  Cosmological simulations predict that during the evolution of galaxies, the
specific star formation rate continuously decreases. In a previous study we
showed that generally this is not caused by the galaxies running out of cold
gas but rather a decrease in the fraction of gas capable of forming stars. To
investigate the origin of this behavior, we use disk galaxies selected from the
cosmological hydrodynamical simulation Magneticum Pathfinder and follow their
evolution in time. We find that the mean density of the cold gas regions
decreases with time. This is caused by the fact that during the evolution of
the galaxies, the star-forming regions move to larger galactic radii, where the
gas density is lower. This supports the idea of inside-out growth of disk
galaxies.
