The H I halo of NGC 891

Neutral hydrogen observations of the nearby, edge-on spiral galaxy NGC 891 reveal the presence of an H I halo extending up to at least 5 kpc from the plane. This halo gas appears to rotate 25 to 100 km s(-1) more slowly than the gas in the plane. If this velocity difference is due to the gradient in the gravitational potential, then it may serve to discriminate between disk and spheroidal mass models. The classic picture of a large outer flare in the H I layer of NGC 891 may no longer be valid. A correlation is seen between the distributions of H I, H alpha and radio continuum emission; this, in accordance with galactic fountain models, supports the picture of a substantial disk-halo circulation related to star formation activity in the disk of NGC 891. There is now also clear evidence for the presence of a rapidly rotating (upsilon(rot) similar or equal to 230 km s(-1)) disk or ring of H I in the central part of NGC 891.