Chemical Abundance Ratios and Gas-phase Mixing in Dwarf Galaxy Evolution

Because of their low gravitational energies dwarf galaxies are greatly exposed to energetical influences by the interstellar medium, like e.g. stellar radiation, winds or explosions, or by their environment. While the metallicity depletion in dwarf galaxies can be explained in general by supernova-driven galactic winds, the reason for their low N/O ratios at low O abundance is not yet completely understood. Stellar yields enrich the different gas phases with elements that are characteristic for the stellar progenitors. Phase transitions are necessary for a mixing of elements, but depend sensitively on the thermal and dynamical state of the interstellar medium. Models of chemical evolution start usually with a high N/O ratio at low O abundance according to a metal enrichment by ancient stellar populations with common yields, but can only reproduce the N/O-O peculiarity by the application of multiple starbursts. Their galactic winds are invoked to reduce O selectively. Chemodynamical models of dwarf galaxies, however, demonstrate that strong evaporation of clouds by the hot supernova gas leads to an almost perfect mixing of the interstellar gas. These models can successfully account for the observed N/O-O values in a self-consistent way without the necessity of starbursts, if new stellar yields are taken into account which provide additional secondary N production from massive stars.