future radio instruments should enable further advances in understanding the influence of radio galaxies on magnetic field properties of groups and clusters. Radio galaxies also have the potential to be used to provide ‘rotation measure grids’ — sufficiently many strongly polarized radio sources across the sky will enable magnetic fields to be determined on a range of scales and cosmic environments (e.g., Beck and Gaensler, 2004; Johnston-Hollitt et al., 2004; Krause et al., 2009). With SKA pathfinders and eventually the SKA itself it should be possible to build grids of background sources that will enable the magnetic field of the Milky Way to be mapped on arcmin scales, as well as enabling detailed investigations of magnetic fields in nearby galaxies via the effect of propagation of emission from background radio galaxies through their ISMs.
Capabilities of next generation telescopes for cosmic magnetism
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