Can the nebular theory be extended to explain the formation of spiral galaxies?
You’re basically right. But galaxy formation differs somewhat from star formation, in that the galaxy is mostly dark matter, and the dynamics of the dark matter dominate. The baryonic matter just “comes along for the ride” until it compresses and shocks. There are two different scenarios for different masses of dark matter halos: adiabatic and isothermal. In smaller galaxies, the infalling gas tends to be isothermal, that is, it cools through radiation as it collapses. In bigger galaxies, the infalling gas heats adiabatically, which mitigates the collapse. The black hole probably forms after the initial formation of the galaxy, and energy from the black hole likely feeds back and modifies the galaxy formation process.
