Elsevier

New Astronomy Reviews

Volume 48, Issues 11–12, December 2004, Pages 1289-1304
New Astronomy Reviews

Observations of magnetic fields in the Milky Way and in nearby galaxies with a Square Kilometre Array

https://doi.org/10.1016/j.newar.2004.09.013Get rights and content

Abstract

The role of magnetic fields in the dynamical evolution of galaxies and of the interstellar medium (ISM) is not well understood, mainly because such fields are difficult to directly observe. Radio astronomy provides the best tools to measure magnetic fields: synchrotron radiation traces fields illuminated by cosmic-ray electrons, while Faraday rotation and Zeeman splitting allow us to detect fields in all kinds of astronomical plasmas, from lowest to highest densities. Here, we describe how fundamental new advances in studying magnetic fields, both in our own Milky Way and in other nearby galaxies, can be made through observations with the proposed Square Kilometre Array. Underpinning much of what we propose is an all-sky survey of Faraday rotation, in which we will accumulate tens of millions of rotation measure measurements toward background radio sources. This will provide a unique database for studying magnetic fields in individual Galactic supernova remnants and Hii regions, for characterizing the overall magnetic geometry of our Galaxy’s disk and halo, and for understanding the structure and evolution of magnetic fields in galaxies. Also of considerable interest will be the mapping of diffuse polarized emission from the Milky Way in many narrow bands over a wide frequency range. This will allow us to carry out Faraday tomography of the Galaxy, yielding a high-resolution three-dimensional picture of the magnetic field within a few kpc of the Sun, and allowing us to understand its coupling to the other components of the ISM. Finally, direct synchrotron imaging of a large number of nearby galaxies, combined with Faraday rotation data, will allow us to determine the magnetic field structure in these sources, and to test both the dynamo and primordial field theories for field origin and amplification.

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:

  • 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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