2019A&A...628A..71H

We present IRAM-30 m Telescope ¹²CO and ¹³CO observations of a sample of 55 luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs) in the local universe. This sample is a subset of the Great Observatory All-Sky LIRG Survey (GOALS), for which we use ancillary multi-wavelength data to better understand their interstellar medium and star formation properties. Fifty-three (96%) of the galaxies are detected in ¹²CO, and 29 (52%) are also detected in ¹³CO above a 3σ level. The median full width at zero intensity (FWZI) velocity of the CO line emission is 661km/s, and ∼54% of the galaxies show a multi-peak CO profile. Herschel photometric data is used to construct the far-IR spectral energy distribution of each galaxy, which are fit with a modified blackbody model that allows us to derive dust temperatures and masses, and infrared luminosities. We make the assumption that the gas-to-dust mass ratio of (U)LIRGs is comparable to local spiral galaxies with a similar stellar mass (i.e., gas/dust of mergers is comparable to their progenitors) to derive a CO-to-H₂ conversion factor of <α> =1.8_–0.8^+1.3M_☉/(K.(km/s).pc²); such a value is comparable to that derived for (U)LIRGs based on dynamical mass arguments. We derive gas depletion times of 400 - 600 Myr for the (U)LIRGs, compared to the 1.3Gyr for local spiral galaxies. Finally, we re-examine the relationship between the ¹²CO/¹³CO ratio and dust temperature, confirming a transition to elevated ratios in warmer systems.