A high-resolution mm and cm study of the obscured LIRG NGC 4418 - A
  compact obscured nucleus fed by in-falling gas?
  The aim of this study is to constrain the dynamics, structure and feeding of
the compact nucleous of NGC4418, and to reveal the nature of the main hidden
power source: starburst or AGN. We obtained high spatial resolution
observations of NGC4418 at 1.4 and 5 GHz with MERLIN, and at 230 and 270 GHz
with the SMA very extended configuration. We use the continuum morphology and
flux density to estimate the size of the emitting region, the star formation
rate and the dust temperature. Emission lines are used to study the kinematics
through position-velocity diagrams. Molecular emission is studied with
population diagrams and by fitting an LTE synthetic spectrum. We detect bright
1mm line emission from CO, HC3N, HNC and C34S, and 1.4 GHz absorption from HI.
The CO 2-1 emission and HI absorption can be fit by two velocity components at
2090 and 2180 km s-1. We detect vibrationally excited HC3N and HNC, with Tvib
300K. Molecular excitation is consistent with a layered temperature structure,
with three main components at 80, 160 and 300 K. For the hot component we
estimate a source size of less than 5 pc. The nuclear molecular gas surface
density of 1e4 Msun pc-2 is extremely high, and similar to that found in the
ultra-luminous infrared galaxy (ULIRG) Arp220. Our observations confirm the the
presence of a molecular and atomic in-flow, previously suggested by Herschel
observations, which is feeding the activity in the center of NGC4418. 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 Arp220, with an age of 3-10 Myr and a star formation rate
>10 Msun yr-1. If an AGN is present, it must be extremely Compton-thick.
