The Evolution of Galactic Fountains

Abstract

"Sem resumo feito pelo autor"; - Gas that escapes from the Galactic disk, rises into the halo, cools and falls back, constitutes a "Galactic fountain". The Galactic fountain includes the outflows that result from isolated as well as clustered supernovae scattered in the Galactic disk, which respectively generate large scale as well as localized outbursts. Up to now such outflows have been considered independently. However, it is proposed here that such independent effects are in fact the result of a global mechanism occuring within the Galactic disk. This mechanism has been studied by means of a three-dimensional hydrodynamical Godunov class scheme. The scheme uses a third order polynomial interpolation algorithm to solve the Riemann problem at the cells interfaces. It allows for radiative cooling and uses a local uniform regridding technique, which automatically refines the flow regions where steep, variations of pressure occur. The numerical results presented here show that the Galactic fountain is formed by a turbulent conective flow that rises into the halo. HI clouds result from Rayleigh-Taylor instabilities at the interface of cool descending gas with the ascending warmer gas, these clouds possibly being identified as the HI complexes previously observed in the halo. Some of these clouds may in turn be disrupted by energetic fountain gas that comes from below and especially from chimneys. These have been supposed to be generated by a large number of type II supernovae. However, the results presented here show that two sucessive supernovae can on their own produce a similar effect. Abstract of the Ph.D. Thesis submitted by Miguel A. de Avillez to the University of Évora and entitled "The Evolution of Galactic Fountains".