Brightest Cluster Galaxy Formation in the z=4.3 Protocluster SPT2349-56: Discovery of a Radio-Loud AGN

We have observed the z = 4.3 protocluster SPT2349 − 56 with the Australia Telescope Compact Array (ATCA) with the aim of detecting radio-loud active galactic nuclei (AGN) amongst the ∼ 30 submillimeter (submm) galaxies (SMGs) identified in the structure. We detect the central complex of submm sources at 2.2 GHz with a luminosity of L 2 . 2 = (4 . 42 ± 0.56) × 10 25 W Hz − 1 . The Australian Square Kilometre Array Pathfinder (ASKAP) also detects the source at 888 MHz, constraining the radio spectral index to α = − 1.6 ± 0.3, consistent