2025A&A...701A.298H

We aim to characterise the multi-phase gas in the SPT2349-56 protocluster at z = 4.3, which is known to host one of the most starbursting and Active Galactic Nuclei (AGN)-rich high redshift environments. For this purpose, we conducted Atacama Pathfinder EXperiment (APEX) single-dish observations of the [C II] 158 μm (hereafter [C II]) line towards the core and north components, which were previously imaged with the Atacama Large Millimeter/submillimeter Array (ALMA). We also present the first [O III] 88 μm (hereafter [O III]) line observations in such a high-redshift protocluster system. We obtain a [C II] line luminosity approximately 1.7 times greater than that recovered by ALMA towards the core, while we recover four times more [C II] line emission than that found in deep ALMA images towards the north component. This suggests that the most massive gas reservoirs lie in the less extreme regions of this protocluster system. A minimum ionised gas mass of M_min(H⁺)∼3.7 × 10¹⁰ M_⊙ is deduced from the [O III] line, which amounts to 30% of the molecular gas mass in the same area, indicating that a full map of the cluster is necessary for determining the large-scale value. Finally, we obtain star formation rate (SFR) estimates using the [O III] line luminosity and the corresponding ionised gas mass. These yield values that can surpass the far-infrared (IR) continuum-derived SFR under the assumption of a standard stellar Initial Mass Function (IMF), which can be reconciled only if non-stellar ionising sources contribute to the [O III] line luminosity, or if a top-heavy stellar IMF produces a larger fraction of O stars per total stellar mass. This is a distinct possibility in high-energy-particle (HEP)-dominated, rather than UV-photon-dominated, environments in clusters. Future work using far-IR fine-structure and molecular or neutral-atomic lines is necessary to determine the thermal and ionisation states of the multi-phase medium in this protocluster, to understand their maintenance, and to resolve the apparent SFR discrepancy. These line ratios must be measured over a wide range of spatial scales, from individual galaxies up to circumgalactic medium (CGM) and intracluster medium (ICM) scales, which ultimately requires combining wide-field single-dish and high-resolution interferometric observations of such lines in protocluster environments where HEP- and UV-dominated ISM phases can co-exist.