Unmasking the history of 3C 293 with LOFAR sub-arcsecond imaging

Abstract

Active galactic nuclei show episodic activity, which can be evident in galaxies that exhibit restarted radio jets. These restarted jets can interact with their environment, leaving signatures on the radio spectral energy distribution. Tracing these signatures is a powerful way to explore the life of radio galaxies. This requires resolved spectral index measurements over a broad frequency range including low frequencies. We present such a study for the radio galaxy 3C 293, which has long been thought to be a restarted galaxy on the basis of its radio morphology. Using the International LOFAR telescope (ILT) we probed spatial scales as fine as ~0.2′′ at 144 MHz, and to constrain the spectrum we combined these data with Multi-Element Radio Linked Interferometer Network and Very Large Array archival data at frequencies up to 8.4 GHz that have a comparable resolution. In the inner lobes (~2 kpc), we detect the presence of a spectral turnover that peaks at ~225 MHz and is most likely caused by free-free absorption from the rich surrounding medium. We confirm that these inner lobes are part of a jet-dominated young radio source (spectral age ≲0.17 Myr), which is strongly interacting with the rich interstellar medium of the host galaxy. The diffuse emission surrounding these lobes on scales of up to ~4.5 kpc shows steeper spectral indices (Δα ~ 0.2–0.5, S ∝ ν−α) and a spectral age of ≲0.27 Myr. The outer lobes (extending up to ~100 kpc) have a spectral index of α ~ 0.6–0.8 from 144–4850 MHz with a remarkably uniform spatial distribution and only mild spectral curvature (Δα ≲ 0.2). We propose that intermittent fuelling and jet flow disruptions are powering the mechanisms that keep the spectral index in the outer lobes from steepening and maintain the spatial uniformity of the spectral index. Overall, it appears that 3C 293 has gone through multiple (two to three) epochs of activity. This study adds 3C 293 to the new sub-group of restarted galaxies with short interruption time periods. This is the first time a spatially resolved study has been performed that simultaneously studies a young source as well as the older outer lobes at such low frequencies. This illustrates the potential of the International LOFAR telescope to expand such studies to a larger sample of radio galaxies.