A sensitive APEX and ALMA CO(1-0), CO(2-1), CO(3-2), and [CI](1-0) spectral survey of 40 local (U)LIRGs

Abstract: We present a high sensitivity spectral line survey of CO(1-0), CO(2-1), CO(3-2) and CI in 40 local (ultra) luminous infrared galaxies ((U)LIRGs), all with previous Herschel OH119 $\mu$m observations. We use single-dish observations (PI and archival) conducted with APEX, complemented with ALMA and ACA data. We study the total emission and pay special attention to the extended low-surface brightness components. We find a tight correlation between low-J CO and [CI] line luminosities suggesting their emission arise from similar regions, at least when averaged over galactic scales. We estimate a median CO-to-H$2$ conversion factor of $1.7\pm 0.5$ M${\odot}$ (K km s$^{-1}$ pc$^2)^{-1}$ for ULIRGs, using [CI] as an independent tracer. We derive median galaxy-integrated CO line ratios ($r_{21}$, $r_{31}$ and $r_{32}$), as well as $r_{CICO}$, significantly higher than normal star forming galaxies, confirming the exceptional molecular gas properties of ULIRGs. We find that $r_{21}$ and $r_{32}$ are poor tracers of CO excitation in ULIRGs, while $r_{31}$ shows a positive trend with $L_{IR}$ and SFR, and a negative trend with the H$2$ gas depletion timescales ($\tau{dep}$). When studying CO line ratios as a function of gas kinematics, we find a positive relation between $r_{21}$ and $\sigma_v$, which can be explained by CO opacity effects. We find that the linewidths of [CI] lines are ~10% narrower than CO lines, which may suggest that the low optical depth of [CI] can challenge its detection in diffuse, low-surface brightness outflows, and so its use as a tracer of CO-dark H$2$ gas in these components. Finally, we find that higher $L{AGN}$ are associated to longer $\tau_{dep}$, consistent with the hypothesis that AGN feedback may reduce the efficiency of star formation.