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Studying molecular gas properties in merging galaxies gives important cluesto the onset and evolution of interaction-triggered starbursts. The CO/13CO 1-0line intensity ratio can be used as a tracer of how dynamics and star formationprocesses impact the gas properties. The Medusa (NGC~4194) merger isparticularly interesting to study since its LFIR/LCO ratio rivals that ofultraluminous galaxies (ULIRGs), despite the comparatively modest luminosity,indicating an exceptionally high star formation efficiency (SFE) in the Medusamerger.  Interferometric OVRO observations of CO and 13CO 1-0 in the Medusa show theCO/13CO intensity ratio increases from normal, quiescent values (7-10) in theouter 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 wherethe line ratio changes by more than a factor of three over 5" (945 pc). Theintegrated 13CO emission peaks in the north-western starburst region while thecentral CO emission is strongly associated with the prominent crossingdust-lane. We discuss the central east-west gradient in the context of gasproperties in the starburst and the central dust lane. We suggest that thecentral gradient is mainly caused by diffuse gas in the dust lane. In thisscenario, the actual molecular mass distribution is better traced by the 13CO1-0 emission than the CO. The possibilities of temperature and abundancegradients are also discussed. We compare the central gas properties of theMedusa to those of other minor mergers and suggest that the extreme andtransient phase of the Medusa star formation activity has similar traits tothose of high-redshift galaxies.

13CO 1-0 imaging of the Medusa merger, NGC4194