Neutral hydrogen observations of the nearby , edge-on spiral galaxy NGC 891 reveal the presence of an \ion H1 halo extending up to at least 5 kpc from the plane .
This halo gas appears to rotate 25 to 100 km/s 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 \ion H1 layer of NGC 891 may no longer be valid .

A correlation is seen between the distributions of \ion H1 , H \alpha and radio continuum emission , which supports , in accordance with galactic fountain models , the picture of a substantial disk-halo circulation related to the star formation activity in the disk of NGC 891 .

There is now also clear evidence for the presence of a rapidly rotating ( v _ { rot } \simeq 230 km/s ) disk or ring of \ion H1 in the central part of NGC 891 .