2010A&A...522A..59A

Studying molecular gas properties in merging galaxies gives important clues to the onset and evolution of interaction-triggered starbursts. The (¹²CO)/(¹³CO) line intensity ratio can be used as a tracer of how dynamics and star formation processes impact the gas properties. The Medusa merger (NGC 4194) is particularly interesting to study since its L_FIR/L_CO ratio rivals that of ultraluminous galaxies (ULIRGs), despite the comparatively modest luminosity, indicating an exceptionally high star formation efficiency (SFE) in the Medusa merger. High resolution OVRO (Owens Valley Radio Observatory) observations of the ¹³CO 1-0 have been obtained and compared with matched resolution OVRO ¹²CO 1-0 data to investigate the molecular gas cloud properties in the Medusa merger. Interferometric observations of ¹²CO and ¹³CO 1-0 in the Medusa (NGC 4194) merger show the ¹²CO/¹³CO 1-0 intensity ratio (R) increases from normal, quiescent values (7-10) in the outer parts (r>2kpc) of the galaxy to high (16 to >40) values in the central (r<1kpc) starburst region. In the central two kpc there is an east-west gradient in R where the line ratio changes by more than a factor of three over 5" (945pc). The integrated ¹³CO emission peaks in the north-western starburst region while the central ¹²CO emission is strongly associated with the prominent crossing dust-lane. We discuss the central east-west gradient in R in the context of gas properties in the starburst and the central dust lane. We suggest that the central gradient in R 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 ¹²CO. 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.