  Most Galaxy-sized systems (M_host ~ 10^12 M_sun) in the LCDM cosmology are
expected to have accreted at least one satellite with a total mass M_sat ~
10^11 M_sun = 3M_disk in the past 8 Gyr. Analytic and numerical investigations
suggest that this is the most precarious type of merger for the survival of
thin galactic disks because more massive accretion events are relatively rare
and less massive ones preserve thin disk components. We use high-resolution,
dissipationless N-body simulations to study the response of an initially-thin,
fully-formed Milky-Way type stellar disk to these cosmologically common events
and show that the thin disk does not survive. Regardless of orbital
configuration, the impacts transform the disks into structures that are roughly
three times as thick and more than twice as kinematically hot as the observed
dominant thin disk component of the Milky Way. We conclude that if the Galactic
thin disk is a representative case, then the presence of a stabilizing gas
component is the only recourse for explaining the preponderance of disk
galaxies in an LCDM universe; otherwise, the disk of the Milky Way must be
uncommonly cold and thin for its luminosity, perhaps as a consequence of an
unusually quiescent accretion history.
