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

TL;DR

This study reveals an exceptional concentration of radio-loud AGN in the z=4.3 SPT2349-56 protocluster, identifying three central radio-AGN (C, A, E) with significant radio excess and jet-like morphologies, plus N1 showing strong CO(11-10) emission indicative of AGN-driven excitation. By combining MeerKAT S-band and 816 MHz data with ATCA 5.5/9 GHz imaging and new ALMA CO measurements, the authors quantify AGN contributions versus star-formation–driven radio emission and constrain jet energetics via the cavity-power relation, obtaining a kinetic power of about $\dot{E}_{\rm kin, radio} \approx (2.6\pm0.3)\times10^{45}$ erg s$^{-1}$ (scaled by $f_{\rm cav}$) and an energy injection of order $\Delta E_{\rm inject} \approx 8.7\times10^{60}$ erg over $t_{\rm radio}\sim100$ Myr, compatible with heating the nascent ICM and the strong tSZ signal observed. The results imply an protocluster AGN fraction of ~13%, rising to ~29% among ULIRGs, and support a scenario where early, cooperative radio-mode feedback from multiple jets heats the intracluster medium, potentially shaping cluster evolution. The combination of resolved jets, steep spectra, and CO-SLED signatures highlights diverse gas excitation states (AGN-dominated vs starburst-like) among members and underscores the role of jet feedback in the early assembly of massive clusters. Overall, the work demonstrates how high-redshift protoclusters can illuminate the origins of ICM heating and the emergence of mature clusters through AGN-driven energetics.

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.

Figures (6)

• Figure 1: Background: MeerKAT 816 MHz robust=-1.2 imaging of the SPT2349$-$56 region, with blue contours highlighting the 110 mJy extended LABOCA source at 870$\mu$m Miller, with ALMA 350 GHz sources shown (green contours). Inset: A $22"\times22"$ zoom-in of ALMA 350 GHz continuum imaging Hill20 with overlays of MeerKAT 2.4 GHz (cyan), ATCA 5.5 GHz (magenta) and 9 GHz (blue). The point source removed UHF 816 MHz observation (highlighting source E) is shown as yellow contours. ALMA sources are named from Miller in order of their 850 $\mu$m flux density. For the faintest ALMA sources M,N, we show [CII] contours (red).
• Figure 2: Spectral energy distributions (SEDs) in the restframe wavelength for the five individually-detected sources from MeerKAT. Also shown are the ALMA flux densities from Hill20, and for 240 GHz from this work (Appendix A). For source C, a new 350$\mu$m measurement from harrington25 is included, suggesting a slightly cooler SED than Arp220 is likely. Arp220 lies near the mean of the local radio-far-IR relation. Normalizing Arp220 to the submm photometry reveals that three of the sources show significant excess in radio above the radio-far-IR correlation for star-forming galaxies, while two (N1, N2) show no significant excess over that expected from SF. Sources A and C are detected in the X-ray vito24. The grey shadings show the 1 and 2$\sigma$ uncertainties in the radio spectral index fit (Appendix B). The red line indicates where restframe L$_{1.4}$ is estimated for Figure 3 from Equation 1.
• Figure 3: Left: Redshift vs. 1.4 GHz radio power using the GOODS-N sample barger2017 and radio-excess candidates from the ALESS sample thomson2014 (green pentagons). Blue symbols represent SPT2349$-$56 radio detections (offset in redshift for clarity). Other symbols are as follows: red circles show sources detected above the 3$\sigma$ level at 850 $\mu$m, while black circles show sources not detected at this level. SPT2349$-$56 source C has about 10 times more radio power than any radio source found in GOODS-N, while A, E, and N1 are amongst the top few percent in radio power found in GOODS-N. The blue line marks the radio flux limit in GOODS-N. Right: Rest-frame 1.4 GHz luminosity over 350 GHz luminosity vs. redshift, with L$_{350}$ derived using the Arp220 SED. Source C is about 125 times higher than the median relation at rest 1.4 GHz, with sources A and E being about 10 times higher. N1 and N2 do not appreciably distinguish themselves as expected. For comparison we show submm sources with spectroscopic redshifts in GOODS-N (red circles), and lower limits on radio sources undetected in the submm (red triangles). The blue dashed line region shows where the submillimeter luminosities and radio luminosities produce consistent estimates of SFRs. None of the GOODS-N SMGs show any excess radio emission over the FIR-radio relation.
• Figure 4: CO $J_{\rm upper}\,{=}\,$11 spectrum of N1, significantly brighter than any other in the protocluster, and much narrower than the CO(4--3) and [CII] lines. Analysis of the CO SLED suggests a possible AGN-driven XDR contribution to the gas excitation (Figure \ref{['fig:sled']}) The red line denotes the aperture-integrated [CII] spectrum of N1.
• Figure 5: SLEDs for the sources detected at $>5\sigma$ in the CO $J_{\rm upper}\,{=}\,$11 transition. New spectral data for the $J_{\rm upper}\,{=}\,$11 lines are shown in Figure \ref{['fig:N1co1110']}. We compare to the $L_{\rm FIR}\,{=}\,3\,{\times}\,10^{12}$ L$_\odot$ AGN-dominated galaxy Mrk231 vanderWerf, and the $L_{\rm FIR}\,{=}\,3\,{\times}\,10^{10}$ L$_\odot$ starburst M82 kamenetzky2012, here normalized to Mrk231 at CO(7--6). Source N1 shows an excess J=11 emission at very high significance, with almost perfect agreement with the Mrk231 SLED modeled with a strong XDR contribution vanderWerf. The radio-AGN A and C, show SLEDs more consistent with starburst galaxies. The radio-AGN E is not significantly detected in CO(11--10), but is also consistent with an M82-like excitation.