The effect of supernova rate on the magnetic field evolution in barred galaxies

Abstract

Context. For the first time, our magnetohydrodynamical numerical calculations provide results for a three-dimensional model of barred galaxies involving a cosmic-ray driven dynamo process that depends on star formation rates. Furthermore, we argue that the cosmic-ray driven dynamo can account for a number of magnetic features in barred galaxies, such as magnetic arms observed along the gaseous arms, magnetic arms in the inter-arm regions, polarized emission that is at the strongest in the central part of the galaxy, where the bar is situated, polarized emission that forms ridges coinciding with the dust lanes along the leading edges of the bar, as well as their very strong total radio intensity. Aims. Our numerical model probes what kind of physical processes could be responsible for the magnetic field topology observed in barred galaxies (modes, etc.). We compare our modelled results directly with observations, constructing models of high-frequency (Faraday rotation-free) polarized radio emission maps out of the simulated magnetic field and cosmic ray pattern in our modeled galaxy. We also take the effects of projection into account as well as the limited resolution. Methods. We applied global 3D numerical calculations of a cosmic-ray driven dynamo in barred galaxies with different physical input parameters such as the supernova (SN) rate. Results. Our simulation results lead to the modelled magnetic field structure similar to the one observed on the radio maps of barred galaxies. Moreover, they cast new light on a number of properties in barred and spiral galaxies, such as fast exponential growth of the total magnetic energy to the present values. The quadrupole modes of magnetic field are often identified in barred galaxies, but the dipole modes (e.g., in NGC 4631) are found very seldom. In our simulations the quadrupole configuration dominates and the dipole configuration only appears once in the case of model S100, apparently as a consequence of the choice of the random number seed. Synthetic radio maps of our models display X-type structure similar to what is observed in real galaxies. Conclusions. We conclude that a cosmic-ray driven dynamo process in barred galaxies can amplify magnetic fields efficiently. The fastest rate of magnetic field increase is 195 yr for a SN frequency of 1/50 yr −1 .The obtained strength of magnetic field corresponds to the observational values (a few μG in spiral arms). The polarization and rotation measure maps also agree with observations. We found the effect of shifting magnetic arms in 4 models (out of the sample of 5).

Cosmic magnetism in centimeter- and meter-wavelength radio astronomy

“…(2010) showed the formation of magnetic arms with low density around the bar. Kulpa-Dybeł et al (2015) and Kulpa-Dybeł et al (2015) simulated galactic dynamo driven by CRs, and presented that a saturation level of magnetic-field strength is 3 − 10 µG and the field structure form the quadrupole-like symmetry with respect to the galactic plane.…”

The global structure of magnetic fields and gas in simulated Milky Way-analogue galaxies

“…We reach roughly an order of magnitude higher mass resolution than even zoom-in cosmological simulations of Milky Way-like galaxies, and, though our resolution is still substantially smaller than that in local patch simulations, we retain the full geometry of the problem, so that we can study field topology and outflows. Relatively few simulations of this type have been published, and those have largely been concerned with studying the magnetisation of the neutral medium (e.g., Wang & Abel 2009) or attempting to explain and interpret the observed Faraday rotation sky (e.g., Kulpa-Dybeł et al 2015;Butsky et al 2017). There have been no previous efforts to use simulations of this type to map out the vertical structure and topology of the magnetic field, or to study how fields interact with galactic winds.…”

On the Combined Role of Cosmic Rays and Supernova-Driven Turbulence for Galactic Dynamos

“…Hanasz & Lesch (2000) indirectly verified such a dynamo action via the numerical simulations of rising magnetic flux tubes and found e-folding times of mean field of the order of 100 Myr . Supplementing this Hanasz et al (2009); Siejkowski et al (2010); Kulpa-Dybeł et al (2015); Girichidis et al (2016) etc. also demonstrated the fast amplification of regular magnetic fields via the direct MHD simulation of global galactic ISM including cosmic ray driven turbulence, along with the differential shear (but excluding the viscous term).…”