Deep ALMA imaging of the merger NGC1614 - Is CO tracing a massive inflow
  of non-starforming gas?
  Observations of the molecular gas over scales of 0.5 to several kpc provide
crucial information on how gas moves through galaxies, especially in mergers
and interacting systems, where it ultimately reaches the galaxy center,
accumulates, and feeds nuclear activity. Studying the processes involved in the
gas transport is an important step forward to understand galaxy evolution.
12CO, 13CO and C18O1-0 high-sensitivity ALMA observations were used to assess
properties of the large-scale molecular gas reservoir and its connection to the
circumnuclear molecular ring in NGC1614. The role of excitation and abundances
were studied in this context. Spatial distributions of the 12CO and 13CO
emission show significant differences. 12CO traces the large-scale molecular
gas reservoir, associated with a dust lane that harbors infalling gas. 13CO
emission is - for the first time - detected in the large-scale dust lane. Its
emission peaks between dust lane and circumnuclear molecular ring. A
12CO-to-13CO1-0 intensity ratio map shows high values in the ring region (~30)
typical for the centers of luminous galaxy mergers and even more extreme values
in the dust lane (>45). This drop in ratio is consistent with molecular gas in
the dust lane being in a diffuse, unbound state while being funneled towards
the nucleus. We find a high 16O-to-18O abundance ratio in the starburst region
(>900), typical of quiescent disk gas - by now, the starburst is expected to
have enriched the nuclear ISM in 18O relative to 16O. The massive inflow of gas
may be partially responsible for the low 18O/16O abundance since it will dilute
the starburst enrichment with unprocessed gas from greater radii. The
12CO-to-13CO abundance is consistent with this scenario. It suggests that the
nucleus of NGC1614 is in a transient phase of evolution where starburst and
nuclear growth are fuelled by returning gas from the minor merger event.
