A rare example of low surface-brightness radio lobes in a gas-rich early-type galaxy: the story of NGC 3998

TL;DR

This study reveals faint, S-shaped radio lobes in NGC 3998, linking their morphology to warped HI gas possibly accreted through a minor merger, providing insights into AGN fueling and jet evolution in gas-rich early-type galaxies.

Contribution

It presents the first detection of low surface-brightness radio lobes in NGC 3998 and connects their structure to gas accretion and AGN activity, highlighting the role of minor mergers.

Findings

Detection of faint, S-shaped radio lobes extending 10 kpc.

Warped HI disc correlates with radio lobe morphology.

Evidence of discrete fueling events causing core variability.

Abstract

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.