NGC 6240: extended CO structures and their association with shocked gas

We present deep CO(1–0) observations of NGC 6240 performed with the IRAM Plateau de Bure Interferometer (PdBI). NGC 6240 is the prototypical example of a major galaxy merger in progress, caught at an early stage, with an extended, strongly-disturbed butterfly-like morphology and a heavily obscured active nucleus in the core of each progenitor galaxy. The CO line shows a skewed profile with very broad and asymmetric wings detected out to velocities of −600 km s −1 and +800 km s −1 with respect to the systemic velocity. The PdBI maps reveal two prominent structures of blueshifted CO emission. One extends eastward, i.e. approximately perpendicular to the line connecting the galactic nuclei, on scales of ∼7 kpc, and it shows velocities up to −400 km s −1 . The other extends southwestward out to ∼7 kpc from the nuclear region, and has a velocity of −100 km s −1 with respect to the systemic one. Interestingly, redshifted emission with velocities 400 to 800 km s −1 is detected around the two nuclei, extending in the east-west direction, and partly overlapping with the eastern blueshifted structure, although tracing a more compact region of size ∼1.7 kpc. The overlap between the southwestern CO blob and the dust lanes seen in HST images, which are interpreted as tidal tails, indicates that the molecular gas is deeply affected by galaxy interactions. The eastern blueshifted CO emission is cospatial with an Hα filament that is associated with strong H2 and soft X-ray emission. The analysis of Chandra X-ray data provides strong evidence of shocked gas at the position of the Hα emission. Its association with outflowing molecular gas supports a scenario where the molecular gas is compressed into a shock wave that propagates eastward from the nuclei. If this is an outflow, the active galactic nuclei are very likely the driving force.