High velocity HI in NGC 6946 and extra-planar gas in NGC 253
  Multi-wavelength observations of nearby spiral galaxies have shown that neutral and ionized gas are present up to a few kpc from the disk and that star formation and supernovae probably play an important role in bringing gas into the halo. We have obtained very sensitive HI observations of the face-on galaxy NGC 6946 and of the nearly edge-on starburst galaxy NGC 253. We find high velocity HI clouds in NGC 6946 and extra-planar gas with anomalous velocities in NGC 253. In both galaxies there seems to be a close connection between the star-forming disk and the halo HI. In the outer parts of NGC 6946 there is also evidence for recent gas accretion. 
Rense Boomsma (1), Tom Oosterloo (2), Filippo Fraternali (3), Thijs   van der Hulst (1), Renzo Sancisi (4 and 1) ((1) Kaptey...