The radio core structure of the luminous infrared galaxy NGC 4418
  The galaxy NGC 4418 contains one of the most compact obscured nuclei within a
luminous infrared galaxy (LIRG) in the nearby Universe. This nucleus contains a
rich molecular gas environment and an unusually high ratio of infrared to radio
luminosity (q-factor). The compact nucleus is powered by either a compact
starburst or an active galactic nucleus (AGN). The aim of this study is to
constrain the nature of the nuclear region (starburst or AGN) within NGC 4418
via very-high-resolution radio imaging. Archival data from radio observations
using the EVN and MERLIN interferometers are imaged. Sizes and flux densities
are obtained by fitting Gaussian intensity distributions to the image. The
average spectral index of the compact radio emission is estimated from
measurements at 1.4 GHz and 5.0 GHz. The nuclear structure of NGC 4418 visible
with EVN and MERLIN consists of eight compact (<49 mas i.e. <8 pc) features
spread within a region of 250 mas, i.e. 41 pc. We derive an inverted spectral
index $\alpha\ge0.7$ ($S_\nu\propto\nu^{\alpha}$) for the compact radio
emission. Brightness temperatures $>10^{4.8}$ K indicate that these compact
features cannot be HII-regions. The complex morphology and inverted spectrum of
the eight detected compact features is evidence against the hypothesis that an
AGN alone is powering the nucleus of NGC 4418. The compact features could be
super star clusters (SSCs) with intense star formation, and their associated
free-free absorption could then naturally explain both their inverted radio
spectrum and the low radio to IR ratio of the nucleus. The required star
formation area density is extreme, however, and close to the limit of what can
be observed in a well-mixed thermal/non-thermal plasma produced by
star-formation, and is also close to the limit of what can be physically
sustained.
