The LOFAR view of NGC 3998, a sputtering AGN

Low-power radio sources dominate the radio sky. They tend to be small in size and core-dominated, but the origin of their properties and the evolution of their radio plasma are not well constrained. Interestingly, there is mounting evidence that low-power radio sources can significantly impact their surrounding gaseous medium and, therefore, may be more relevant for galaxy evolution than previously thought. In this paper, we present low radio frequency observations obtained with LOFAR at 147 MHz of the radio source hosted by NGC 3998. This is a rare example of a low-power source which is extremely core-dominated, but which has two large-scale lobes of low surface brightness. We combine the new 147 MHz image with available 1400 MHz data to derive the spectral index over the source. Despite the low surface brightness, reminiscent of remnant structures, the lobes show an optically thin synchrotron spectral index (~ 0.6). We interpret this as being due to rapid decollimation of the jets close to the core, to high turbulence of the plasma flow, and entrainment of thermal gas. This could be the result of intermittent activity of the central AGN, or, more likely, temporary disruption of the jet due to the interaction of the jet with the rich circumnuclear ISM. Both would result in sputtering energy injection from the core which would keep the lobes fed, albeit at a low rate. We discuss these results in connection with the properties of low-power radio sources in general. Our findings show that amorphous, low surface brightness lobes should not be interpreted, by default, as remnant structures. Large, deep surveys (in particular the LOFAR 150 MHz LoTSS and the recently started 1400 MHz Apertif survey) will identify a growing number of objects similar to NGC 3998 where these ideas can be further tested.