  We present results of a study of large-scale neutral hydrogen (HI) gas in
nearby radio galaxies. We find that the early-type host galaxies of different
types of radio sources (compact, FR-I and FR-II) appear to contain
fundamentally different large-scale HI properties: enormous regular rotating
disks and rings are present around the host galaxies of a significant fraction
of low power compact radio sources, while no large-scale HI is detected in low
power, edge-darkened FR-I radio galaxies. Preliminary results of a study of
nearby powerful, edge-brightened FR-II radio galaxies show that these systems
generally contain significant amounts of large-scale HI, often distributed in
tail- or bridge-like structures, indicative of a recent galaxy merger or
collision. Our results suggest that different types of radio galaxies may have
a different formation history, which could be related to a difference in the
triggering mechanism of the radio source. If confirmed by larger studies with
the next generation radio telescopes, this would be in agreement with previous
optical studies that suggest that powerful FR-II radio sources are likely
triggered by galaxy mergers and collisions, while the lower power FR-I sources
are fed in other ways (e.g. through the accretion of hot IGM). The giant HI
disks/rings associated with some compact sources could - at least in some cases
- be the relics of much more advanced mergers.

Astrophysics