Three-dimensional simulations of the interstellar medium in dwarf galaxies - II. Galactic winds

Abstract

We study the hydrodynamical evolution of galactic winds in discy dwarf galaxies moving through an intergalactic medium (IGM). In agreement with previous investigations, we find that when the ram pressure stripping does not disrupt the interstellar medium (ISM), it usually has a negligible effect on the galactic wind dynamics. Only when the IGM ram pressure is comparable to the central ISM thermal pressure do the stripping and the superwind influence each other, increasing the gas removal rate. In this case, several parameters regulate the ISM ejection process, such as the original distribution of the ISM and the geometry of the IGM–galaxy interaction. When the ISM is not removed by the ram pressure or the wind, it loses memory of the starburst episode and recovers almost its pre-burst distribution in a time-scale of 50–200 Myr. After this time, another star formation episode becomes, in principle, possible. Evidently, galactic winds are consistent with the star formation history of recurrent bursts. Contrary to the ISM content, the amount of the metal-rich ejecta retained by the galaxy is more sensitive to the ram pressure action. Part of the ejecta is first trapped in a low-density, extraplanar gas produced by the IGM–ISM interaction, and then pushed back on to the galactic disc. The amount of trapped metals in a moving galaxy may be up to three times larger than in a galaxy at rest. This prediction may be tested by comparing metallicity of dwarf galaxies in nearby poor clusters or groups, such as Virgo or Fornax, with the field counterpart. The sensitivity of the metal entrapment efficiency on the geometry of the interaction may explain part of the observed scatter in the metallicity–luminosity relation for dwarf galaxies.