Little is known about how primordial magnetic fields are generated nor about their initial configurations. Their exact primordial normalization, coherence length and spectral index are undetermined but they are widely believed to be many orders of magnitude lower in strength (3 x 10-20 G < B0 < 3 x 10-10 G) than their microGauss values in present-day galaxies. What is known is that magnetic fields play as large a role in galaxies as turbulence, thermal and cosmic ray energy densities and that therefore they have a crucial role to play in shaping a galaxy’s evolution. To understand how different configurations of primordial magnetic fields might affect the global morphological and dynamical properties of galaxies, we used DiRAC to perform a suite of high-resolution constrained transport magneto-hydrodynamic cosmological zoom simulations where we varied the initial magnetic field strength and configuration along with the prescription for stellar feedback. In Martin-Alvarez et al, 2020, MNRAS, 495, 4475 we report our findings that strong primordial magnetic fields delay the onset of star formation and drain the rotational support of the galaxy, diminishing the radial size of the galactic disk and driving a higher amount of gas towards the centre. A possible mechanism behind such a reduction in angular momentum is magnetic braking. The figure above shows the effect of increasing primordial magnetic field strength on the stars and gas of a galaxy from our simulations. For the strongest primordial magnetic field studied (B0 = 3 x 10-10 G ), the gas radial scale length is halved compared to the simulation where the primordial magnetic field is weakest. New Scientist magazine reported on our work: https://www.newscientist.com/article/2244840-the-milky-way-may-have-been-shrunk-down-by-ancient-magnetic-fields/
