Ram pressure effects in the galactic plane and galactic dynamos in the no-zapproximation

Abstract

Context. The magnetic field of galaxies is believed to be produced by internal dynamo action, but can be affected by motion of the galaxy through the surrounding medium. Observations of polarized radio emission of galaxies located in galaxy clusters have revealed noticeable features of large-scale magnetic configurations, including displacements of the magnetic structures from the optical images and tails, which are possible imprints of ram pressure effects arising from motion of the galaxies through the intracluster medium. Aims. We present a quantitative dynamo model, which attempts to describe the above effects. In contrast, to the traditional problem of a wind affecting a body with a prescribed magnetic field, we investigate how a non-magnetized wind flow affects a magnetic field that is being self-excited by galactic dynamo action. Methods. To isolate the leading physical effects, we exploit a simple dynamo model that can describe relevant effects. In particular, we use what is known as the "no-z" approximation for the mean-field dynamo equations. Results. In a suitable parametric range we obtain displacements of the large-scale magnetic field, as well as magnetic tails. However, the specific details of their locations are quite counterintuitive. The direction of displacement is perpendicular to, rather than parallel to, the wind direction. The point at which the tail emerges from the galaxy depends on details of the model. The tail is eventually directed downstream. In the simplest case the magnetic tail begins in the region where the wind decreases the total gas velocity. Any wind that penetrates the galaxy modifies the intrinsic dynamo action. These features are different from those found in ram-pressure models. Conclusions. Any determination of galactic motion through the cluster medium from observational data needs to take the effects of dynamo action into account.

The NARROW X-Ray TAIL AND DOUBLE Hα TAILS OF ESO 137-002 IN A3627

“…However, more work needs to be done to compare this folding of the magnetic field with previous studies on magnetic draping (e.g., Lyutikov 2006;Dursi & Pfrommer 2008;Pfrommer & Dursi 2010). Future simulations should consider a varying wind, as well as include the effect of the interstellar (as opposed to intracluster) magnetic field, which evolves due to compression and shear motions (e.g., Otmianowska-Mazur & Vollmer 2003) or dynamo processes (e.g., Moss et al 2012) induced by the ram pressure. They should also consider the back reaction exerted by the magnetic field on the process of stripping (e.g., magnetic draping).…”

Magnetic Fields Around Galactic Discs

“…An example of a rather unexpected magnetic configuration produced by dynamo action (axisymmetric within the circle x 2 + y 2 < 1, which is shown) and a streaming velocity in the negative x-direction. This figure appeared in [44] where more examples can be found. The figure is reproduced by permission of A&A.…”

“…As a result, instead of prescribing the magnetic field, it is better to consider dynamo generated galactic magnetic fields where the dynamo governing parameters are affected by the galactic motion through an external medium. A pilot research in this direction presented in [44] demonstrates that the effects of such a system can be quite pronounced and rather unexpected (an example of such an unexpected magnetic configuration is given in Figure 1). In particular, galactic tails can be substantially asymmetric with respect to the galactic motion.…”

Seeking large-scale magnetic fields in a pure-disk dwarf galaxy NGC 2976

“…Moreover, maxima of the gas distribution outside of the disk should occur in this phase, which is actually observed in the case of NGC 2976. Another possibility, related to the ram pressure, is the influence of draping flows which modify the large-scale dynamo action and lead to an asymmetric PI distribution (Moss et al 2012). However, in this case, relatively strong ram pressure is also needed.…”