Galactic winds are high speed flows of charged (and neutral) gas observed blowing out of galaxies.  Explosions of Supernovae and stellar winds are the most likely sources of the energy that drives these flows.  There are numerous examples of local galaxies showing clear signatures of large-scale outflows (Veilleux et al. 2005), but perhaps the best examples of large-scale outflows are at large redshifts (Pettini et al. 1998, 2001), mostly due to the larger star formation rates (and, consequently, of the larger rates of Supernova explosions).

Given their shallow potential well, dwarf galaxies are supposed to be more prone to the formation of galactic winds.  Through these winds, a large fraction of heavy elements freshly produced in the stellar populations of the galaxy can be lost.  The consequences for the chemical evolution can be very significant (the most important being that these lost elements cannot contribute to the chemical enrichment of the galaxy, whose metallicity remains low).

As an example of the development of galactic winds in dwarf galaxies, Figure 1 shows the time evolution of a galaxy model in which clouds are constantly falling towards the center of a galaxy. These clouds delay the development of galactic winds but do not prevent it.

The development of galactic winds and the fate of freshly produced metals can be better analyzed by means of a systematic study of the effect of initial set-ups, star formation recipes and environmental effects.  This study is underway in our group.  It turns out that the most significant parameters affecting the evolution of a galaxy are the initial mass and initial gas distribution (geometry).  The mass determines the depth of the potential well, therefore low-mass objects can develop galactic winds more easily.  In a disk-like galaxy, the pressure gradient perpendicular to the disk is much steeper than the gradient along the disk.  The galactic wind will develop preferentially along this direction and the horizontal transport of matter can be very limited.  Consequently, such a galaxy can expel a large fraction of the freshly produced heavy elements but most of the disk gas will remain bound to the galaxy.  In a roundish object the development of galactic wind is less likely (the pressure gradients, along any direction, are not that steep).  Figure 2 shows the gas density maps for 9 model galaxies differing on the initial baryonic mass and the initial geometry.  The effect of mass and geometry on the development of galactic wind can be noticed from this figure.