The MAGPI survey: Stellar population radial trends and mass assembly in star-forming galaxies at z~0.3

Context . The evolution of galaxies from cosmic noon to the present day provides a key window through which to probe the balance between early, rapid bulge formation and prolonged disk growth. In this context, the epoch at z ~ 0.3 (intermediate redshift) marks a crucial transitional phase between the peak of cosmic star formation and the predominantly quiescent local Universe, where these processes can be directly observed. Aims . In this work, we examine the spatially resolved stellar populations of 34 galaxies at z ~ 0.3 to quantify radial gradients in age, stellar metallicity, and star formation activity, and disentangle the distinct evolutionary pathways of inner and outer galactic components. Methods . We utilised MUSE integral-field spectroscopy data cubes from the MAGPI survey at redshifts of 0.28 < z < 0.35. Stellar population properties were derived using the spectral synthesis codes FADO and Starlight , and radial profiles were constructed by fitting isophotal annuli to the galaxy continuum emission, thereby preserving galaxy morphology and enabling the separation between components. We further reconstructed star formation histories and cumulative mass assembly curves for inner and outer regions. Results . We find pronounced negative radial gradients in age and negative to flat gradients in stellar metallicity. Inner regions are systematically older and more metal-rich than their surrounding outskirts, with age differences of up to 3–4 Gyr in the most massive systems. H α equivalent width profiles reveal centrally suppressed specific star formation in most galaxies, consistent with inside-out quenching. Star formation histories and mass assembly curves demonstrate that galaxy cores formed 80% of their stellar mass rapidly, within the first 2–3 Gyr of cosmic time; while areas outside 1 R eff assembled more gradually and sustained star formation to later epochs. These findings support a scenario in which early dissipative processes and mergers dominate inner formation. At the same time, outskirts evolve primarily through extended, secular star formation, establishing the centrally concentrated quenching and inside-out growth that link high-redshift systems to the quiescent galaxies of the local Universe.