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We have updated our radially-resolved SAMs of galaxy formation, which trackboth the atomic and molecular gas phases of the ISM. The models are adaptedfrom those of Guo et al. using similar methodology as in Fu et al. and are runon halo merger trees from the MS and MS II with the following main changes: (1)We adopt a simple star formation law where \Sigma_SFR \propto \Sigma_H2 (2) Weinject the heavy elements produced by supernovae directly into the halo hotgas, instead of first mixing them with the cold gas in the disk. (3) We includeradial gas inflows in disks using a model of the form v_inflow = \alpha r. Themodels are used to study the radial profiles of star formation rate andgas-phase metallicity in present-day galaxies. The \Sigma_H2 profiles in L*galaxies place strong constraints on inflow velocities, favouring models wherev_inflow~7km/s at a galactocentric radius of 10kpc. Radial gas inflow haslittle influence on gas-phase and stellar metallicity gradients, which areaffected much more strongly by the fraction of metals that are directlyinjected into the halo gas, rather than mixed with the cold gas. Metals ejectedout of the galaxy in early epochs result in late infall of pre-enriched gas andflatter present-day gas-phase metallicity gradients. A prescription in which80% of the metals are injected into the halo gas results in good fits to theflat observed metallicity gradients in galaxies with stellar masses greaterthan 10^10 M_sun, as well as the relations between gas-phase metallicity andsSFR in the outer parts of galactic disks. We examine the correlation betweengas-phase metallicity gradient and global galaxy properties, finding that it ismost strongly correlated with the bulge-to-total ratio of the galaxy. This isbecause gas is consumed when the bulge forms during galaxy mergers, and thegas-phase metallicity gradient is then set by newly-accreted gas.

Star Formation and Metallicity Gradients in Semi-analytic Models of Disk Galaxy Formation