On the Kinematics of Extraplanar Gas

A review of the observational constraints and theoretical models of extra-planar rotation (lagging halos) is presented. It is becoming clear that lagging halos, with vertical gradients in rotation speeds ranging from dvΘ/dz = −8to − 60km s−1kpc−1, are a ubiquitous phenomenon. The observational data so far does not allow one to say definitively whether this number changes with radius or with vertical height in a galaxy, but it is pointed out that such measurements, along with constraints on the presence of radial flows, will provide a crucial test that may allow one to distinguish between the different physical models that produce lagging halos. Some challenges in kinematically distinguishing between Galactic circulation and extragalactic accretion are also discussed.