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

TL;DR

This spectral survey of 40 local (U)LIRGs using APEX and ALMA reveals their molecular gas properties, correlations between line emissions, and the impact of AGN activity on star formation efficiency, providing new insights into their interstellar medium.

Contribution

First comprehensive high-sensitivity CO and [CI] spectral survey of 40 local (U)LIRGs combining single-dish and interferometric data, analyzing molecular gas properties and excitation.

Findings

Strong correlation between CO and [CI] line luminosities.

Median CO-to-H2 conversion factor of 1.7±0.5 for ULIRGs.

Higher CO line ratios than normal star-forming galaxies, indicating unique molecular gas conditions.

Abstract

We present a high sensitivity spectral line survey of CO(1-0), CO(2-1), CO(3-2) and [CI](1-0) 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.