2024ApJ...976...64R

We present deep, wideband multifrequency radio observations (144 MHz-8 GHz) of the remarkable galaxy group NGC 741, which yield crucial insights into the interaction between the infalling head-tail radio galaxy (NGC 742) and the main group. Our new data provide an unprecedentedly detailed view of the NGC 741-742 system, including the shock cone, disrupted jets from NGC 742, the long (∼255 kpc) braided southern radio tail, and the eastern lobe-like structure (∼100 kpc), and reveal, for the first time, complex radio filaments throughout the tail and lobe, and a likely vortex ring behind the shock cone. The cone traces the bow shock caused by the supersonic (${ \mathcal M }\sim 2$) interaction between the head-tail radio galaxy NGC 742 and the intragroup medium (IGrM), while the ring may have been formed by the interaction between the NGC 742 shock and a previously existing lobe associated with NGC 741. This interaction plausibly compressed and reaccelerated the radio plasma. We estimate that shock-heating by NGC 742 has likely injected ∼2-5 × 10⁵⁷ erg of thermal energy into the central 10 kpc cooling region of the IGrM, potentially affecting the cooling and feedback cycle of NGC 741. A comparison with Chandra X-ray images shows that some of the previously detected thermal filaments align with radio edges, suggesting compression of the IGrM as the relativistic plasma of the NGC 742 tail interacts with the surrounding medium. Our results highlight that multifrequency observations are key to disentangling the complex, intertwined origins of the variety of radio features seen in the galaxy group NGC 741, and the need for simulations to reproduce all the detected features.