Evolution of the ISM of Starburst Galaxies and the formation of galactic superwinds

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. SN ejecta are transported out of the galaxies by such winds therefore affecting the chemical evolution of the galaxies. The effectiveness of the processes mentioned above depends on the heating efficiency (HE) of the SNe, and its value is still a matter of debate. We have developed a semi‐analytic model, considering the essential ingredients of a SB environment which is able to qualitatively trace the thermalisation history of the ISM in a SB region and determine the HE evolution. Our study has been also accompanied by fully 3‐D radiative cooling, hydrodynamical simulations of SNR‐SNR and SNR‐ clouds interactions. We have found that HE is very sensitive to the amount of ambient gas and clouds of the SB and may remain very small at least during part of the SB lifetime, therefore preventing or postponing the formation of a superwind. As long as the efficiency remains small, the cold gas remains confined to the system and can promote new generations of star‐formation, or increase the gas in‐fall to the central regions of the SB. As the ambient density decreases, the gas can finally heat and expand very rapidly and leave the galaxy as a superwind. A magneto‐centrifugal mechanism to accelerate and collimate these superwinds as a function of the SNe heating efficiency will be also discussed.