Non-thermal filaments and AGN recurrent activity in the galaxy group Nest200047: A LOFAR, uGMRT, MeerKAT, and VLA radio spectral analysis

Nest200047 is one of the clearest examples of multiple radio bubbles from an active galactic nucleus (AGN) observed in a galaxy group. It also features a complex system of non-thermal filaments, likely shaped by buoyancy and gas motions in the group and stabilised by large-scale magnetic fields. In this study we present a new set of high-quality data obtained from our dedicated observational campaign using uGMRT, MeerKAT, and VLA. By combining this with existing LOFAR data, we perform a detailed morphological and spectral analysis of the system over a broad frequency range (53−1518 MHz) using various complementary techniques. Our images reveal new filamentary emission in the inner 60 kpc of the system, surrounding and extending from the inner bubbles and jets, suggesting a complex dynamical evolution of the non-thermal plasma in the group core as well. Overall, all filaments in the group have a width of a few kiloparsec and lengths from a few tens up to a few hundred kiloparsec. They all show a steep (α = 1 ∼ 2) and curved (spectral curvature up to 1) radio spectrum. Interestingly, the filaments exhibit a constant radio spectral index profile along their length, indicating that particles are not cooling along them. This suggests that the particles were either (re-)accelerated together and evolved in a similar magnetic field or they are moving along the filaments at super-Alfvenic speeds. A spectral ageing analysis based solely on radiative losses provides age estimates for the three different pairs of bubbles of 130 Myr, 160−170 Myr, and > 220 Myr, with jet active times ranging between 50 and 100 Myr, and very short inactive times. This supports the idea of a nearly continuous energy injection, typical of the maintenance mode of AGN feedback, particularly in galaxy groups. By taking into account adiabatic expansion, we also find that the outermost northern bubble cannot be considered a simple evolution of the inner bubbles without invoking some re-acceleration or mixing with the external gas or, alternatively, different outburst properties. In conclusion, our study clearly shows the potential of the combined use of high-quality new-generation radio data for understanding recurrent jet activity and feedback, and anticipates the new opportunities that will be offered by the SKA observatory.