SN Heating Efficiency of the ISM of Starburst Galaxies

Abstract

The interstellar medium heated by supernova explosions (SN) may acquire an expansion velocity larger than the escape velocity and leave the galaxy through a supersonic wind. Galactic winds are effectively observed in many local starburst galaxies (Lehnert & Heckman 1996). The SN ejecta are transported out of the galaxies by such winds which must affect the chemical evolution of the galaxies. The effectiveness of the processes mentioned above depends on the heating efficiency (HE) of the SNs, i.e., the ratio between the kinetic plus internal energy density of the ambient gas and the SN energy density. In a starburst region, several SN explosions occur at a large rate inside a relatively small volume. If the successive generations of SN remnants (SNRs) interact with each other very fast, then a superbubble of high temperature and low density will rapidly develop, before a significant increase of the ambient gas density that could lead to substantial losses of energy by radiation. In this case, it is common to assume a value for HE of the order of unity, since most of the available energy of the SNs will be transferred to the ambient gas in the form of kinetic and internal energy, instead of being radiated away. However this assumption fails to reproduce both the chemical and dynamical characteristics of most starburst (SB) galaxies. In order to solve this paradigm, we have constructed a simple semi-analytical model, considering the essential ingredients of a SB environment, i.e., a three-phase medium composed by hot diffuse gas, SNRs and clouds, which is able to qualitatively trace the thermalisation history of the ISM in a SB region and determine the HE evolution (Melioli, de Gouveia Dal Pino, & D'Ercole, A&A, 2003, submitted). Our study has also been accompanied by fully 3-D radiative cooling, hydrodynamical simulations of SNR-SNR and SNR-clouds interactions (see Melioli, de Gouveia Dal Pino, & Raga 2003, in preparation).