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Superwind-Driven Intense H_2 Emission in NGC 6240
We have performed a long-slit K-band spectroscopic observation of theluminous infrared galaxy NGC 6240. Spatially extended H_2 emission isdetected over 3.3 kpc around the two nuclei. The peak position of theH_2 v=1-0 S(1) emission is located at ~ 0.3''-0.'' north of the southernnucleus. Based on line-ratio analyses, we suggest that the excitationmechanism of H_2 is pure thermal at most positions. We find thefollowing three velocity components in the H_2 emission around thesouthern nucleus and its south region: a blue-shifted component (\approx-250 km s^{-1} with respect to V_sys), which is recognized as a distinctC-shape distortion in the velocity field around the southern nucleus; ahigh-velocity blue-shifted ``wing'' component (\sim -1000 km s^{-1} withrespect to V_sys); and a component indicating possible line splitting of\sim 500 km s^{-1}. We show that these kinematic properties can bereproduced by expanding motion of a shell-like structure around thesouthern nucleus. The offset peak position of the H_2 emission can beunderstood if we assume that the shell expanding to the north interactswith the extragalactic molecular gas which has been transferred duringthe merging of the two nuclei. At the interface the cloud-crushingmechanism may work efficiently, and shock-excited intense H_2 emissionis thus expected there. With this mechanism, the H_2 luminosity can beexplained without global shock driven by the collision of two nuclei.All of these findings lead us to propose a model in which most of theH_2$ emission is attributed to shock excitation driven by the superwindactivity of the southern nucleus.
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