Gas accretion and star formation in disk galaxies

Numerical simulations predict that metal-poor gas accretion from the cosmic web fuels the
formation of disk galaxies. My presentation will summarize observational evidence
for this gas accretion process driving star formation. In particular, I will discuss
a number of properties characterizing large samples of star-forming galaxies that
can be explained by metal-poor gas accretion (e.g., the fundamental metallicity relation,
or the relationship between galaxy morphology and metallicity). Some properties of the
(neutral and ionized) gas around galaxies will be examined too. I will also review the
properties of the extremely metal poor galaxies. They show chemical and dynamical
inhomogeneities that can be naturally explained as due to star-formation feeding from
infall of external metal-poor gas.