2013A&A...556A..66C

Understanding the nature of the power-source in luminous infrared galaxies (LIRG) is difficult due to their extreme obscuration. Observations at radio and mm wavelengths can penetrate large columns of dust and gas and provide unique insights into the properties of the compact obscured nuclei of LIRGs. The aim of this study is to constrain the dynamics, structure, and feeding of the compact nucleus of NGC 4418, and to reveal the nature of the main hidden power-source: starburst or active galactic nucleus (AGN). We obtained high spatial resolution observations of NGC 4418 at 1.4 and 5GHz with MERLIN, and at 230 and 270GHz with the SMA in very extended configuration. We used the continuum morphology and flux density to estimate the size of the emitting region, the star formation rate, and the dust temperature. Emission lines were used to study kinematics through position-velocity diagrams. Molecular emission was studied with population diagrams and by fitting a local thermal equilibrium (LTE) synthetic spectrum. We detect bright 1-mm-line emission from CO, HC₃N, HNC, and C³⁴S and 1.4GHz absorption from HI. The CO 2-1 emission and HI absorption can be fit by two velocity components at 2090 and 2180km/s. We detect vibrationally excited HC₃N and HNC, with T_vib∼300K. Molecular excitation is consistent with a layered temperature structure, with three main components at 80, 160, and 300K. For the hot component we estimate a source size of less than 5 pc. The nuclear molecular gas surface density of 10⁴M_☉/pc² is extremely high and similar to that found in the ultra-luminous infrared galaxy (ULIRG) Arp220. Our observations confirm the presence of a molecular and atomic in-flow, previously suggested by Herschel observations, which is feeding the activity in the center of NGC 4418. Molecular excitation confirms the presence of a very compact, hot dusty core. If a starburst is responsible for the observed IR flux, this has to be at least as extreme as the one in the ULIRG Arp 220, with an age of 3-10Myr and a star formation rate >10M_☉/yr. If an AGN is present, it must be extremely Compton-thick.