Superwind-driven Intense H2 Emission in NGC 6240 II: Detailed Comparison of Kinematical and Morphological Structures of the Warm and Cold Molecular Gas

We report on our new analysis of the spatial and kinematical distribution of warm and cold molecular gas in the nearby, prototypical, luminous infrared galaxy NGC 6240, which was undertaken 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) in the radio and H2 in the near infrared, we are able to search for the spatial and kinematical conditions under which efficient H2 emission is produced in much more detail than has previously been possible. In particular, 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. We derive this by utilizing the recent high-resolution three-dimensional data presented by Tecza et al. (2000). The integrated H2 emitting efficiency is calculated by integrating the velocity profile of H2 emitting efficiency in blue, red, and total (blue + red) velocity regions of the profile. We find that (1) 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 (2) the H2 emitting efficiency and the kinematical conditions in the warm molecular gas are closely related to each other. We compare two possible models that might explain 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