Large-scale HI in nearby radio galaxies (II): the nature of classical
  low-power radio sources
  An important aspect of solving the long-standing question as to what triggers
various types of Active Galactic Nuclei involves a thorough understanding of
the overall properties and formation history of their host galaxies. This is
the second in a series of papers that systematically study the large-scale
properties of cold neutral hydrogen (HI) gas in nearby radio galaxies. The main
goal is to investigate the importance of gas-rich galaxy mergers and
interactions among radio-loud AGN. In this paper we present results of a
complete sample of classical low-power radio galaxies. We find that extended
Fanaroff & Riley type-I radio sources are generally not associated with
gas-rich galaxy mergers or ongoing violent interactions, but occur in
early-type galaxies without large (> 10^8 M_sun) amounts of extended neutral
hydrogen gas. In contrast, enormous discs/rings of HI gas (with sizes up to 190
kpc and masses up to 2 x 10^10 M_sun) are detected around the host galaxies of
a significant fraction of the compact radio sources in our sample. This
segregation in HI mass with radio source size likely indicates that these
compact radio sources are either confined by large amounts of gas in the
central region, or that their fuelling is inefficient and different from the
fuelling process of classical FR-I radio sources. To first order, the overall
HI properties of our complete sample (detection rate, mass and morphology)
appear similar to those of radio-quiet early-type galaxies. If confirmed by
better statistics, this would imply that low-power radio-AGN activity may be a
short and recurrent phase that occurs at some point during the lifetime of many
early-type galaxies.
