The nuclear PAH emission of merger system NGC 1614: rings within rings
  It is important to understand the interplay between nuclear star-formation
and nuclear activity when studying the evolution of gas rich galaxy mergers. We
present here new spatially resolved L-band integral field unit observations of
the inner kpc of the luminous IR galaxy NGC 1614. A broad ring of 3.3 um PAH
emission is found at a distance of approximately 200 pc from the core. This
ring overlaps with a previously established star-forming ring detected with
Pa_alpha and radio continuum observations, but peaks outside it, especially if
determined using the PAH equivalent width. Using the characteristics of the PAH
emission and the ionised gas emission we argue that NGC 1614 features an
outward propagating ring of star formation, where the equivalent width of the
PAH emission localises the regions where the current star formation is just
expanding into the molecular gas outward of the nucleus. The core itself shows
a highly luminous, slightly resolved (at ~80 pc) L-band continuum source. We
find no evidence of AGN activity and rule out the presence of an obscured AGN
using L-band diagnostics. Furthermore, we detect the likely companion galaxy
from archival HST/ACS imaging. The star formation and dynamical characteristics
of the system are consistent with a relatively major merger just after its
second passage. An outstanding question is how a gas-rich advanced merger such
as this one, with strong LIRG level nuclear starburst and major merger-like
tidal features, has not yet developed an active nucleus.
