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

Abstract

We present a high sensitivity, ground-based spectral line survey of low-J carbon monoxide (CO(J<SUB>up</SUB> → J<SUB>up</SUB> − 1) with J<SUB>up</SUB> = 1, 2, 3) and neutral carbon [CI] <SUP>3</SUP>P<SUB>1</SUB>−<SUP>3</SUP>P<SUB>0</SUB> ([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<SUB>2</SUB> conversion factor of ⟨α<SUB>CO</SUB>⟩ = 1.7 ± 0.5 M<SUB>⊙</SUB> (K km s<SUP>−1</SUP>pc<SUP>2</SUP>)<SUP>−1</SUP> for ULIRGs. We derive median galaxy-integrated CO line ratios of «r<SUB>21</SUB>» = L<SUB>CO(2-1)</SUB><SUP>'</SUP>/L<SUB>CO(1-0)</SUB><SUP>'</SUP> = 1.09, «r<SUB>31</SUB>» = L<SUB>CO(3-2)</SUB><SUP>'</SUP>/L<SUB>CO(1-0)</SUB><SUP>'</SUP> = 0.76, and «r<SUB>32</SUB>» = L<SUB>CO(3-2)</SUB><SUP>'</SUP>/L<SUB>CO(2-1)</SUB><SUP>'</SUP> = 0.76, significantly higher than normal star-forming galaxies, confirming the exceptional molecular gas properties of ULIRGs. We find that the r<SUB>21</SUB> and r<SUB>32</SUB> ratios are poor tracers of CO excitation in ULIRGs, while r<SUB>31</SUB> shows a positive trend with L<SUB>IR</SUB> and star formation rates and a negative trend with the H<SUB>2</SUB> gas depletion timescales (τ<SUB>dep</SUB>). Our investigation of CO line ratios as a function of gas kinematics shows no clear trends, except for a positive relation between r<SUB>21</SUB> and σ<SUB>v</SUB>, which can be explained by CO opacity effects. These ULIRGs are also characterized by high L<SUB>[CI](1-0)</SUB><SUP>'</SUP>/L<SUB>CO(1-0)</SUB><SUP>'</SUP> ratios, with a measured median value of ⟨r<SUB>CICO</SUB>⟩ = 0.18, higher than values from previous interferometric studies that were affected by missing [CI] line flux. The r<SUB>CICO</SUB> 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<SUB>v</SUB> > 150 km s<SUP>−1</SUP>) 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<SUB>2</SUB> gas in these components. Finally, we find that higher L<SUB>AGN</SUB> are associated with longer τ<SUB>dep</SUB>, 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<SUB>2</SUB> 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....