Galaxy Formation at its Peak

The way galaxies form and evolve is a major open question in cosmology. Rapid progress is made based on observations and simulations of galaxies during the peak of their assembly and star-formation activity, in the first few billion years of cosmic evolution. The emerging picture highlights the formation of galaxies at the nodes of the cosmic web. These galaxies are steadily fed by cold streams along dark-matter filaments. The streams, including a smooth component and merging galaxies, penetrate through hot gas in dark-matter halos to form extended, turbulent, rotating disks and spheroidal bulges with central black holes. The streams determine the disc structure by transporting angular momentum from large distances into the disk. The intense gas input drives a self-regulated, violent gravitational disk instability with giant clumps where stars form rapidly, without appealing to the rare major mergers between galaxies. The violent instability induces clump migration and gas inflow to the centre, complementing mergers as a mechanism for spheroid formation and for feeding the central black hole. We address the implications of this developing picture on different aspects of galaxy formation theory and its observable features.