How primordial magnetic fields shrink galaxies
  As one of the prime contributors to the interstellar medium energy budget,
magnetic fields naturally play a part in shaping the evolution of galaxies.
Galactic magnetic fields can originate from strong primordial magnetic fields
provided these latter remain below current observational upper limits. To
understand how such magnetic fields would affect the global morphological and
dynamical properties of galaxies, we use a suite of high-resolution constrained
transport magneto-hydrodynamic cosmological zoom simulations where we vary the
initial magnetic field strength and configuration along with the prescription
for stellar feedback. We find 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. This is also reflected in mock UVJ observations by an
increase in the light profile concentration of the galaxy. We explore the
possible mechanisms behind such a reduction in angular momentum, focusing on
magnetic braking. Finally, noticing that the effects of primordial magnetic
fields are amplified in the presence of stellar feedback, we briefly discuss
whether the changes we measure would also be expected for galactic magnetic
fields of non-primordial origin.
