EXCITATION MECHANISM OF MOLECULAR-HYDROGEN EMISSION IN NGC-6240

We have obtained H- and K-band spectra simultaneously in the central r egion of the merging galaxy system NGC 6240. From the observed line fl ux ratios of molecular hydrogen (H-2) emission, including sensitive up per limits on highly excited H-2 lines, we conclude that the emission is excited purely by thermal mechanisms. Previously suggested contribu tions of UV fluorescence are negligible, at least in the central regio n. The H-2 lines are most likely excited by shocks, nondissociative or partially dissociative ones, although we cannot rule out completely X -ray heating. The H-2 lines are emitted between the double nuclei. Fro m its excitation mechanism and its peak position, we conclude that the emission is from a global shock caused by a galaxy-galaxy collision. The comparison between the timescale of the H-2 emission and the dynam ical timescale of the double nuclei suggests that the galaxy-galaxy me rging is in a very early phase, such as would be the case for the firs t close encounter.