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

Frank, Bradley S; Morganti, Raffaella; Oosterloo, Tom; Nyland, Kristina; Serra, Paolo

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

Journal Article

2016

Journal Title

Astronomy and Astrophysics

Link to Journal

https://dx.doi.org/10.1051/0004-6361/201628282

Department

Department of Astronomy

Faculty

Faculty of Science

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 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.