Complex molecular gas structure in the Medusa merger

Abstract

High resolution OVRO aperture synthesis maps of the 12CO 1-0 emission in the `Medusa' galaxy merger (NGC4194) reveal the molecular emission being surprisingly extended. It is distributed on a total scale of 25$''$ (4.7 kpc) - despite the apparent advanced stage of the merger. The complex, striking CO morphology occupies mainly the center and the north-eastern part of the main optical body. The extended 12CO flux is tracing two prominent dust lanes: one which is crossing the central region at right angle and a second which curves to the north-east and then into the beginning of the northern tidal tail. The bulk of the 12CO emission (67%) can be found in a complex starburst region encompassing the central 2kpc. The molecular gas is distributed in five major emission regions of typical size 300pc. About 15% of the total 12CO flux is found in a bright region 1.5'' south of the radio continuum nucleus. We suggest that this region together with the kpc sized central starburst is being fueled by gas flows along the central dust lane. We discuss the merger history of NGC4194 and suggest that it may be the result of an early-type/spiral merger with a shell emerging to the south of the center. The total molecular mass in the system is estimated to be at most 2 X 10^9 M_sun (depending on the CO - H_2 conversion factor). The high 12CO/13CO 1-0 intensity ratio, ~20, indicates highly excited physical conditions in the interstellar medium showing that the starburst has a big impact on its surrounding ISM. At the current rate of star formation, the gas will be consumed within 40 million years.