Abstract
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 \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).
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