Shock excitation in interacting galaxies: Markarian 266

We present near-infrared data on the luminous interacting system Mrk 266 (NGC 5256), comprising 2 pm continuum, and Br gamma and 1-0 S(1) emission-line images, together with K-band spectra. We have fitted stellar templates to the continuum, allowing us to account for all the stellar features and hence detect even faint gas excitation emission lines, including eight and 11 H-2 lines in the southwest and northeast nuclei, respectively. Population diagrams for the excited H-2 molecules indicate that most of the 1-0 S(1) in each of the nuclei has a thermal origin. We discuss this with reference to the observed morphologies, especially that of the 1-0 S(1) line. In particular, the core of 1-0 S(1) in the northeast nucleus is more compact than the 2 mu m continuum, while in the southwest nucleus the 1-0 S(1) is significantly offset by 500 pc from the continuum land other) emission. Last, we address the issue of the region midway between the two nuclei, where previously a strong source of radio continuum has been observed. These results are set in the context of interacting galaxies where shock-excited emission might be expected to occur as a direct consequence of the interaction.