A large thermal energy reservoir in the nascent intracluster medium at a redshift of 4.3

Technical Summary: A Large Thermal Energy Reservoir in the Nascent Intracluster Medium at Redshift 4.3

Problem Statement
Cosmological simulations predict that the intracluster medium (ICM) in galaxy clusters is a hot gas reservoir ($\gtrsim 10^7$ K) whose mass and temperature decrease significantly at high redshifts as clusters are still assembling. The thermal Sunyaev-Zeldovich (tSZ) effect, caused by the inverse Compton scattering of cosmic microwave background (CMB) photons by energetic electrons in the hot ICM, serves as a direct probe of this thermal energy. Prior to this study, the tSZ effect had been detected in only three systems at or above $z \sim 2$, leaving the assembly timeline and heating mechanisms of the ICM in the early Universe uncertain. Theoretical models suggest that at $z \sim 4.3$ (1.4 billion years after the Big Bang), the ICM should be cooler and less massive, with any detected signal expected to be weak compared to the self-similar evolution observed in mature clusters.

Methodology
The authors investigated the protocluster SPT2349−56, a massive dark matter halo ($\sim 10^{13} M_\odot$) at $z = 4.3$ containing at least 30 dusty star-forming galaxies (DSFGs) and three radio-loud active galactic nuclei (AGN) within a 100-kpc region. This system was selected from the 2,500 deg² South Pole Telescope survey as a bright protocluster candidate.

To detect the tSZ signal, the team utilized deep Band-3 (3 mm) observations from the Atacama Large Millimeter/submillimeter Array (ALMA), combining data from the 12-m main array and the 7-m Atacama Compact Array (ACA). The methodology involved:

1. Data Reduction: Processing data from four ALMA cycles (3, 5, 9, and 10) using the latest CASA pipelines, including specific corrections for phase errors and cloud contamination.
2. Continuum Subtraction: Employing a Fourier-space technique to subtract the dust continuum emission from the DSFGs. This was necessary because the tSZ signal manifests as a decrement (negative intensity) that is contaminated by the positive dust emission of the galaxies.
3. Signal Extraction: Imaging the residual data using short baselines ($uv < 10k\lambda$) to maximize sensitivity to extended emission while minimizing the contribution from point sources.
4. Validation: Conducting interferometric simulations (using simalma) and jackknife tests to rule out imaging artifacts, negative sidelobes, or calibration errors as the source of the observed decrement.
5. Comparative Analysis: Comparing the derived Compton-$Y$ parameter against the universal mass–$Y$ relation and predictions from the TNG-Cluster cosmological simulation suite.

Key Results
The observations revealed a strong, extended decrement in the core region of SPT2349−56:

• Detection Significance: The tSZ signal peaks at $8.4\sigma$ in the image plane and $10.4\sigma$ in Fourier space, with an integrated flux density of $-157 \pm 16$ $\mu$Jy.
• Thermal Parameters: The signal corresponds to a Compton-$y$ parameter of $(5.6 \pm 0.8) \times 10^{-6}$ and an integrated Compton-$Y$ parameter of $(2.0 \pm 0.2) \times 10^{-6}$ arcmin$^2$.
• Thermal Energy: The inferred total thermal energy of the system is $\sim 10^{61}$ erg.
• Deviation from Theory: This thermal energy exceeds the possible energy of a virialized ICM by an order of magnitude. Even assuming a fully collapsed system with an abundant ICM fraction ($f_{ICM} \approx 0.06$), the expected decrement would be only $\lesssim 30$ $\mu$Jy. The observed signal is at least five times larger than theoretical expectations for a system at this redshift.
• Simulation Comparison: When compared to TNG-Cluster simulations, the mass-normalized Compton-$Y$ in SPT2349−56 lies $6.4\sigma$ above the predicted evolutionary track. Simulations generally predict cooler ICMs at $z \gtrsim 3$, whereas SPT2349−56 exhibits a significantly hotter, over-pressurized state.

Significance and Claims
The paper claims that the detection of such a large thermal energy reservoir in a protocluster at $z = 4.3$ challenges current theoretical expectations regarding the timing and mechanism of ICM assembly. Specifically:

• Overheating Mechanism: The results demonstrate that substantial heating can occur and deposit a large amount of thermal energy within growing galaxy clusters two billion years before the first mature clusters emerged at $z \sim 2$.
• Role of Feedback: The authors argue that the observed excess cannot be explained by gravitational shock heating alone, as it would require an implausibly massive halo ($> 5 \times$ the dynamical mass). Instead, they propose that intense pre-heating from non-gravitational processes—specifically strong AGN feedback (both kinetic and radiative modes) and potentially star formation—is required. The high ambient gas pressure at $z > 4$ likely confines AGN jets, enhancing shock heating and reducing energy loss to expansion, thereby storing energy in an over-pressurized ICM.
• Breakdown of Self-Similarity: The finding implies a breakdown of the self-similar evolution of the ICM mass–$Y$ relation at early epochs, suggesting that the physics of galaxy formation and feedback in nascent clusters is more complex than currently modeled in state-of-the-art hydrodynamical simulations.
• Rarity vs. General Stage: While SPT2349−56 is a unique system from a large survey, the authors suggest it may represent a short-lived but important stage of cluster assembly characterized by extreme heating, a phase that current simulations fail to predict.