Four phases of angular-momentum buildup in high-z galaxies:from cosmic-web streams to an extended tilted ring, disc and bulge

We study the buildup of angular momentum (AM) in high-z galaxies using zoom-in hydro-cosmological simulations. The disc AM originates in a few co-planar streams of cold gas and merging galaxies tracing filaments of the cosmic web and undergo 4 phases of evolution. In phase I, outside the halo virial radius (Rv), the elongated streams gain AM by tidal torques with a specific AM (sAM) ∼ 1.7 times that of the dark matter (DM) due to the gas’ higher quadrupole moment. This AM is expressed as stream impact parameters, from ∼0.3Rv to occasional counter rotation. In phase II, in the outer halo, while the incoming DM mixes with the existing halo of lower sAM to a spin λdm∼0.04, the cold streams transport the AM to the inner halo such that their spin in the halo is ∼ 3λdm. In phase III, near pericenter, the streams dissipate and form a non-uniform, rotating ring extending to ∼ 0.3Rv and tilted relative to the inner disc. Torques exerted partly by the disc make the gas ring lose AM, spiral in, and settle into the disc within one orbit. The ring is observable with 30% probability as a damped Lyman-α absorber. In phase IV, within the disc, torques associated with violent disc instability drive AM out and baryons in to a central bulge, while outflows remove low-spin gas. Despite the different AM histories of gas and dark matter, the spin of the disc is only moderately smaller than that of the DM halo.