A sensitive APEX and ALMA CO(1–0), CO(2–1), CO(3–2), and [CI](1–0) spectral survey of 40 local (ultra-)luminous infrared galaxies

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We present a high sensitivity, ground-based spectral line survey of low- J carbon monoxide (CO( J up → J up − 1) with J up = 1, 2, 3) and neutral carbon [CI] 3 P 1 − 3 P 0 ([CI](1–0)) in 36 local ultra-luminous infrared galaxies (ULIRGs) and 4 additional LIRGs, all of which have previous Herschel OH 119 μm observations. The study is based on new single-dish observations conducted with the Atacama Pathfinder Experiment (APEX) and complemented with archival APEX and Atacama Large Millimeter Array (ALMA and ACA) data. Our methods are optimized for a multi-tracer study of the total molecular line emission from these ULIRGs, including any extended low-surface-brightness components. We find a tight correlation between the CO and [CI] line luminosities, which suggests that the emission from CO(1–0) (and CO(2–1)) arises from similar regions as the [CI](1–0), at least when averaged over galactic scales. By using [CI] to compute molecular gas masses, we estimate a median CO-to-H 2 conversion factor of ⟨ α CO ⟩ = 1.7 ± 0.5 M ⊙ (K km s −1 pc 2 ) −1 for ULIRGs. We derive median galaxy-integrated CO line ratios of 〈 r 21 〉 = L CO(2-1) ′ / L CO(1-0) ′ = 1.09, 〈 r 31 〉 = L CO(3-2) ′ / L CO(1-0) ′ = 0.76, and 〈 r 32 〉 = L CO(3-2) ′ / L CO(2-1) ′ = 0.76, significantly higher than normal star-forming galaxies, confirming the exceptional molecular gas properties of ULIRGs. We find that the r 21 and r 32 ratios are poor tracers of CO excitation in ULIRGs, while r 31 shows a positive trend with L IR and star formation rates and a negative trend with the H 2 gas depletion timescales ( τ dep ). Our investigation of CO line ratios as a function of gas kinematics shows no clear trends, except for a positive relation between r 21 and σ v , which can be explained by CO opacity effects. These ULIRGs are also characterized by high L [CI](1-0) ′ / L CO(1-0) ′ ratios, with a measured median value of ⟨ r CICO ⟩ = 0.18, higher than values from previous interferometric studies that were affected by missing [CI] line flux. The r CICO values do not show a significant correlation with any of the galaxy properties investigated, including OH outflow velocities and equivalent widths. We find that the widths of [CI](1–0) lines are ∼10% smaller than those of CO lines, and that this discrepancy becomes more significant in ULIRGs with broad lines ( σ v > 150 km s −1 ) and when considering the high- v wings of the lines. This suggests that the low optical depth of [CI] can challenge its detection in diffuse, low-surface-brightness outflows and, therefore, its use as a tracer of CO-dark H 2 gas in these components. Finally, we find that higher L AGN are associated with longer τ dep , consistent with the hypothesis that active galactic nucleus feedback may reduce the efficiency of star formation. Our study highlights the need for sensitive single-dish multi-tracer H 2 surveys of ULIRGs that are able to recover the flux that is missed by interferometers, especially in the high-frequency lines such as [CI]. The Atacama Large Aperture Submillimeter Telescope (AtLAST) will be transformational for this field.