This work is a theoretical investigation of the role of collisions between protogalactic clouds of gas and stars in the formation of galaxies. We assume that galaxy formation begins sometime after recombination in the standard hot big bang cosmology, and that the galaxies form from a turbulent and fragmented gas consisting of primordial hydrogen and helium. The first part of this study consists of detailed numerical models of gas cloud collisions in the earliest stages of galaxy formation. In the second part more schematic analytic models of the formation of the global properties of elliptical galaxies are constructed. The numerical hydrodynamic simulations explicitly calculate the effects of heating, cooling, ionization, recombination, gravity, molecule formation and destruction. Magnetic fields have not been included. In these calculations the high compression and efficient atomic lime cooling create a thin, high-density layer, an ideal environment for the formation of a small amount of molecular hydrogen via the two-step associative detachment reaction. In the absence of a strong external flux of molecule dissociating photons, molecular hydrogen line radiation further cools the gas before the end of the collision. The cloud temperature is then more than an order of magnitude lower than the pre-collision temperature. The clouds will be subject to a strong gravitational instability. The ensuing collapse and fragmentation probably trigger early star formation in galaxies. On a larger scale, elliptical galaxy formation can be modeled analytically as a series of idealized mergers between protogalactic subunits. Assuming physically reasonable amounts of star formation and gaseous energy dissipation result from each merger, correlations between the mass, average metallicity and velocity dispersion of ellipticals are predicted by these models. In particular, the models account for the observed luminosity - velocity dispersion relation in ellipticals as a result of the dissipation in mergers. These merger models are based on a number of rather ad hoc assumptions, but their success in accounting for the observed correlations between the average properties of ellipticals as the result of an evolutionary buildup is encouraging.
- Pub Date:
- June 1981
- Physics: Astronomy and Astrophysics