The cold gaseous halo of NGC 891

Abstract

We present HI observations of the edge-on galaxy NGC 891. These are among the deepest ever performed on an external galaxy. They reveal a huge gaseous halo, much more extended than seen previously and containing almost 30 % of the HI. This HI halo shows structures on various scales. On one side, there is a filament extending (in projection) up to 22 kpc vertically from the disk. Small (M_HI ~ 10^6 Msol) halo clouds, some with forbidden (apparently counter-rotating) velocities, are also detected. The overall kinematics of the halo gas is characterized by differential rotation lagging with respect to that of the disk. The lag, more pronounced at small radii, increases with height from the plane. There is evidence that a significant fraction of the halo is due to a galactic fountain. Accretion from intergalactic space may also play a role in building up the halo and providing low angular momentum material needed to account for the observed rotation lag. The long HI filament and the counter-rotating clouds may be direct evidence of such accretion.