Radio jets in NGC 4151: where eMERLIN meets HST
  We present high-sensitivity eMERLIN radio images of the Seyfert galaxy NGC
4151 at 1.5 GHz. We compare the new eMERLIN images to those from archival
MERLIN observations in 1993 to determine the change in jet morphology in the 22
years between observations. We report an increase by almost a factor of 2 in
the peak flux density of the central core component, C4, thought to host the
black hole, but a probable decrease in some other components, possibly due to
adiabatic expansion. The core flux increase indicates an AGN which is currently
active and feeding the jet. We detect no significant motion in 22 years between
C4 and the component C3, which is unresolved in the eMERLIN image. We present a
spectral index image made within the 512 MHz band of the 1.5 GHz observations.
The spectrum of the core, C4, is flatter than that of other components further
out in the jet. We use HST emission line images (H$\alpha$, [O III] and [O II])
to study the connection between the jet and the emission line region. Based on
the changing emission line ratios away from the core and comparison with the
eMERLIN radio jet, we conclude that photoionisation from the central AGN is
responsible for the observed emission line properties further than 4" (360 pc)
from the core, C4. Within this region, several evidences (radio-line
co-spatiality, low [O III]/H$\alpha$ and estimated fast shocks) suggest
additional ionisation from the jet.
