A twofold approach to magnetic fields in nearby galaxies

Magnetic fields are a fundamental component of galaxy evolution: they regulate star formation, shape interstellar turbulence, and channel feedback through galactic outflows; yet their amplification mechanisms and coupling to the multiphase interstellar medium (ISM) remain poorly constrained. This doctoral thesis investigates magnetic fields in nearby galaxies through two complementary but independent approaches: parsec-scale polarimetric observations that resolve magnetic geometry and depolarization in a nuclear starburst, and cosmological magnetohydrodynamical (MHD) simulations that quantify how magnetic energy scales with star formation and ISM conditions. We review large and small-scale dynamo theory, energy-equipartition arguments, and the main observational tracers (synchrotron and dust polarimetry, Faraday rotation, Zeeman splitting), establishing the physical and methodological context for the analyses that follow. We present full-Stokes ALMA polarimetry of the nuclear starburst in NGC~253 at Band~4 ($\approx145$\,GHz; $\approx25$ pc resolution) and Band~7 ($\approx345$\,GHz; $\approx5$\ pc resolution). Both bands reveal a consistent plane-of-sky magnetic geometry aligned with the base of the biconical outflow. The polarization fraction anti-correlates with column density and with polarization-angle dispersion, indicating depolarization dominated by field tangling on $\sim$25 pc scales, with additional effects emerging at $\sim$5 pc. Minima near super star clusters point to feedback-driven magnetic complexity. Using 19 zoom-in galaxies from the \textsc{Azahar} suite, we quantify magnetic, turbulent, thermal, and cosmic-ray energies and their links to star formation. The simulations yield near-universal scalings $B\propto\mathrm{SFR}^{0.2\text{--}0.3}$ and $B\propto\Sigma_{\mathrm{SFR}}^{1/3}$, consistent with supernova-driven, turbulence-regulated amplification. Neutral gas is typically turbulence-dominated, with magnetic and cosmic-ray energies approaching equipartition in vigorously star-forming systems. These trends match observed slopes and normalizations across spirals, dwarfs, and starbursts. These theoretical, observational, and numerical results converge toward a coherent picture in which magnetic fields act as active agents that both trace and regulate star formation and feedback across multiple scales and environments.