1303.5586 (Jian Fu et al.)

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

Jian Fu, Guinevere Kauffmann, Meiling Huang, Robert M. Yates, Sean Moran, Timothy M. Heckman, Romeel Davé, Qi Guo

We updated our radially-resolved SAMs of galaxy formation to track the radial distribution of stars, metals, atomic and molecular gas in galactic disks. The models are run on both MS and MS II using the recipes outlined in Fu et al. (2010) and Guo et al. (2011), with 3 main changes: (1) We adopt a simple star formation law where \Sigma_SFR \propto \Sigma_H2. (2) We inject the heavy elements produced by SNe directly into the halo, instead of first mixing them with the disk cold gas. (3) We include radial gas inflows in disks using a model of the form v_inflow=alpha r. The average \Sigma_H2 profiles in L_* galaxies strongly constrains the inflow velocities, favoring models where v_inflow ~ 7 km/s at r=10 kpc. The radial inflow model has little influence on the gas and stellar metallicity gradients in the outer disks. Gas metallicity gradients are affected much more strongly by the fraction of metals that are directly injected into the halo gas, rather than mixed with the interstellar cold gas. Metals ejected out of the galaxy at early epochs result in late infall of pre-enriched gas and flatter present-day gas metallicity gradients. A prescription in which 80% of the metals produced by stars are injected into the halo gas provides the best fit to the relatively flat observed metallicity gradients of galaxies with stellar masses greater than 10^10 M_sun. Such a prescription also results in a good fit to the relation between gas metallicity and sSFR in the outer parts of disks. We examine the correlation between gas metallicity gradient and some global galaxy properties, finding that it is most strongly correlated with the B/T ratio of the galaxy. This is because gas is consumed when the bulge forms during the galaxy merger, and the gas metallicity gradient is then set by newly-accreted gas. These model predictions appear to be in good agreement with observations from Moran et al. (2012).

View original: http://arxiv.org/abs/1303.5586