The diverse morphologies and evolution of low-luminosity edge-brightened radio galaxies

Original Abstract

Fanaroff-Riley class I (FRI) radio galaxies show centre-brightened emission from disrupted lower power jets, while traditionally more luminous class II (FRIIs), are edge-brightened, with relativistic jets terminating in hotspots. Population studies of radio-loud AGN (RLAGN) with low frequency, deep, wide-field surveys have revealed FRII-like radio structures at lower luminosities. We present the first high-resolution morphological investigation of a representative LOFAR-selected sample of low-luminosity FRIIs, to determine whether this new population is physically distinct from traditional high-luminosity FRIIs. Using new $1.5$-GHz Jansky Very Large Array (VLA) observations for a sample of 19 low-luminosity FRIIs, from the LOFAR Two Metre Sky Survey Data Release 1 (LoTSS DR1), with luminosities up to three orders of magnitude lower than the typical FR break ($L_{150} = 10^{26}$ W Hz$^{-1}$). We examine the compact features and perform spectral index analysis to identify hotspots, cores and signatures of restarting or remnant activity. We find a higher prevalence of cores and a comparable number of hotspots in the low-luminosity FRII sample compared to a randomly-selected sample of luminous ($L_{150}>10^{26}$ W Hz$^{-1}$) FRIIs selected from the same parent LOFAR sample. Approximately 32 per cent of low-luminosity FRIIs show restarting or remnant behaviour, while $\sim 32$ per cent are active FRIIs with compact hotspots. Our results show that FRII source dynamics occur at low radio luminosities, but reinforce earlier conclusions that the population of low-luminosity edge-brightened RLAGN is highly diverse. Binary morphological classifications should be used cautiously as a first step towards more nuanced investigations of the complexity of jet life cycles and evolution.