Delayed metal recycling in galaxies: the inefficiency of cold gas
  enrichment in colliding supershell simulations
  The fate of metals ejected by young OB associations into the Interstellar
Medium (ISM) is investigated numerically. In particular, we study the
enrichment of the cold gas phase, which is the material that forms molecular
clouds. Following previous work, the expansion and collision of two supershells
in a diffuse ISM is simulated, in this case also introducing an advected
quantity which represents the metals expelled by the young stars. We adopt the
simplest possible approach, not differentiating between metals coming from
stellar winds and those coming from supernovae. Even though the hot, diffuse
phase of the ISM receives a significant amount of metals from the stars, the
cold phase is efficiently shielded, with very little metal enrichment.
Significant enrichment of the cold ISM will therefore be delayed by at least
the cooling time of this hot phase. No variations in cloud metallicity with
distance from the OB association or with direction are found, which means that
the shell collision does little to enhance the metallicity of the cold clumps.
We conclude that the stellar generation that forms out of molecular structures,
triggered by shell collisions cannot be significantly enriched.
