Cosmic rays and the magnetic field in the nearby starburst galaxy NGC 253 - II. The magnetic field structure

V. Heesen; M. Krause; R. Beck; R.-J. Dettmar

doi:10.1051/0004-6361/200911698

Free Access

Issue		A&A Volume 506, Number 3, November II 2009


Page(s)		1123 - 1135
Section		Extragalactic astronomy
DOI		https://doi.org/10.1051/0004-6361/200911698
Published online		27 August 2009

Online Material

Appendix A: Magnetic field models

In order to find the best model for the magnetic field in NGC 253 we studied the expected polarized intensities and RMs of various possibilities of the disk and halo magnetic field alone and combinations of them. The setup of the models is described in Sect. 5.1 with the parameters of Table 3. In Figs. A.1-A.12 we present the polarized intensity and the magnetic field orientation (not corrected for Faraday rotation) for $\lambda \lambda$ $6.2~\rm cm$ (a) and $3.6~\rm cm$ (b) at $30\hbox {$^{\prime \prime }$ }$ resolution. Note that the Faraday corrected magnetic field orientation is identical for all models consisting both of the disk and halo field (Figs. A.5-A.12). The difference in the models is only in the direction of the magnetic field that becomes visible in the RM distribution (c) shown with $144\hbox {$^{\prime \prime }$ }$ resolution. Sketches (d) show the direction of the magnetic field in the disk and in the halo. Here, a `` $\cdot$ '' indicates that the field points to the observer and a ``+'' denotes a field pointing away.

Figures A.1 and A.2 show the disk magnetic field alone for the even and odd case. They are similar but the odd field has a smaller amplitude in the RM distribution. This can be understood as the scaleheight of the disk is smaller than the projected minor axis. We see therefore only one side of the disk (the southern one). Figures A.3 and A.4 show the halo magnetic field alone for the even and odd cases. Note the asymmetry of both the polarized intensity and the RM. Because the magnetic field orientation changes along the line-of-sight, there is some depolarization as the magnetic field vectors are not parallel to each other. If the Faraday rotation has the same sense as the rotation of the magnetic field, the depolarization is stronger. If they have opposite senses, the depolarization is weaker. This can explain the asymmetry although the magnetic field is axisymmetric.

Figures A.5-A.8 show the models with the even disk magnetic field. Among them Figs. A.5 and A.7 are the models whose RM distribution agrees best with the observations. The two models only differ in the direction of the northern halo field. The even halo magnetic field (Fig. A.5) is our best-fit model. Figures A.9-A.12 show the models for an odd disk magnetic field. They all have strong gradients in the RM which are not observed.

$\begin{figure} \includegraphics[width=18cm,clip]{11698fA1.eps}\end{figure}$

Figure A.1:

Even disk magnetic field. For Figs. A.1-A.12: a) polarized flux density and magnetic field orientation at $\lambda$ $6.2~\rm cm$ with $30\hbox {$^{\prime \prime }$ }$ resolution. b) likewise but for $\lambda$ $3.6~\rm cm$ . c) RM distribution between $\lambda \lambda$ $6.2~\rm cm$ and $3.6~\rm cm$ with $144\hbox {$^{\prime \prime }$ }$ resolution. d) sketch of the magnetic field direction.

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$\begin{figure}\includegraphics[width=18cm,clip]{11698fA2.eps} \end{figure}$	Figure A.2: Odd disk magnetic field.
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$\begin{figure}\includegraphics[width=18cm,clip]{11698fA3} \end{figure}$	Figure A.3: Even halo magnetic field. The halo field points away from the disk.
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$\begin{figure}\includegraphics[width=18cm,clip]{11698fA4} \end{figure}$	Figure A.4: Odd halo magnetic field. The halo field points away from the disk in the southern halo.
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$\begin{figure}\includegraphics[width=18cm,clip]{11698fA5} \end{figure}$	Figure A.5: Even disk magnetic field and even halo magnetic field. The halo field points away from the disk.
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$\begin{figure}\includegraphics[width=18cm,clip]{11698fA6} \end{figure}$	Figure A.6: Even disk magnetic field and even halo magnetic field. The halo field points towards the disk.
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$\begin{figure}\includegraphics[width=18cm,clip]{11698fA7} \end{figure}$	Figure A.7: Even disk magnetic field and odd halo magnetic field. The halo field points away from the disk in the southern halo.
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$\begin{figure}\includegraphics[width=18cm,clip]{11698fA8} \end{figure}$	Figure A.8: Even disk magnetic field and odd halo magnetic field. The halo field points towards the disk in the southern halo.
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$\begin{figure}\includegraphics[width=18cm,clip]{11698fA9} \end{figure}$	Figure A.9: Odd disk magnetic field and even halo magnetic field. The halo field points away from the disk.
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$\begin{figure}\includegraphics[width=18cm,clip]{11698fA10} \end{figure}$	Figure A.10: Odd disk magnetic field and even halo magnetic field. The halo field points towards the disk.
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$\begin{figure}\includegraphics[width=18cm,clip]{11698fA11} \end{figure}$	Figure A.11: Odd disk magnetic field and odd halo magnetic field. The halo field points away from the disk in the southern halo.
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$\begin{figure}\includegraphics[width=18cm,clip]{11698fA12} \end{figure}$	Figure A.12: Odd disk magnetic field and odd halo magnetic field. The halo field points towards the disk in the southern halo.
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