Speaker
Mr
Sergio Martin-Alvarez
(University of Oxford)
Description
Regardless of being acknowledged as a relevant factor in several astrophysical processes, the study of magnetic fields has been one of the most elusive areas of cosmology and galaxy evolution up to this day. The reason for it are both the complications associated with their observation and measurements, and the intricate difficulty to model them. However, the resources required to tackle them are becoming available and new discoveries are starting to take place. On the observational side, recent results suggest that the magnetic field was already on the order of microGauss in galaxies at high redshifts. Meanwhile on the numerical side, computers start to possess the required power and memory to model these magnetic fields efficiently, allowing various amplification mechanisms to be tested.
One of the major puzzles to be explained is precisely the rapid dynamo mechanism that transforms the weak values of the magnetic field expected at the beginning of the reionization era into the strong fields found in galaxies. Amongst the most favoured explanations is a fast amplification by small-scale dynamos, due to the low e-folding time characteristic of turbulence eddies.
To shed some light on this question, we present in this study the first magnetohydrodynamical cosmological zoom-in simulations of a Milky-Way-like galaxy with enough resolution to capture the small-scale dynamo (i.e. down to ~10pc cell size) operating on the galactic Interstellar Medium (ISM). The simulations, performed with the code RAMSES, also include realistic turbulent star formation and dynamical supernovae (SNe) feedback. Starting from various physically motivated initial strengths for the magnetic field, we find that amplification does take place in a cosmological context, where the processes of interaction and accretion of pristine gas are not strong enough to halt it. This amplification typically follows an exponential law, where the magnetic energy per unit mass grows as E_(mag,0) e^(Gamma t). After an initial epoch of fast increase during the initial collapse where Gamma ~ 5 Gyr^-1, the process slows down to Gamma=1.2 Gyr^-1. This value of Gamma is still in the same order of magnitude as the galaxy’s angular rotation frequency Omega ~ 5. We also find that the kinematic energy spectrum confirms the presence of Kolmogorov turbulence and the magnetic energy density spectrum display different periods of activity of the dynamo. The magnetic field profiles show a tangled structure around the disk plane, with the toroidal and the radial components dominating along the z-axis away from the centre. This is in agreement with previous studies whenever feedback is included. The radial profile however, displays a more organised structure, and in the central region of the galaxy the magnetic field is dominated by the gas kinematics. The star formation is also affected by strong magnetic fields, as the presence of a strong initial comoving magnetic seed (B_0 = 10^(-12) Gauss) triggers a reduction of the stellar content by 16% with respect to a weak initial comoving magnetic concordant with a Biermann battery seeded field (B_0 = 10^(-20) Gauss) after 1 Gyr.
This study takes the first steps to study the evolution of the magnetic field in galaxies within a cosmological context and finds that amplification is already occurring with ~10pc resolution and turbulent star formation and dynamical SNe feedback models.
Primary author
Mr
Sergio Martin-Alvarez
(University of Oxford)
Co-authors
Prof.
Adrianne Slyz
(University of Oxford)
Prof.
Julien Devriendt
(University of Oxford)
