  We have modelled the observed color-magnitude diagram (CMD) at one location
in M33's outskirts under the framework of a simple chemical evolution scenario
which adopts instantaneous and delayed recycling for the nucleosynthetic
products of Type II and Ia supernovae. In this scenario, interstellar gas forms
stars at a rate modulated by the Kennicutt-Schmidt relation and gas outflow
occurs at a rate proportional to the star formation rate (SFR). With this
approach, we put broad constraints on the role of gas flows during this
region's evolution and compare its [alpha/Fe] vs. [Fe/H] relation with that of
other Local Group systems. We find that models with gas inflow are
significantly better than the closed box model at reproducing the observed
distribution of stars in the CMD. The best models have a majority of gas inflow
taking place in the last 7 Gyr, and relatively little in the last 3 Gyr. These
models predict most stars in this region to have [alpha/Fe] ratios lower than
the bulk of the Milky Way's halo. The predictions for the present-day SFR, gas
mass, and oxygen abundance compare favorably to independent empirical
estimates. Our results paint a picture in which M33's outer disc formed from
the protracted inflow of gas over several Gyr with at least half of the total
inflow occurring since z ~ 1.

Astrophysics