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Abstract 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 atz =  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 $\stackrel{\text{″}}{.}$0  ×  12 $\stackrel{\text{″}}{.}$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 $\stackrel{\text{″}}{.}$0  ×  0 $\stackrel{\text{″}}{.}$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 [Cii] spectrum obtained by APEX/FLASH supports the ACA CO(4–3) result, revealing a large excess in [Cii] 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. 

Abstract The CO(1–0) line has been carefully calibrated as a tracer of molecular gas mass. However, recent studies often favor higherJtransitions of the CO molecule, which are brighter and accessible for redshift ranges where CO(1–0) is not. These lines are not perfect analogs for CO(1–0), owing to their more stringent excitation conditions, and must be calibrated for use as molecular gas tracers. Here, we introduce the Arizona Molecular ISM Survey with the SMT, a multi-CO line survey ofz∼ 0 galaxies conducted to calibrate the CO(2–1) and CO(3–2) lines. The final survey includes CO(2–1) spectra of 176 galaxies and CO(3–2) spectra for a subset of 45. We supplement these with archival CO(1–0) spectra from xCOLD GASS for all sources and additional CO(1–0) observations with the Kitt Peak 12 m Telescope. Targets were selected to be representative of the 10⁹M_⊙≤M_*≤ 10^11.5M_⊙galaxy population. Our project emphasized careful characterization of statistical and systematic uncertainties to enable studies of trends in CO line ratios. We show that optical and CO disk sizes are on average equal, for both the CO(1–0) and CO(2–1) line. We measure the distribution of CO line luminosity ratios, finding medians (16th–84th percentile) of 0.71 (0.51–0.96) for the CO(2–1)-to-CO(1–0) ratio, 0.39 (0.24–0.53) for the CO(3–2)-to-CO(1–0) ratio, and 0.53 (0.41–0.74) for the CO(3–2)-to-CO(2–1) ratio. A companion paper presents our study of CO(2–1)'s applicability as a molecular gas mass tracer and search for trends in the CO(2–1)-to-CO(1–0) ratio. Our catalog of CO line luminosities is publicly available.