Effects of the Cooling Epoch on Formation of Spiral Galaxies With High Angular Momenta

While large-scale cosmological simulations indicate that regions with typical galaxy masses collapse around z=5, these regions are clumpy, suggesting that smooth collapse is rare. The ensuing evolution is composed of several merger events in which often less than 50% of the final mass has been assembled by z=1. In previous simulations of galaxy formation in hierarchically clustering universes, gas can form a thin disk. In subsequent mergers, gas disks are disrupted but efficient conversion of kinetic to thermal energy and rapid cooling allows a new disk to form in a short time. However, due to the transportation of angular momentum outward into the halo during merging, these gas disks have angular momenta some two orders of magnitude below those of observed spirals. When efficient star formation is included, stars are produced before the dominant object assembles most of its mass. When merging occurs, the stars form a spheroid with no strong disk structure or rotation. We present numerical simulations from a code which evolves a high-resolution mixture of dark matter, gas, and stars in a CDM universe. Our intention was to determine whether spiral galaxies with angular momenta similar to those observed could be produced for realistic radiative cooling epochs. Because the effects of feedback from supernovae explosions are not well understood, an alternative to applying empirical prescriptions was used. In order to mimic the effects of transfering thermal and kinetic energy from supernovae into the gas, radiative cooling and star formation were not switched on until z=1. Additional simulations were performed with the cooling initiated at z=4 and z=0.6, and using an empirical formula that suppresses cooling. In contrast with previous simulations, we produce stellar disks with angular momenta at z=0 comparable to those of real spirals. To survive, the disks of progenitor spiral galaxies require a relatively quiescent period of evolution after the majority of stars have formed. These galaxies must either be in uncommonly smooth regions or efficient star formation must begin after merging has nearly ceased. Our results suggest that only haloes in which a significant amount of mass has been assembled before the gas cools and forms stars can produce robust spirals. A delay in cooling allows formation of disks with high J/M.