A rare example of low surface-brightness radio lobes in a gas-rich
  early-type galaxy: the story of NGC 3998
  We study the nearby lenticular galaxy NGC 3998. This galaxy is known to host
a low-power radio AGN with a kpc-size one-sided jet and a large, nearly polar
HI disc. It is therefore a good system to study to understand the relation
between the availability of cold-gas and the triggering of AGNs in galaxies.
Our new WSRT data reveal two faint, S-shaped radio lobes extending out to
$\sim$10 kpc from the galaxy centre. Remarkably, we find that the inner HI disc
warps back towards the stellar mid-plane in a way that mirrors the warping of
the radio lobes. We suggest that the polar HI disc was accreted through a minor
merger, and that the torques causing it to warp in the inner regions are also
responsible for feeding the AGN. The "S" shape of the radio lobes would then be
due to the radio jets adapting to the changing angular momentum of the accreted
gas. The extended radio jets are likely poorly collimated, which would explain
their quick fading and, therefore, their rarity in galaxies similar to NGC
3998. The fuelling of the central super-massive black hole is likely occurring
via "discrete events", suggested by the observed variability of the radio core
and the extremely high core dominance, which we attribute to the formation and
ejection of a new jet resulting from a recent fuelling event.
