Excitation Mechanism of Molecular Hydrogen Emission in NGC 6240

Abstract

We have obtained H- and K-band spectra simultaneously in the central region of the merging galaxy system NGC 6240. From the observed line flux ratios of molecular hydrogen (H2) emission, including sensitive upper limits on highly excited H2 lines, we conclude that the emission is excited purely by thermal mechanisms. Previously suggested contributions of UV fluorescence are negligible, at least in the central region. The H2 lines are most likely excited by shocks, nondissociative or partially dissociative ones, although we cannot rule out completely X-ray heating. The H2 lines are emitted between the double nuclei. From 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 H2 emission and the dynamical timescale of the double nuclei suggests that the galaxy-galaxy merging is in a very early phase, such as would be the case for the first close encounter.