The H i Halo of NGC 891

Abstract

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 di †erence is due to the gradient in the gravita-tional potential, then it may serve to discriminate between disk and spheroidal mass models. The classic picture of a large outer Ñare in the H I layer of NGC 891 may no longer be valid. A correlation is seen between the distributions of H I, Ha 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 km s~1) disk or ring (v rot ^ 230 of H I in the central part of NGC 891.