Orateur
Description
We use high sensitivity, beam-matched APEX, ALMA, and ACA (sub)-mm observations to study the state of the cold molecular gas in a sample of ~40 local ultra luminous infrared galaxies (ULIRGs), hence probing mostly galaxy mergers characterized by strong starburst and AGN activity. We aim to understand how such energetic feedback processes affect the kinematics and excitation of the molecular ISM of ULIRGs. To do so, we use proxy tracers such as low-J CO (up to J=3) and neutral atomic carbon ([CI](1-0)) emission lines, which allow us to study their different line ratios and in turn get insight on the physical properties of the ISM, such as its excitation temperature and density, which in turn allow us to constrain the H2 mass using two independent gas tracers. Additionally, by performing a multi-Gaussian component fit to the high S/N CO spectral line observations, we can study separately the spectral line components that are characterised by different line-of-sight velocities and velocity dispersions, comparing the CO line excitation of the quiescent and the fast-moving molecular gas. We find extremely high average line ratios, however no correlation with SFR, AGN luminosity or SFE, indicating that these low-J CO lines are poor tracers of ISM heating mechanisms in such IR luminous mergers. We, however, suggest that large gas velocity gradients caused by turbulence and/or massive molecular outflows are at the origin of the most extremely high CO line ratios measured the our sample. Lastly, we find that these sources are very bright in [CI](1-0) compared to other galaxies and this enhancement seems to correlate with the line widths (turbulence/outflows), which are on average narrower for [CI] than CO lines in our sample of local ULIRGs, which may suggest different behaviours for different gas tracers.
