Magnetic fields in ring galaxies

Abstract

Context. Many galaxies contain magnetic fields supported by galactic dynamo action. The investigation of these magnetic fields can be helpful for understanding galactic evolution; however, nothing definitive is known about magnetic fields in ring galaxies. Aims. Here we investigate large-scale magnetic fields in a previously unexplored context, namely ring galaxies, and concentrate our efforts on the structures that appear most promising for galactic dynamo action, i.e. outer star-forming rings in visually unbarred galaxies. Methods. We use tested methods for modelling α − Ω galactic dynamos, taking into account the available observational information concerning ionized interstellar matter in ring galaxies. Results. Our main result is that dynamo drivers in ring galaxies are strong enough to excite large-scale magnetic fields in the ring galaxies studied. The variety of dynamo driven magnetic configurations in ring galaxies obtained in our modelling is much richer than that found in classical spiral galaxies. In particular, various long-lived transients are possible. An especially interesting case is that of NGC 4513, where the ring counter-rotates with respect to the disc. Strong shear in the region between the disc and the ring is associated with unusually strong dynamo drivers in such counter-rotators. The effect of the strong drivers is found to be unexpectedly moderate. With counter-rotation in the disc, a generic model shows that a steady mixed parity magnetic configuration that is unknown for classical spiral galaxies, may be excited, although we do not specifically model NGC 4513. Conclusions. We deduce that ring galaxies constitute a morphological class of galaxies in which identification of large-scale magnetic fields from observations of polarized radio emission, as well as dynamo modelling, may be possible. Such studies have the potential to throw additional light on the physical nature of rings, their lifetimes, and evolution.

Global enhancement and structure formation of the magnetic field in spiral galaxies

“…The growth of magnetic field strength is clearly demonstrated in dynamo theory simulations of the interstellar medium in spiral galaxies (Frick et al 2016), barred galaxies (Beck et al 1994) and ring galaxies (Moss et al 2016). Numerical models of evolving galaxies show fast field amplification and ordering due to differential rotation (Kotarba et al 2009;Wang & Abel 2009), possibly supported by the magneto-rotational instability (Gressel et al 2013;Pakmor & Springel 2013).…”

Far-infrared–radio correlation and magnetic field strength in star-forming early-type galaxies

“…Regular fields are not expected here due to lack of ordered motions in elliptical galaxies. However, it may be noted that a possibility for field regularization over a few giga-year of time-scales in some dynamical components such as the rings in ETGs has been discussed (Moss et al 2016). Large-scale magnetic fields can also be generated via cosmicray driven dynamos as discussed in Hanasz et al (2009).…”

Possible formation of ring galaxies by torus-shaped magnetic wormholes

“…repeat the shape of the entrance (the shape of a doughnut) which roughly corresponds to the fields observed in spiral galaxies [43][44][45]. It is necessary to point out that the magnetic fields in the galaxies cannot exactly repeat the shape of a doughnut.…”