The high-redshift radio galaxy 3C 294 at low frequencies: radio detection of the X-ray ghost

ABSTRACT We report on the very first radio detection associated with the peculiar hourglass-morphology X-rays surrounding 3C 294 at $z=1.8$. Using International Low Frequency Array (LOFAR) data at 144 MHz and Chandra data at 0.3–6 keV, we find that the co-spatial diffuse radio and X-ray emission is well described by synchrotron and inverse-Compton processes by the same electron population. Through modelling of this rare low-energy plasma, we find that the most defining property of the electrons up-scattering CMB photons at this redshift is very low electron Lorentz factors ($\gamma _{\text{max}}\ll 10^{4}$ and $\gamma _{\text{break}}\lesssim 10^{3}$) in the lobe: deep low frequency ($< 150$ MHz) observations are critical to the detection of radio lobes at high redshift. The physical conditions imply a total energy in the diffuse emission significantly greater than that implied by the temperature of the protocluster gas: 3C 294 is one of the most powerful known radio-loud systems in a dense protocluster environment. Through resolved spectral analysis of archival radio data up to 15 GHz, we find evidence that the inner hotspots are due to restarted activity, while the outer hotspots remain energetic, suggesting a rapid duty cycle while the jet precesses. This allowed the low-energy aged plasma driving the X-rays to remain spatially distinct from the high-energy plasma. Together, our results promise a revelation of active galactic nucleus-related radio emission at high redshift using future low-frequency arrays such as SKA-LOW.