摘要：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 H i 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 ~10 kpc from the galaxy centre. Remarkably, we find that the
inner H i disc warps back towards the stellar mid-plane in a way that mirrors the
warping of the radio lobes. We suggest that the polar H i 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.
关键词：galaxies: active;galaxies: individual: NGC 3998;ISM: jets and outflows;radio continuum: galaxies;radio lines: galaxies;history and philosophy of astronomy
