Abstract

--- - 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] P-3(1) - P-3(0) ([CI](1-0)) in 36 local ultra-luminous infrared galaxies (ULIRGs) and 4 additional LIRGs, all of which have previous Herschel OH 119 mu m 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 = 1.7 +/- 0.5M(circle dot) (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 LIR and star formation rates and a negative trend with the H-2 gas depletion timescales (tau(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 sigma(nu), which can be explained by CO opacity e ffects. 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 a ffected 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 similar to 10% smaller than those of CO lines, and that this discrepancy becomes more significant in ULIRGs with broad lines (sigma(nu) > 150 km s(-1)) and when considering the high-nu wings of the lines. This suggests that the low optical depth of [CI] can challenge its detection in di ffuse, 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 tau(dep), consistent with the hypothesis that active galactic nucleus feedback may reduce the e fficiency 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.