Recognizing magnetic structures by present and future radio telescopes with Faraday rotation measure synthesis

Abstract

Context. Modern radio telescopes allow us to record a large number of spectral channels. The application of a Fourier transform to spectropolarimetric data in radio continuum, Faraday rotation measure (RM) synthesis, yields the "Faraday spectrum", which hosts valuable information about the magneto-ionic medium along the line of sight. Aims. We investigate whether the method of wavelet-based RM synthesis can help us to identify structures of regular and turbulent magnetic fields in extended magnetized objects, such as galaxies and galaxy clusters. Methods. The analysis of spectropolarimetric radio observations of multi-scale targets calls for a corresponding mathematical technique. Wavelets allow us to reformulate the RM synthesis method in a scale-dependent way and to visualize the data as a function of Faraday depth and scale.Results. We present observational tests to recognize magnetic field structures. A region with a regular magnetic field generates a broad "disk" in Faraday space, with two "horns" when the distribution of cosmic-ray electrons is broader than that of the thermal electrons. Each field reversal generates one asymmetric "horn" on top of the "disk". A region with a turbulent field can be recognized as a "Faraday forest" of many components. These tests are applied to the spectral ranges of various synthesis radio telescopes. We argue that the ratio of maximum to minimum wavelengths determines the range of scales that can be identified in Faraday space. Conclusions. A reliable recognition of magnetic field structures in spiral galaxies or galaxy clusters requires the analysis of data cubes in position-position-Faraday depth space ("PPF cubes"), observed over a wide and continuous frequency range, allowing the recognition of a wide range of scales as well as high resolution in Faraday space. The planned Square Kilometre Array (SKA) will fulfill this condition and will be close to representing a perfect "Faraday telescope". The combination of data from the Low Frequency Array (LOFAR, at low frequencies) and the Expanded Very Large Array (EVLA, at high frequencies) appears to be a promising approach for the recognition of magnetic structures on all scales. The addition of data at intermediate frequencies from the Westerbork Synthesis Radio Telescope (WSRT) or the Giant Meterwave Radio Telescope (GMRT) would fill the gap between the LOFAR and EVLA frequency ranges. The Global Magneto-Ionic Medium Survey (GMIMS), planned with several single-dish telescopes at low angular resolution, will also provide good scale recognition and high resolution in Faraday space.

Study of the Vertical Magnetic Field in Face-on Galaxies Using Faraday Tomography

“…The latter part, however, often turns out to be complicated, mainly because of the turbulent component of the magnetic field; it causes the relation between the Faraday depth and the physical depth to be non-trivial and produces the Faraday forest (Frick et al 2011;Beck et al 2012), many smallscale features, in F(f). Our previous work showed that F(f) calculated with a realistic model for the Milky Way (Akahori et al 2013) has the Faraday forest superposed on the large-scale diffuse emission.…”

“…While it would be straightforward to estimate, for instance, the number of sources of synchrotron radiation and their Faraday depths, the Faraday spectrum could be used to obtain more information. The properties of F(f) were previously studied (e.g., Bell et al 2011;Frick et al 2011;Beck et al 2012;Ideguchi et al 2014). For instance, the characteristic features in F(f) caused by various configurations of the large-scale LOS magnetic field such as field reversal were examined using simple models.…”

“…Also, the effects of a small-scale, turbulent field were taken into account, and how the effects would be superposed on the features due to the large-scale field were studied. It was shown that turbulent field basically appears as many small-scale components in F(f), which are called "Faraday forest" (Beck et al 2012).…”

Broadband Polarimetry with the Square Kilometre Array: A Unique Astrophysical Probe

“…However, we now realise that even quite simple scenarios (e.g., a source consisting of two spatially unresolved polarised components, each with different RMs) can produce an apparent linear relationship between θ and λ 2 , but corresponding to a spurious value of RM; only when one considers the fractional polarisation, Π ≡ (Q 2 +U 2 )/I, as a function of λ 2 does the observer realise that something is amiss (Farnsworth et al 2011). In such cases, and also in sources for which there is no linear dependence of θ on λ 2 , we also now better appreciate the various ways in which q ≡ Q/I, u ≡ U/I and Π, all as a function of λ 2 , can provide detailed information on magnetic fields, ionised gas and turbulence (Bell et al 2011;Farnsworth et al 2011;Beck et al 2012;Bernet et al 2012;Farnes et al 2014a;Brandenburg & Stepanov 2014;Horellou & Fletcher 2014). Furthermore, it is now clear that over a fractional bandwidth of 25% as assumed by , there are considerable degeneracies as to the nature of the observed Faraday rotation (Law et al 2011;O'Sullivan et al 2012).…”

Radio Continuum Surveys with Square Kilometre Array Pathfinders

“…Other techniques for identifying magnetic field structures from RM synthesis observations continue to be developed (e.g. Beck et al 2012).…”