The Impact of Mergers on the Survival and Abundance of Disk-Dominated
  Galaxies
  We study the formation of disk-dominated galaxies in a LCDM universe. Their
existence is considered to be a challenge for the LCDM cosmology, because
galaxy mergers isotropize stellar disks and trigger angular momentum transport
in gas disks, thus fostering the formation of central stellar spheroids. Here,
we postulate that the formation of stellar spheroids from gasrich disks is
controlled by two parameters that characterize galaxy mergers, the mass ratio
of merging dark matter halos, and the virial velocity of the larger merging
halo. We utilize merger histories generated from realizations of the
cosmological density field to calculate the fraction of dark matter halos that
have avoided spheroid formation, and compare the derived statistics with the
spheroid occupation fractions in surveys of nearby galaxies. We find, for
example, that the survival rate of disk-dominated galaxies in LCDM is just high
enough to explain the observed fractional representation of disk-dominated
galaxies in the universe if the only mergers which lead to central spheroid
formation are those with mass ratios M2/M1 > 0.3 and virial velocities Vvir,1 >
55 km/s. We discuss the physical origin of this criterion, and show that the
dependence of the disk-dominated fraction on galaxy mass provides a further
test of the merger hypothesis. [For additional details, see, Koda et al.
(2009).]
