THE DESTRUCTION OF THIN STELLAR DISKS VIA COSMOLOGICALLY COMMON SATELLITE ACCRETION EVENTS

Most Galaxy-sized systems (M(host) similar or equal to 10(12) M(circle dot)) in the Lambda CDM cosmology are expected to have interacted with at least one satellite with a total mass M(sat) similar or equal to 10(11) M(circle dot) similar or equal to 3 M(disk) in the past 8 Gyr. Analytic and numerical investigations suggest that this is the most precarious type of accretion 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 satellite accretion 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 a Lambda CDM 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.