3D global simulations of a cosmic-ray-driven dynamo in dwarf galaxies

Abstract

Context. Star-forming dwarf galaxies can be seen as the local proxies of the high-redshift building blocks of more massive galaxies according to the current paradigm of the hierarchical galaxy formation. They are low-mass objects, and therefore their rotation speed is very low. Several galaxies are observed to show quite strong magnetic fields. These cases of strong ordered magnetic fields seem to correlate with a high, but not extremely high, star formation rate. Aims. We investigate whether these magnetic fields could be generated by the cosmic-ray-driven dynamo. The environment of a dwarf galaxy is unfavourable for the large-scale dynamo action because of the very slow rotation that is required to create the regular component of the magnetic field. Methods. We built a 3D global model of a dwarf galaxy that consists of two gravitational components: the stars and the dark-matter halo described by the purely phenomenological profile proposed previously. We solved a system of magnetohydrodynamic equations that include an additional cosmic-ray component described by the fluid approximation. Results. We found that the cosmic-ray-driven dynamo can amplify the magnetic field with an exponential growth rate. The e-folding time is correlated with the initial rotation speed. The final mean value of the azimuthal flux for our models is on the order of few µG and the system reaches its equipartition level. The results indicate that the cosmic-ray-driven dynamo is a process that can explain the magnetic fields in dwarf galaxies.

Enhancement of magnetic fields arising from galactic encounters

“…We note that our results are broadly consistent with studies of cosmic ray driven dynamos (e.g. Siejkowski et al 2014, and references therein). These studies find that for, modest values of the SFR, galactic fields are enhanced, but that the dynamo ceases to operate for very high values of SFR.…”

Nonthermal particles and photons in starburst regions and superbubbles

“…One should have in mind the existence of a feedback effect of supernova and superbubbles on the global galactic ecology. This is the effect of large scale galactic magnetic field amplification by the cosmic-ray-driven dynamo on the timescale of about 150 Myr simulated by Hanasz et al (2009), Siejkowski et al (2014). Price and Bate (2008) demonstrated that even a very low magnetic field is able to significantly influence the star formation process.…”

Ab Initio Simulations of a Supernova-driven Galactic Dynamo in an Isolated Disk Galaxy

“…In this prescription, supernova events occur in proportion to star formation activity, and provide localized injections of thermal energy, metal-rich material, and a low level of toroidal magnetic field. This is in contrast to a number of earlier studies in which supernovae were assumed to function strictly as sources of energetic cosmic rays (Hanasz et al 2009;Siejkowski et al 2014;Kulpa-Dybeł et al 2011. In this study, we will neglect the influence of cosmic rays, and focus on the role of turbulence driven by supernovae through their thermal feedback.…”

“…In their approach, only 10% of supernova events included a magnetic field. The same approach was adopted in Kulpa-Dybeł et al (2011 to simulate dynamo activity in barred spiral galaxies, and by Siejkowski et al (2014) to study dwarf galaxies. Beck et al (2013) also utilized a magnetic dipole configuration as supernova source terms.…”