13CO 1-0 imaging of the Medusa merger, NGC4194
  Studying molecular gas properties in merging galaxies gives important clues
to the onset and evolution of interaction-triggered starbursts. The CO/13CO 1-0
line intensity ratio can be used as a tracer of how dynamics and star formation
processes impact the gas properties. The Medusa (NGC~4194) merger is
particularly interesting to study since its LFIR/LCO ratio rivals that of
ultraluminous galaxies (ULIRGs), despite the comparatively modest luminosity,
indicating an exceptionally high star formation efficiency (SFE) in the Medusa
merger.
  Interferometric OVRO observations of CO and 13CO 1-0 in the Medusa show the
CO/13CO intensity ratio increases from normal, quiescent values (7-10) in the
outer parts (r>2 kpc) of the galaxy to high (16 to >40) values in the central
(r<1 kpc) starburst region. In the centre there is an east-west gradient where
the line ratio changes by more than a factor of three over 5" (945 pc). The
integrated 13CO emission peaks in the north-western starburst region while the
central CO emission is strongly associated with the prominent crossing
dust-lane. We discuss the central east-west gradient in the context of gas
properties in the starburst and the central dust lane. We suggest that the
central gradient is mainly caused by diffuse gas in the dust lane. In this
scenario, the actual molecular mass distribution is better traced by the 13CO
1-0 emission than the CO. The possibilities of temperature and abundance
gradients are also discussed. We compare the central gas properties of the
Medusa to those of other minor mergers and suggest that the extreme and
transient phase of the Medusa star formation activity has similar traits to
those of high-redshift galaxies.
