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

Abstract

Faraday tomography allows astronomers to probe the distribution of the magnetic field along the line of sight (LOS), but that can be achieved only after the Faraday spectrum is interpreted. However, the interpretation is not straightforward, mainly because the Faraday spectrum is complicated due to a turbulent magnetic field; it ruins the one-to-one relation between the Faraday depth and the physical depth, and appears as many small-scale features in the Faraday spectrum. In this paper, by employing "simple toy models" for the magnetic field, we describe numerically as well as analytically the characteristic properties of the Faraday spectrum. We show that the Faraday spectrum along "multiple LOSs" can be used to extract the global properties of the magnetic field. Specifically, considering face-on spiral galaxies and modeling turbulent magnetic field as a random field with a single coherence length, we numerically calculate the Faraday spectrum along a number of LOSs and its shape-characterizing parameters, that is, the moments. When multiple LOSs cover a region of (10 coherence length) 2 , the shape of the Faraday spectrum becomes smooth and the shape-characterizing parameters are well specified. With the Faraday spectrum constructed as a sum of Gaussian functions with different means and variances, we analytically show that the parameters are expressed in terms of the regular and turbulent components of the LOS magnetic field and the coherence length. We also consider the turbulent magnetic field modeled with a power-law spectrum, and study how the magnetic field is revealed in the Faraday spectrum. Our work suggests a way to obtain information on the magnetic field from a Faraday tomography study.

The Disk-Halo Distinction of Galaxies Using Faraday Tomography

“…As implied in the previous subsection, the global properties of galaxies (especially, this is the coherent, vertical magnetic field in this paper) can affect the shape of F(φ). As with the previous works [10,12], I use the width (standard deviation), skewness and kurtosis of F(φ) as shape-characterizing parameters to quantify the shape of F(φ). Here, I try to calculate the parameters only from the disk for the purpose to compare them with the simple galactic models.…”

“…As emphasized by Ideguchi et al [10], the study using the realistic Galactic model provides the realistic F(φ) of galaxies, but therefore, it is not straightforward to interpret the results due to its complexity. Following the studies by Ideguchi et al [10], Ideguchi et al [12] employ "simple, toy models" for the magnetic field in order for the deeper understanding of the behavior of F(φ). They found that the shape of F(φ) will converge to a well defined average by observing multiple LOSs covering a region of (10 coherent length of turbulence) 2 .…”

“…In addition, I study the presence of coherent, vertical magnetic field by separately looking at each contributions from the disk and halos on F(φ). Then, I try to compare the realistic F(φ) with the study in Ideguchi et al [12]. Since Ideguchi et al [12] considered the only disk to make the situation as simple as possible, I calculate the realistic F(φ) by considering only the contribution from disk.…”

“…Then, I try to compare the realistic F(φ) with the study in Ideguchi et al [12]. Since Ideguchi et al [12] considered the only disk to make the situation as simple as possible, I calculate the realistic F(φ) by considering only the contribution from disk. Interpretation of F(φ) of external galaxies is useful not only to study magnetic fields associated with the galaxies themselves, but also could be useful to probe intergalactic magnetic field embedded between two galaxies as a gap between two components in φ space (e.g., [13,14]).…”

Workshop Summary “The Power of Faraday Tomography”

“…The workshop had 64 participants. The biggest group of these were Japanese (24), there were 19 from Europe (6 Germany,4 Netherlands,4 Italy, 2 UK, 1 France, 1 Spain), 8 from North America (5 Canada, 3 US), 8 from Australia, and 4 from Asia other than Japan (2 South-Korea, 1 Russia, 1 India). A relatively large part of these were young researchers: 28 postdocs, 12 PhD students and 2 MSc students.…”