  We discuss a model for treating chemical enrichment by SNII and SNIa
explosions in simulations of cosmological structure formation. Our model
includes metal-dependent radiative cooling and star formation in dense
collapsed gas clumps. Metals are returned into the diffuse interstellar medium
by star particles using a local SPH smoothing kernel. A variety of chemical
abundance patterns in enriched gas arise in our treatment owing to the
different yields and lifetimes of SNII and SNIa progenitor stars. In the case
of SNII chemical production, we adopt metal-dependent yields. Because of the
sensitive dependence of cooling rates on metallicity, enrichment of galactic
haloes with metals can in principle significantly alter subsequent gas infall
and the build up of the stellar components. Indeed, in simulations of isolated
galaxies we find that a consistent treatment of metal-dependent cooling
produces 25% more stars outside the central region than simulations with a
primordial cooling function. In the highly-enriched central regions, the
evolution of baryons is however not affected by metal cooling, because here the
gas is always dense enough to cool. A similar situation is found in
cosmological simulations because we include no strong feedback processes which
could spread metals over large distances and mix them into unenriched diffuse
gas. We demonstrate this explicitly with test simulations which adopt
super-solar cooling functions leading to large changes both in the stellar mass
and in the metal distributions. We also find that the impact of metallicity on
the star formation histories of galaxies may depend on their particular
evolutionary history. Our results hence emphasise the importance of feedback
processes for interpreting the cosmic metal enrichment.
