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Y. Ohyama (Subaru Telescope, NAOJ), M. Yoshida (Okayama Astrophys. Obs., NAOJ), T. Takata (Subaru Telescope, NAOJ)
We report on our new analysis of the spatial and kinematical distribution of warm and cold molecular gas in the nearby luminous infrared galaxy NGC 6240 to explore the origin of its unusually luminous H2 emission. The gas components are known to be distributed between the two merging nuclei, forming an off-nuclear molecular gas concentration. By comparing three-dimensional emission-line data (in space and velocity) of CO (J=2-1) and H2, we search for the spatial and kinematical conditions under which efficient H2 emission is produced. We focus on the H2 emitting efficiency, defined in terms of the intensity ratio of H2 to CO [I(H2)/I(CO)], as a function of velocity. The integrated efficiency is calculated by integrating the velocity profile of the efficiency in blue, red, and total (blue+red) velocity regions of the profile. We find that both the total H2 emitting efficiency and the blue-to-red ratio of the efficiency are larger in regions surrounding the CO and H2 intensity peaks, and the efficiency and the kinematical conditions in the warm molecular gas are closely related to each other. We compare two possible models for these characteristics: a large-scale collision between the molecular gas concentration and the merging nuclei, and a collision between the molecular gas concentration and the external superwind outflow from the southern nucleus. The latter model seems more plausible, since it can reproduce the enhanced emitting efficiency of blueshifted H2 around the molecular gas concentration, if the superwind blows from the southern nucleus toward us, hitting the entire gas concentration from behind. Internal cloud-cloud collisions within the molecular gas concentration are enhanced by the interaction with the superwind outflow, and efficient and intense shock-excited H2 emission is expected as a result of the cloud-crushing mechanism.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.