Galactoseismology: Discovery of Vertical Waves in the Galactic Disk
  We present evidence for a Galactic North-South asymmetry in the number
density and bulk velocity of solar neighborhood stars. The number density
profile, which is derived from main-sequence stars in the Sloan Digital Sky
Survey, shows a (North - South)/(North + South) deficit at |z| ~ 400 pc and an
excess at |z| ~ 800 pc. The bulk velocity profile, which is derived from the
Sloan Extension for Galactic Understanding and Exploration, shows a gradual
trend across the Galactic midplane as well as smaller-scale features. We
speculate that the North-South asymmetry, which has the appearance of a
wavelike perturbation, is intrinsic to the disk. We explore the physics of this
phenomenon through an analysis of the linearized Boltzmann and Poisson
equations and through one-dimensional simulations. The perturbation may be
excited by the passage of a satellite galaxy or dark matter subhalo through the
Galactic disk, in which case we are witnessing a recent disk-heating event.
