Gas and Metal Contents of Galaxies and Gaseous Halos: Preventive versus
  Ejective Feedback
  Using a semi-analytical approach we investigate the characteristics of
predictions for the masses and metallicities of the baryonic matter in and
around galaxies made by three galaxy formation models. These models represent
three different feedback scenarios: one model with purely ejective feedback,
one model with ejective feedback with reincorporation of ejected gas, and one
preventative model. We find that, when the model parameters are adjusted to
predict the correct stellar masses for a range of halo masses between 10^{10}
to 10^{12}Msun, these three scenarios have very different predictions for the
masses and metallicities of the interstellar and circum-galactic media.
Compared with current observational data, the model implementing preventative
feedback has a large freedom to match a broad range of observational data,
while the ejective models have difficulties to match a number of observational
constraints simultaneously, independent of how the ejection and reincorporation
are implemented. Our results suggest that the feedback process which regulates
the amounts of stars and cold gas in low-mass galaxies is preventative in
nature.
