  (Abridged) The violent hierarchical nature of the LCDM cosmology poses
serious difficulties for the formation of disk galaxies. To help resolve these
issues, we describe a new, merger-driven scenario for the cosmological
formation of disk galaxies at high redshifts that supplements the standard
model based on dissipational collapse.In this picture, large gaseous disks may
be produced from high-angular momentum mergers of systems that are
gas-dominated, i.e. M_gas/(M_gas +M_star > 0.5 at the height of the merger.
Pressurization from the multiphase structure of the interstellar medium
prevents the complete conversion of gas into stars during the merger, and if
enough gas remains to form a disk, the remnant eventually resembles a disk
galaxy. We perform numerical simulations of galaxy mergers to study how
supernovae feedback strength, supermassive black hole growth and feedback,
progenitor gas fraction, merger mass-ratio, and orbital geometry impact the
formation of remnant disks. We find that disks can build angular momentum
through mergers and the degree of rotational support of the baryons in the
merger remnant is primarily related to feedback processes associated with star
formation. Disk-dominated remnants are restricted to form in mergers that are
gas-dominated at the time of final coalescence and gas-dominated mergers
typically require extreme progenitor gas fractions (>80%). We also show that
the formation of rotationally-supported stellar systems in mergers is not
restricted to idealized orbits, or major or minor mergers. We suggest that the
hierarchical nature of the LCDM cosmology and the physics of the interstellar
gas may act together to form spiral galaxies by building the angular momentum
of disks through early, gas-dominated mergers.
