Gas accretion in galactic disks

Abstract: Evidence for the accretion of material in spiral galaxies has grown over the past years and clear signatures can be found in HI observations of galaxies. We describe here new detailed and sensitive HI synthesis observations of a few nearby galaxies (NGC 3359, NGC 4565 and NGC 6946) which show that indeed accretion of small amounts of gas is taking place. These should be regarded as examples illustrating a general phenomenon of gas infall in galaxies. Such accretion may also be at the origin of the gaseous halos which are being found around spirals. Probably it is the same kind of phenomenon of material infall as observed in the stellar streams in the halo and outer parts of our galaxy and M 31