Abstract
ABSTRACT We study the prospects of detecting magnetic helicity in galaxies by observing the dust polarization of the edge-on galaxy NGC 891. Our numerical results of mean-field dynamo calculations show that there should be a large-scale component of the rotationally invariant parity-odd B polarization that we predict to be negative in the first and third quadrants, and positive in the second and fourth quadrants. The large-scale parity-even E polarization is predicted to be negative near the axis and positive further away in the outskirts. These properties are shown to be mostly a consequence of the magnetic field being azimuthal and the polarized intensity being maximum at the centre of the galaxy and are not a signature of magnetic helicity.
Highlights
The magnetic fields of spiral galaxies possess a clear largescale component along with a fluctuating component of comparable strength (Beck et al 1996; Han 2017)
The primary reason for this is connected just with the spatial variation of the polarized intensity. This is demonstrated in Appendix A, where we show the E and B polarizations for a polarization sigc 0000 RAS, MNRAS 000, 000–000
Our preliminary investigation based on simple models suggests that the E polarization is positive near the disc midplane and away from the axis, where it tends to outline star-like patterns in the magnetic field, while in the halo near the axis it is negative, corresponding to ring-like patterns in the magnetic field; see the top panel of Fig. 2
Summary
The magnetic fields of spiral galaxies possess a clear largescale component along with a fluctuating component of comparable strength (Beck et al 1996; Han 2017). Owing to the presence of turbulence in the interstellar medium (ISM), there is significant turbulent diffusion, which would destroy the large scale magnetic field on a time scale of less than a billion years (Shukurov 1998), unless there is a correspondingly strong anti-diffusive mechanism. The best known mechanism for explaining the origin and maintenance of galactic large-scale magnetic fields is the α effect (Parker 1955; Steenbeck et al 1966). It is important to assess the validity of the dynamo models toward a more comprehensive understanding of how a galaxy forms its structure and how magnetic fields regulate the cycle of the ISM by enhancing and suppressing star-formation activity in a galaxy. There is no explicit evidence that the α effect really does operate in galaxies
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