A Merger-driven Scenario for Cosmological Disk Galaxy Formation

The violent hierarchical nature of the Λ-Cold Dark Matter 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 highangular momentum mergers of systems that are gas-dominated, i.e. Mgas/(Mgas + M⋆) & 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. Nearly every simulated gas-rich merger remnant contains rapidlyrotating stellar substructure, while disk-dominated remnants are restricted to form in mergers that are gas-dominated at the time of final coalescence. Typically, gas-dominated mergers require extreme progenitor gas fractions (fgas > 0.8). We also show that the formation of rotationally-supported stellar systems in mergers is not restricted to idealized orbits, and both gas-rich major and minor mergers can produce disk-dominated stellar remnants. We suggest that the hierarchical nature of the Λ-Cold Dark Matter 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. Our proposed scenario may be especially important for galaxy formation at high redshifts, where gas-dominated mergers are believed to be more common than in the local Universe. Subject headings: galaxies: formation – galaxies: evolution