The formation of galaxy discs

Galaxy disc formation must incorporate the multiphase nature of the interst ellar medium. The resulting two-phase structure is generated and maintained by gravitational instability and supernova energy input, which yield a sou rce of turbulent viscosity that is able to compete effectively in the proto disc phase with early angular momentum loss of the baryonic component via d ynamical friction in the dark halo. Provided that star formation occurs on the viscous drag time-scale, this mechanism provides a means of accounting for disc sizes and radial profiles. The star formation feedback is self-reg ulated by turbulent gas pressure limited percolation of the supernova remna nt heated hot phase, but can run away in gas-rich protodiscs to generate co mpact starbursts. A simple analytic model is derived for a Schmidt-like glo bal star formation law in terms of the cold gas volume density.