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Effects of Starbursts on the Structure of Young Galaxies

We have investigated the effects of primeval starbursts on the galactic structure, showing that the dynamical structure of young galaxies is changed by relaxation of a stellar system formed by a starburst-driven outflowing gas (superwind). When a superwind interacts with halo gas, an expanding dense supershell of shocked gas is formed. Using a similarity solution for the expanding shell and a condition for the gravitational instability of a gaseous shell, we show that an expanding gaseous shell with a mass of several $10^{10} M_\odot$ and a radius of several kpc becomes gravitationally unstable to form stars. A stellar shell is thus formed, relaxes and will evolve into a fat stellar system. \par In order to investigate the fate of the stellar shell and its dynamical influence on the host galaxy, we performed three dimensional $N$-body simulations of a stellar shell plus disk system. The evolution of the stellar shell was computed under two types of environment: an external flattened potential and a live stellar disk. We find that the relaxation process and resultant structure are significantly affected by the external disk potential, and also depend on the velocity dispersion of the initial shell stars and the mass of the shell. The final galactic structure ranges from a thick disk and a compact bulge with a high density core to a thin disk with a diffuse bulge. Isodensity maps of the computed disk plus bulge system resemble isophotes of S0 galaxies. We suggest that the effects of primeval starbursts would explain some observational properties of S0 galaxies.