Radio jets in NGC 4151: where eMERLIN meets HST

TL;DR

This study uses high-sensitivity eMERLIN radio images and HST emission line data to analyze the jet morphology, activity, and ionization mechanisms in the Seyfert galaxy NGC 4151 over a 22-year period.

Contribution

It provides the first detailed comparison of radio jet morphology in NGC 4151 over two decades and links jet activity to ionization processes in the surrounding emission line regions.

Findings

Central core flux increased, indicating active feeding of the jet.

No significant motion detected in jet components over 22 years.

Jet-related ionization influences emission line properties within 360 pc.

Abstract

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.