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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.