The Kinematics of the Disk-Halo Interaction in Spiral Galaxies

Moderateto high-velocity resolution optical spectroscopic observations of the gaseous halos of three galaxies (NGC 5775, NGC 891, and NGC 4302) have shown that they do not rotate cylindrically; rather, the rotation speed is observed to fall off roughly linearly with increasing height above the midplane. The measurements of these velocity gradients, which have magnitudes of approximately 8, 15, and 30 km/s/kpc, respectively, are briefly summarized. Correlations with other observables are presented. Signs point to the kinematics of gaseous halos being related to the star formation activity in the underlying disk, as well as to the distribution and morphology of the halo gas. Of particular interest is the apparent connection between the magnitude of the velocity gradient and the vertical scale height of the gas distribution: suggesting that the decrease in rotation speed per unit scale height may be a constant parameter of gaseous halos. Such a constant parameter may help constrain theoretical models of the disk-halo interaction in spiral galaxies. The implications of this possibility on the idea that gaseous halos are supported by disk star formation are briefly discussed.