An Overabundance of Radio Active Galactic Nuclei in the SPT2349–56 Protocluster: Preheating the Intracluster Medium

Abstract Following the detection of a radio-loud active galactic nucleus (AGN) in the z = 4.3 protocluster SPT2349−56, we have obtained additional observations with the South African MeerKAT radio telescope in the S band (2.4 GHz) with the aim of further characterizing radio emission from amongst the ∼30 submillimeter galaxies (SMGs) identified in the structure. In addition to the previous radio-AGN, we newly identify two of the protocluster SMGs individually at 2.4 GHz as having a radio excess. Two of these radio-AGN are now known to be X-ray luminous AGN. Two additional members are also detected with radio emission consistent with their star formation rate (SFR). Archival MeerKAT Ultra-High Frequency (UHF; 816 MHz) observations further constrain luminosities and radio spectral indices of these five galaxies. The Australia Telescope Compact Array is used to detect and resolve the central two sources at 5.5 and 9.0 GHz finding elongated, bipolar jet-like morphologies. The excess radio luminosities range from L 1.4,rest = (1–20) × 10 25 W Hz −1 , ∼10–100× higher than expected from the SFRs, assuming the usual far-infrared–radio correlation. Of the known cluster members, only the SMG “N1” shows signs of AGN in any other diagnostics, namely a large and compact excess in 12 CO(11–10) line emission. We compare these results to field samples of radio sources and SMGs. The overdensity of radio-loud AGN in the compact core region of the cluster may be providing significant heating to the recently discovered nascent intracluster medium in SPT2349−56.