2025ApJ...982L..17Z

We present Atacama Compact Array (ACA) Band-3 observations of the protocluster SPT2349-56, an extreme system hosting >10 ultraluminous infrared galaxies (ULIRGs; L_IR ≳ 10¹²L_⊙) in a 200 kpc diameter region at z = 4.3, to study its integrated molecular gas content via CO(4-3) and the long-wavelength dust continuum. The ∼30 hr integration represents one of the longest exposures yet taken on a single pointing with the ACA 7 m. The low-resolution ACA data (21$\mathop{.}\limits^{\unicode{x02033}}$0 × 12$\mathop{.}\limits^{\unicode{x02033}}$2) reveal a 75% excess CO(4-3) flux compared to the sum of individual sources detected in higher-resolution Atacama Large Millimeter/submillimeter Array (ALMA) data (1$\mathop{.}\limits^{\unicode{x02033}}$0 × 0$\mathop{.}\limits^{\unicode{x02033}}$8). Our work also reveals a similar result by tapering the ALMA data to 10″. In contrast, the 3.2 mm dust continuum shows little discrepancy between ACA and ALMA. A single-dish [C II] spectrum obtained by APEX/FLASH supports the ACA CO(4-3) result, revealing a large excess in [C II] emission relative to ALMA. The missing flux is unlikely due to undetected faint sources but instead suggests that high-resolution ALMA observations might miss extended and low-surface-brightness gas. Such emission could originate from the circumgalactic medium or the preheated protointracluster medium (proto-ICM). If this molecular gas reservoir replenishes the star formation fuel, the overall depletion timescale will exceed 400 Myr, reducing the requirement for the simultaneous ULIRG activity in SPT2349-56. Our results highlight the role of an extended gas reservoir in sustaining a high star formation rate in SPT2349-56 and potentially establishing the ICM during the transition phase to a mature cluster.