An Overabundance of Radio-AGN in the SPT2349-56 Protocluster: Preheating the Intra-Cluster Medium

TL;DR

This study identifies an unusually high number of radio-loud AGN in the SPT2349-56 protocluster, suggesting significant preheating of the nascent intra-cluster medium due to AGN activity.

Contribution

It provides detailed radio observations of multiple SMGs in a high-redshift protocluster, revealing an overabundance of radio-loud AGN and their potential role in ICM preheating.

Findings

Three SMGs have radio excess indicating AGN activity.

Two SMGs are X-ray luminous AGN.

Radio luminosities are 10-100 times higher than expected from SFRs.

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 MeerKAT in S-band (2.4 GHz) with the aim of further characterizing radio emission from amongst the ~30 submillimeter (submm) galaxies (SMGs) identified in the structure. We newly identify three of the protocluster SMGs individually at 2.4GHz as having a radio-excess, two of which 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 UHF (816 MHz) observations further constrain luminosities and radio spectral indices of these five galaxies. The Australia Telescope Compact Array (ATCA) is used to detect and resolve the central two sources at 5.5 and 9.0 GHz finding elongated, jet-like morphologies. The excess radio luminosities range from L1.4,rest = (1-20)x10^25 W/Hz, ~10-100x 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 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 intra-cluster medium (ICM) in SPT2349-56.