2021A&A...651A..42P

We analyze new high-resolution (400 pc) ∼220GHz continuum and CO(2-1) Atacama Large Millimeter Array (ALMA) observations of a representative sample of 23 local (z < 0.165) ultra-luminous infrared systems (ULIRGs; 34 individual nuclei) as part of the "Physics of ULIRGs with MUSE and ALMA" (PUMA) project. The deconvolved half-light radii of the ∼220GHz continuum sources, r_cont, are between < 60 pc and 350 pc (median 80-100 pc). We associate these regions with the regions emitting the bulk of the infrared luminosity (L_IR). The good agreement, within a factor of 2, between the observed ∼220GHz fluxes and the extrapolation of the infrared gray-body as well as the small contributions from synchrotron and free-free emission support this assumption. The cold molecular gas emission sizes, r_CO, are between 60 and 700 pc and are similar in advanced mergers and early interacting systems. On average, r_CO are ∼2.5 times larger than r_cont. Using these measurements, we derived the nuclear L_IR and cold molecular gas surface densities (Σ_LIR=10^11.5-10^14.3L_☉/kpc² and Σ_H2=10^2.9-10^4.2M_☉/pc², respectively). Assuming that the L_IR is produced by star formation, the median Σ_LIR corresponds to Σ_SFR=2500M_☉/yr/kpc². This Σ_SFR implies extremely short depletion times, Σ_H2/Σ_SFR<1-15Myr, and unphysical star formation efficiencies >1 for 70% of the sample. Therefore, this favors the presence of an obscured active galactic nucleus (AGN) in these objects that could dominate the L_IR. We also classify the ULIRG nuclei in two groups: (a) compact nuclei (r_cont<120pc) with high mid-infrared excess emission (ΔL_6–20µm/L_IR) found in optically classified AGN; and (b) nuclei following a relation with decreasing ΔL_6–20µm/L_IR for decreasing r_cont. The majority, 60%, of the nuclei in interacting systems lie in the low-r_cont end (<120pc) of this relation, while this is the case for only 30% of the mergers. This suggests that in the early stages of the interaction, the activity occurs in a very compact and dust-obscured region while, in more advanced merger stages, the activity is more extended, unless an optically detected AGN is present. Approximately two-thirds of the nuclei have nuclear radiation pressures above the Eddington limit. This is consistent with the ubiquitous detection of massive outflows in local ULIRGs and supports the importance of the radiation pressure in the outflow launching process.