AGN feedback in an infant galaxy cluster: LOFAR-Chandra view of the giant FRII radio galaxy J103025+052430 at z = 1.7

In the nearby universe, jets from active galactic nuclei (AGN) are observed to have a dramatic impact on their surrounding extragalactic environment. The effect of jets at high redshift (z > 1.5) is instead much more poorly constrained. However, studying the jet impact at cosmic noon, the epoch in which both star formation and AGN activity peak, is crucial for fully understanding galaxy evolution. Here we present a study of the giant (∼750 kpc) radio galaxy 103025+052430 located at the centre of a protocluster at redshift z = 1.7, with a focus on its interaction with the external medium. We present new LOFAR observations at 144 MHz, which we combine with VLA 1.4 GHz data and 0.5-7 keV Chandra archival data. The new radio map at 144 MHz confirms that the source has a complex morphology, which can possibly fit the hybrid morphology radio galaxy classification. The large size of the source enabled us to perform a resolved radio spectral index analysis, a very unique opportunity for a source at this high redshift. This revealed a tentative unexpected flattening of the radio spectral index at the edge of the backflow in the western lobe, which might be indicating plasma compression. The spatial coincidence between this region and the thermal X-ray bubble C suggests a causal connection between the two. In contrast to previous estimates for the bright X-ray component A, we find that inverse Compton scattering between the radio-emitting plasma of the eastern lobe and cosmic microwave background photons can account for a large fraction (∼45%-80%) of its total 0.5-7 keV measured flux. Finally, the X-ray bubble C, which is consistent with a thermal origin, is found to be significantly overpressurised with respect to the ambient medium. This suggests that it will tend to expand and release its energy into the surroundings, contributing to the overall intracluster medium heating. Overall, 103025+052430 enables us to investigate the interaction between AGN jets and the surrounding medium in a system that is likely the predecessor of the rich galaxy clusters we all know well at z = 0.